mirror of
https://github.com/boostorg/optional.git
synced 2026-04-29 02:03:19 +02:00
Merge branch 'develop'
This commit is contained in:
Andrzej Krzemienski
+554
-337
@@ -1,4 +1,4 @@
|
||||
name: GitHub Actions CI
|
||||
name: CI
|
||||
|
||||
on:
|
||||
pull_request:
|
||||
@@ -6,17 +6,11 @@ on:
|
||||
branches:
|
||||
- master
|
||||
- develop
|
||||
- githubactions*
|
||||
- feature/**
|
||||
- fix/**
|
||||
- pr/**
|
||||
|
||||
concurrency:
|
||||
group: ${{format('{0}:{1}', github.repository, github.ref)}}
|
||||
cancel-in-progress: true
|
||||
|
||||
env:
|
||||
NET_RETRY_COUNT: 5
|
||||
UBSAN_OPTIONS: print_stacktrace=1
|
||||
ACTIONS_ALLOW_USE_UNSECURE_NODE_VERSION: true
|
||||
|
||||
jobs:
|
||||
posix:
|
||||
@@ -24,373 +18,596 @@ jobs:
|
||||
fail-fast: false
|
||||
matrix:
|
||||
include:
|
||||
- name: "TOOLSET=gcc COMPILER=g++ CXXSTD=03,11 Job 0"
|
||||
buildtype: "boost"
|
||||
packages: ""
|
||||
packages_to_remove: ""
|
||||
os: "ubuntu-20.04"
|
||||
container: "ubuntu:16.04"
|
||||
cxx: "g++"
|
||||
sources: ""
|
||||
llvm_os: ""
|
||||
llvm_ver: ""
|
||||
toolset: "gcc"
|
||||
compiler: "g++"
|
||||
- toolset: gcc-4.8
|
||||
cxxstd: "03,11"
|
||||
- name: "TOOLSET=gcc COMPILER=g++-4.7 CXXSTD=03,11 Job 1"
|
||||
buildtype: "boost"
|
||||
packages: "g++-4.7"
|
||||
packages_to_remove: ""
|
||||
os: "ubuntu-20.04"
|
||||
container: "ubuntu:16.04"
|
||||
cxx: "g++-4.7"
|
||||
sources: ""
|
||||
llvm_os: ""
|
||||
llvm_ver: ""
|
||||
toolset: "gcc"
|
||||
compiler: "g++-4.7"
|
||||
cxxstd: "03,11"
|
||||
- name: "TOOLSET=gcc COMPILER=g++-4.8 CXXSTD=03,11 Job 2"
|
||||
buildtype: "boost"
|
||||
packages: "g++-4.8"
|
||||
packages_to_remove: ""
|
||||
os: "ubuntu-20.04"
|
||||
container: "ubuntu:16.04"
|
||||
cxx: "g++-4.8"
|
||||
sources: ""
|
||||
llvm_os: ""
|
||||
llvm_ver: ""
|
||||
toolset: "gcc"
|
||||
compiler: "g++-4.8"
|
||||
cxxstd: "03,11"
|
||||
- name: "TOOLSET=gcc COMPILER=g++-4.9 CXXSTD=03,11 Job 3"
|
||||
buildtype: "boost"
|
||||
packages: "g++-4.9"
|
||||
packages_to_remove: ""
|
||||
os: "ubuntu-20.04"
|
||||
container: "ubuntu:16.04"
|
||||
cxx: "g++-4.9"
|
||||
sources: ""
|
||||
llvm_os: ""
|
||||
llvm_ver: ""
|
||||
toolset: "gcc"
|
||||
compiler: "g++-4.9"
|
||||
cxxstd: "03,11"
|
||||
- name: "TOOLSET=gcc COMPILER=g++-5 CXXSTD=03,11,14,1z Job 4"
|
||||
buildtype: "boost"
|
||||
packages: "g++-5"
|
||||
packages_to_remove: ""
|
||||
os: "ubuntu-20.04"
|
||||
container: "ubuntu:16.04"
|
||||
cxx: "g++-5"
|
||||
sources: ""
|
||||
llvm_os: ""
|
||||
llvm_ver: ""
|
||||
toolset: "gcc"
|
||||
compiler: "g++-5"
|
||||
os: ubuntu-latest
|
||||
container: ubuntu:18.04
|
||||
install: g++-4.8-multilib
|
||||
address-model: 32,64
|
||||
- toolset: gcc-5
|
||||
cxxstd: "03,11,14,1z"
|
||||
- name: "TOOLSET=gcc COMPILER=g++-6 CXXSTD=03,11,14,1z Job 5"
|
||||
buildtype: "boost"
|
||||
packages: "g++-6"
|
||||
packages_to_remove: ""
|
||||
os: "ubuntu-20.04"
|
||||
container: "ubuntu:16.04"
|
||||
cxx: "g++-6"
|
||||
sources: ""
|
||||
llvm_os: ""
|
||||
llvm_ver: ""
|
||||
toolset: "gcc"
|
||||
compiler: "g++-6"
|
||||
os: ubuntu-latest
|
||||
container: ubuntu:18.04
|
||||
install: g++-5-multilib
|
||||
address-model: 32,64
|
||||
- toolset: gcc-6
|
||||
cxxstd: "03,11,14,1z"
|
||||
- name: "TOOLSET=gcc COMPILER=g++-7 CXXSTD=03,11,14,17 Job 6"
|
||||
buildtype: "boost"
|
||||
packages: "g++-7"
|
||||
packages_to_remove: ""
|
||||
os: "ubuntu-20.04"
|
||||
container: "ubuntu:16.04"
|
||||
cxx: "g++-7"
|
||||
sources: ""
|
||||
llvm_os: ""
|
||||
llvm_ver: ""
|
||||
toolset: "gcc"
|
||||
compiler: "g++-7"
|
||||
os: ubuntu-latest
|
||||
container: ubuntu:18.04
|
||||
install: g++-6-multilib
|
||||
address-model: 32,64
|
||||
- toolset: gcc-7
|
||||
cxxstd: "03,11,14,17"
|
||||
- name: "TOOLSET=clang COMPILER=clang++ CXXSTD=03,11 Job 7"
|
||||
buildtype: "boost"
|
||||
packages: "clang"
|
||||
packages_to_remove: ""
|
||||
os: "ubuntu-22.04"
|
||||
container: "ubuntu:18.04"
|
||||
cxx: "clang++"
|
||||
sources: ""
|
||||
llvm_os: ""
|
||||
llvm_ver: ""
|
||||
toolset: "clang"
|
||||
compiler: "clang++"
|
||||
cxxstd: "03,11"
|
||||
- name: "TOOLSET=clang COMPILER=clang++-3.5 CXXSTD=03, Job 8"
|
||||
buildtype: "boost"
|
||||
packages: "clang-3.5 libstdc++-4.9-dev"
|
||||
packages_to_remove: ""
|
||||
os: "ubuntu-20.04"
|
||||
container: "ubuntu:16.04"
|
||||
cxx: "clang++-3.5"
|
||||
sources: ""
|
||||
llvm_os: "precise"
|
||||
llvm_ver: "3.5"
|
||||
toolset: "clang"
|
||||
compiler: "clang++-3.5"
|
||||
os: ubuntu-latest
|
||||
container: ubuntu:18.04
|
||||
install: g++-7-multilib
|
||||
address-model: 32,64
|
||||
- toolset: gcc-8
|
||||
cxxstd: "03,11,14,17,2a"
|
||||
os: ubuntu-20.04
|
||||
install: g++-8-multilib
|
||||
address-model: 32,64
|
||||
- toolset: gcc-9
|
||||
cxxstd: "03,11,14,17,2a"
|
||||
os: ubuntu-20.04
|
||||
install: g++-9-multilib
|
||||
address-model: 32,64
|
||||
- toolset: gcc-10
|
||||
cxxstd: "03,11,14,17,2a"
|
||||
os: ubuntu-20.04
|
||||
install: g++-10-multilib
|
||||
address-model: 32,64
|
||||
- toolset: gcc-11
|
||||
cxxstd: "03,11,14,17,20"
|
||||
os: ubuntu-22.04
|
||||
install: g++-11-multilib
|
||||
address-model: 32,64
|
||||
- toolset: gcc-12
|
||||
cxxstd: "03,11,14,17,20,2b"
|
||||
os: ubuntu-22.04
|
||||
install: g++-12-multilib
|
||||
address-model: 32,64
|
||||
- toolset: gcc-13
|
||||
cxxstd: "03,11,14,17,20,2b"
|
||||
os: ubuntu-latest
|
||||
container: ubuntu:23.04
|
||||
install: g++-13-multilib
|
||||
address-model: 32,64
|
||||
- toolset: clang
|
||||
compiler: clang++-3.9
|
||||
cxxstd: "03,11,14"
|
||||
- name: "TOOLSET=clang COMPILER=clang++-3.6 CXXSTD=03, Job 9"
|
||||
buildtype: "boost"
|
||||
packages: "clang-3.6"
|
||||
packages_to_remove: ""
|
||||
os: "ubuntu-20.04"
|
||||
container: "ubuntu:16.04"
|
||||
cxx: "clang++-3.6"
|
||||
sources: ""
|
||||
llvm_os: "precise"
|
||||
llvm_ver: "3.6"
|
||||
toolset: "clang"
|
||||
compiler: "clang++-3.6"
|
||||
os: ubuntu-latest
|
||||
container: ubuntu:18.04
|
||||
install: clang-3.9
|
||||
- toolset: clang
|
||||
compiler: clang++-4.0
|
||||
cxxstd: "03,11,14"
|
||||
- name: "TOOLSET=clang COMPILER=clang++-3.7 CXXSTD=03, Job 10"
|
||||
buildtype: "boost"
|
||||
packages: "clang-3.7"
|
||||
packages_to_remove: ""
|
||||
os: "ubuntu-20.04"
|
||||
container: "ubuntu:16.04"
|
||||
cxx: "clang++-3.7"
|
||||
sources: ""
|
||||
llvm_os: "precise"
|
||||
llvm_ver: "3.7"
|
||||
toolset: "clang"
|
||||
compiler: "clang++-3.7"
|
||||
cxxstd: "03,11,14"
|
||||
- name: "TOOLSET=clang COMPILER=clang++-3.8 CXXSTD=03, Job 11"
|
||||
buildtype: "boost"
|
||||
packages: "clang-3.8 libstdc++-4.9-dev"
|
||||
packages_to_remove: ""
|
||||
os: "ubuntu-20.04"
|
||||
container: "ubuntu:16.04"
|
||||
cxx: "clang++-3.8"
|
||||
sources: ""
|
||||
llvm_os: "precise"
|
||||
llvm_ver: "3.8"
|
||||
toolset: "clang"
|
||||
compiler: "clang++-3.8"
|
||||
cxxstd: "03,11,14"
|
||||
- name: "TOOLSET=clang COMPILER=clang++-3.9 CXXSTD=03, Job 12"
|
||||
buildtype: "boost"
|
||||
packages: "clang-3.9 libstdc++-4.9-dev"
|
||||
packages_to_remove: ""
|
||||
os: "ubuntu-20.04"
|
||||
container: "ubuntu:16.04"
|
||||
cxx: "clang++-3.9"
|
||||
sources: ""
|
||||
llvm_os: "precise"
|
||||
llvm_ver: "3.9"
|
||||
toolset: "clang"
|
||||
compiler: "clang++-3.9"
|
||||
cxxstd: "03,11,14"
|
||||
- name: "TOOLSET=clang COMPILER=clang++-4.0 CXXSTD=03, Job 13"
|
||||
buildtype: "boost"
|
||||
packages: "clang-4.0"
|
||||
packages_to_remove: ""
|
||||
os: "ubuntu-20.04"
|
||||
container: "ubuntu:14.04"
|
||||
cxx: "clang++-4.0"
|
||||
sources: ""
|
||||
llvm_os: "trusty"
|
||||
llvm_ver: "4.0"
|
||||
toolset: "clang"
|
||||
compiler: "clang++-4.0"
|
||||
cxxstd: "03,11,14"
|
||||
- name: "TOOLSET=clang COMPILER=clang++-5.0 CXXSTD=03, Job 14"
|
||||
buildtype: "boost"
|
||||
packages: "clang-5.0"
|
||||
packages_to_remove: ""
|
||||
os: "ubuntu-22.04"
|
||||
container: "ubuntu:18.04"
|
||||
cxx: "clang++-5.0"
|
||||
sources: ""
|
||||
llvm_os: ""
|
||||
llvm_ver: ""
|
||||
toolset: "clang"
|
||||
compiler: "clang++-5.0"
|
||||
os: ubuntu-latest
|
||||
container: ubuntu:18.04
|
||||
install: clang-4.0
|
||||
- toolset: clang
|
||||
compiler: clang++-5.0
|
||||
cxxstd: "03,11,14,1z"
|
||||
os: ubuntu-latest
|
||||
container: ubuntu:18.04
|
||||
install: clang-5.0
|
||||
- toolset: clang
|
||||
compiler: clang++-6.0
|
||||
cxxstd: "03,11,14,17"
|
||||
os: ubuntu-20.04
|
||||
install: clang-6.0
|
||||
- toolset: clang
|
||||
compiler: clang++-7
|
||||
cxxstd: "03,11,14,17"
|
||||
os: ubuntu-20.04
|
||||
install: clang-7
|
||||
- toolset: clang
|
||||
compiler: clang++-8
|
||||
cxxstd: "03,11,14,17"
|
||||
os: ubuntu-20.04
|
||||
install: clang-8
|
||||
- toolset: clang
|
||||
compiler: clang++-9
|
||||
cxxstd: "03,11,14,17,2a"
|
||||
os: ubuntu-20.04
|
||||
install: clang-9
|
||||
- toolset: clang
|
||||
compiler: clang++-10
|
||||
cxxstd: "03,11,14,17,2a"
|
||||
os: ubuntu-20.04
|
||||
- toolset: clang
|
||||
compiler: clang++-11
|
||||
cxxstd: "03,11,14,17,2a"
|
||||
os: ubuntu-20.04
|
||||
- toolset: clang
|
||||
compiler: clang++-12
|
||||
cxxstd: "03,11,14,17,20"
|
||||
os: ubuntu-20.04
|
||||
- toolset: clang
|
||||
compiler: clang++-13
|
||||
cxxstd: "03,11,14,17,20,2b"
|
||||
container: ubuntu:22.04
|
||||
os: ubuntu-latest
|
||||
install: clang-13
|
||||
- toolset: clang
|
||||
compiler: clang++-14
|
||||
cxxstd: "03,11,14,17,20,2b"
|
||||
container: ubuntu:22.04
|
||||
os: ubuntu-latest
|
||||
install: clang-14
|
||||
- toolset: clang
|
||||
compiler: clang++-15
|
||||
cxxstd: "03,11,14,17,20,2b"
|
||||
container: ubuntu:22.04
|
||||
os: ubuntu-latest
|
||||
install: clang-15
|
||||
- toolset: clang
|
||||
compiler: clang++-16
|
||||
cxxstd: "03,11,14,17,20,2b"
|
||||
container: ubuntu:23.04
|
||||
os: ubuntu-latest
|
||||
install: clang-16
|
||||
- toolset: clang
|
||||
compiler: clang++-17
|
||||
cxxstd: "03,11,14,17,20,2b"
|
||||
container: ubuntu:23.10
|
||||
os: ubuntu-latest
|
||||
install: clang-17
|
||||
- toolset: clang
|
||||
cxxstd: "03,11,14,17,20,2b"
|
||||
os: macos-12
|
||||
- toolset: clang
|
||||
cxxstd: "03,11,14,17,20,2b"
|
||||
os: macos-13
|
||||
|
||||
runs-on: ${{ matrix.os }}
|
||||
container: ${{ matrix.container }}
|
||||
runs-on: ${{matrix.os}}
|
||||
container: ${{matrix.container}}
|
||||
|
||||
defaults:
|
||||
run:
|
||||
shell: bash
|
||||
|
||||
steps:
|
||||
- name: Check if running in container
|
||||
if: matrix.container != ''
|
||||
run: echo "GHA_CONTAINER=${{ matrix.container }}" >> $GITHUB_ENV
|
||||
- name: If running in container, upgrade packages
|
||||
if: matrix.container != ''
|
||||
- uses: actions/checkout@v3
|
||||
|
||||
- name: Setup container environment
|
||||
if: matrix.container
|
||||
run: |
|
||||
if [ -f "/etc/debian_version" ]; then
|
||||
echo "DEBIAN_FRONTEND=noninteractive" >> $GITHUB_ENV
|
||||
export DEBIAN_FRONTEND=noninteractive
|
||||
apt-get -o Acquire::Retries=$NET_RETRY_COUNT update
|
||||
apt-get -o Acquire::Retries=$NET_RETRY_COUNT install -y sudo software-properties-common curl wget apt-transport-https ca-certificates
|
||||
# Need (newer) git, and the older Ubuntu container may require requesting the key manually using port 80
|
||||
curl -sSL --retry ${NET_RETRY_COUNT:-5} 'http://keyserver.ubuntu.com/pks/lookup?op=get&search=0xE1DD270288B4E6030699E45FA1715D88E1DF1F24' | sudo gpg --dearmor > /etc/apt/trusted.gpg.d/git-core_ubuntu_ppa.gpg
|
||||
for i in {1..${NET_RETRY_COUNT:-3}}; do sudo -E add-apt-repository -y ppa:git-core/ppa && break || sleep 10; done
|
||||
apt-get -o Acquire::Retries=$NET_RETRY_COUNT update
|
||||
apt-get -o Acquire::Retries=$NET_RETRY_COUNT install -y g++ python libpython-dev git unzip libssl-dev build-essential libc++-helpers gcc-multilib g++-multilib pkgconf ccache
|
||||
fi
|
||||
apt-get update
|
||||
apt-get -y install sudo python3 git g++
|
||||
|
||||
- uses: actions/checkout@v2
|
||||
|
||||
- name: linux
|
||||
shell: bash
|
||||
env:
|
||||
CXX: ${{ matrix.cxx }}
|
||||
SOURCES: ${{ matrix.sources }}
|
||||
LLVM_OS: ${{ matrix.llvm_os }}
|
||||
LLVM_VER: ${{ matrix.llvm_ver }}
|
||||
PACKAGES: ${{ matrix.packages }}
|
||||
PACKAGES_TO_REMOVE: ${{ matrix.packages_to_remove }}
|
||||
JOB_BUILDTYPE: ${{ matrix.buildtype }}
|
||||
TOOLSET: ${{ matrix.toolset }}
|
||||
COMPILER: ${{ matrix.compiler }}
|
||||
CXXSTD: ${{ matrix.cxxstd }}
|
||||
TRAVIS_BRANCH: ${{ github.base_ref }}
|
||||
TRAVIS_OS_NAME: "linux"
|
||||
- name: Install packages
|
||||
if: matrix.install
|
||||
run: |
|
||||
echo '==================================> SETUP'
|
||||
echo '==================================> PACKAGES'
|
||||
set -e
|
||||
if [ -n "$PACKAGES_TO_REMOVE" ]; then sudo apt-get purge -y $PACKAGES_TO_REMOVE; fi
|
||||
echo ">>>>> APT: REPO.."
|
||||
for i in {1..3}; do sudo -E apt-add-repository -y "ppa:ubuntu-toolchain-r/test" && break || sleep 2; done
|
||||
|
||||
if test -n "${LLVM_OS}" ; then
|
||||
wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
|
||||
if test -n "${LLVM_VER}" ; then
|
||||
sudo -E apt-add-repository "deb http://apt.llvm.org/${LLVM_OS}/ llvm-toolchain-${LLVM_OS}-${LLVM_VER} main"
|
||||
else
|
||||
# Snapshot (i.e. trunk) build of clang
|
||||
sudo -E apt-add-repository "deb http://apt.llvm.org/${LLVM_OS}/ llvm-toolchain-${LLVM_OS} main"
|
||||
fi
|
||||
fi
|
||||
echo ">>>>> APT: UPDATE.."
|
||||
sudo -E apt-get -o Acquire::Retries=3 update
|
||||
if test -n "${SOURCES}" ; then
|
||||
echo ">>>>> APT: INSTALL SOURCES.."
|
||||
for SOURCE in $SOURCES; do
|
||||
sudo -E apt-add-repository ppa:$SOURCE
|
||||
done
|
||||
fi
|
||||
echo ">>>>> APT: INSTALL ${PACKAGES}.."
|
||||
sudo -E DEBIAN_FRONTEND=noninteractive apt-get -o Acquire::Retries=3 -y --no-install-suggests --no-install-recommends install ${PACKAGES}
|
||||
sudo apt-get update
|
||||
sudo apt-get -y install ${{matrix.install}}
|
||||
|
||||
echo '==================================> INSTALL AND COMPILE'
|
||||
set -e
|
||||
export TRAVIS_BUILD_DIR=$(pwd)
|
||||
export TRAVIS_BRANCH=${TRAVIS_BRANCH:-$(echo $GITHUB_REF | awk 'BEGIN { FS = "/" } ; { print $3 }')}
|
||||
export VCS_COMMIT_ID=$GITHUB_SHA
|
||||
export GIT_COMMIT=$GITHUB_SHA
|
||||
export REPO_NAME=$(basename $GITHUB_REPOSITORY)
|
||||
export USER=$(whoami)
|
||||
export CC=${CC:-gcc}
|
||||
export PATH=~/.local/bin:/usr/local/bin:$PATH
|
||||
|
||||
if [ "$JOB_BUILDTYPE" == "boost" ]; then
|
||||
|
||||
echo '==================================> INSTALL'
|
||||
|
||||
BOOST_BRANCH=develop && [ "$TRAVIS_BRANCH" == "master" ] && BOOST_BRANCH=master || true
|
||||
- name: Setup Boost
|
||||
run: |
|
||||
echo GITHUB_REPOSITORY: $GITHUB_REPOSITORY
|
||||
LIBRARY=${GITHUB_REPOSITORY#*/}
|
||||
echo LIBRARY: $LIBRARY
|
||||
echo "LIBRARY=$LIBRARY" >> $GITHUB_ENV
|
||||
echo GITHUB_BASE_REF: $GITHUB_BASE_REF
|
||||
echo GITHUB_REF: $GITHUB_REF
|
||||
REF=${GITHUB_BASE_REF:-$GITHUB_REF}
|
||||
REF=${REF#refs/heads/}
|
||||
echo REF: $REF
|
||||
BOOST_BRANCH=develop && [ "$REF" == "master" ] && BOOST_BRANCH=master || true
|
||||
echo BOOST_BRANCH: $BOOST_BRANCH
|
||||
cd ..
|
||||
git clone -b $BOOST_BRANCH --depth 1 https://github.com/boostorg/boost.git boost-root
|
||||
cd boost-root
|
||||
git submodule update --init tools/build
|
||||
git submodule update --init libs/config
|
||||
cp -r $GITHUB_WORKSPACE/* libs/$LIBRARY
|
||||
git submodule update --init tools/boostdep
|
||||
cp -r $TRAVIS_BUILD_DIR/* libs/optional
|
||||
python tools/boostdep/depinst/depinst.py optional
|
||||
python3 tools/boostdep/depinst/depinst.py --git_args "--jobs 3" $LIBRARY
|
||||
./bootstrap.sh
|
||||
./b2 headers
|
||||
./b2 -d0 headers
|
||||
|
||||
echo '==================================> SCRIPT'
|
||||
- name: Create user-config.jam
|
||||
if: matrix.compiler
|
||||
run: |
|
||||
echo "using ${{matrix.toolset}} : : ${{matrix.compiler}} ;" > ~/user-config.jam
|
||||
|
||||
echo "using $TOOLSET : : $COMPILER ;" > ~/user-config.jam
|
||||
./b2 libs/optional/test toolset=$TOOLSET cxxstd=$CXXSTD
|
||||
- name: Run tests
|
||||
run: |
|
||||
cd ../boost-root
|
||||
./b2 -j3 libs/$LIBRARY/test toolset=${{matrix.toolset}} cxxstd=${{matrix.cxxstd}} ${ADDRMD:+address-model=$ADDRMD} variant=debug,release
|
||||
|
||||
fi
|
||||
osx:
|
||||
windows:
|
||||
strategy:
|
||||
fail-fast: false
|
||||
matrix:
|
||||
include:
|
||||
- name: "TOOLSET=clang COMPILER=clang++ CXXSTD=03,11,1 Job 15"
|
||||
buildtype: "boost"
|
||||
packages: ""
|
||||
os: "macos-11"
|
||||
cxx: "clang++"
|
||||
sources: ""
|
||||
llvm_os: ""
|
||||
llvm_ver: ""
|
||||
toolset: "clang"
|
||||
compiler: "clang++"
|
||||
- toolset: msvc-14.0
|
||||
cxxstd: 14,latest
|
||||
addrmd: 32,64
|
||||
os: windows-2019
|
||||
- toolset: msvc-14.2
|
||||
cxxstd: "14,17,20,latest"
|
||||
addrmd: 32,64
|
||||
os: windows-2019
|
||||
- toolset: msvc-14.3
|
||||
cxxstd: "14,17,20,latest"
|
||||
addrmd: 32,64
|
||||
os: windows-2022
|
||||
- toolset: clang-win
|
||||
cxxstd: "14,17,20,latest"
|
||||
addrmd: 32,64
|
||||
os: windows-2022
|
||||
- toolset: gcc
|
||||
cxxstd: "03,11,14,17,2a"
|
||||
addrmd: 64
|
||||
os: windows-2019
|
||||
|
||||
runs-on: ${{ matrix.os }}
|
||||
runs-on: ${{matrix.os}}
|
||||
|
||||
steps:
|
||||
- uses: actions/checkout@v2
|
||||
- uses: actions/checkout@v3
|
||||
|
||||
- name: "osx"
|
||||
shell: bash
|
||||
env:
|
||||
CXX: ${{ matrix.cxx }}
|
||||
SOURCES: ${{ matrix.sources }}
|
||||
LLVM_OS: ${{ matrix.llvm_os }}
|
||||
LLVM_VER: ${{ matrix.llvm_ver }}
|
||||
PACKAGES: ${{ matrix.packages }}
|
||||
JOB_BUILDTYPE: ${{ matrix.buildtype }}
|
||||
TOOLSET: ${{ matrix.toolset }}
|
||||
COMPILER: ${{ matrix.compiler }}
|
||||
CXXSTD: ${{ matrix.cxxstd }}
|
||||
TRAVIS_BRANCH: ${{ github.base_ref }}
|
||||
TRAVIS_OS_NAME: "osx"
|
||||
- name: Setup Boost
|
||||
shell: cmd
|
||||
run: |
|
||||
echo '==================================> SETUP'
|
||||
set -e
|
||||
sudo mv /Library/Developer/CommandLineTools /Library/Developer/CommandLineTools.bck
|
||||
echo '==================================> PACKAGES'
|
||||
echo '==================================> INSTALL AND COMPILE'
|
||||
set -e
|
||||
export TRAVIS_BUILD_DIR=$(pwd)
|
||||
export TRAVIS_BRANCH=${TRAVIS_BRANCH:-$(echo $GITHUB_REF | awk 'BEGIN { FS = "/" } ; { print $3 }')}
|
||||
export VCS_COMMIT_ID=$GITHUB_SHA
|
||||
export GIT_COMMIT=$GITHUB_SHA
|
||||
export REPO_NAME=$(basename $GITHUB_REPOSITORY)
|
||||
export USER=$(whoami)
|
||||
export CC=${CC:-gcc}
|
||||
export PATH=~/.local/bin:/usr/local/bin:$PATH
|
||||
echo GITHUB_REPOSITORY: %GITHUB_REPOSITORY%
|
||||
for /f %%i in ("%GITHUB_REPOSITORY%") do set LIBRARY=%%~nxi
|
||||
echo LIBRARY: %LIBRARY%
|
||||
echo LIBRARY=%LIBRARY%>>%GITHUB_ENV%
|
||||
echo GITHUB_BASE_REF: %GITHUB_BASE_REF%
|
||||
echo GITHUB_REF: %GITHUB_REF%
|
||||
if "%GITHUB_BASE_REF%" == "" set GITHUB_BASE_REF=%GITHUB_REF%
|
||||
set BOOST_BRANCH=develop
|
||||
for /f %%i in ("%GITHUB_BASE_REF%") do if "%%~nxi" == "master" set BOOST_BRANCH=master
|
||||
echo BOOST_BRANCH: %BOOST_BRANCH%
|
||||
cd ..
|
||||
git clone -b %BOOST_BRANCH% --depth 1 https://github.com/boostorg/boost.git boost-root
|
||||
cd boost-root
|
||||
xcopy /s /e /q %GITHUB_WORKSPACE% libs\%LIBRARY%\
|
||||
git submodule update --init tools/boostdep
|
||||
python tools/boostdep/depinst/depinst.py --git_args "--jobs 3" %LIBRARY%
|
||||
cmd /c bootstrap
|
||||
b2 -d0 headers
|
||||
|
||||
if [ "$JOB_BUILDTYPE" == "boost" ]; then
|
||||
- name: Run tests
|
||||
shell: cmd
|
||||
run: |
|
||||
cd ../boost-root
|
||||
b2 -j3 libs/%LIBRARY%/test toolset=${{matrix.toolset}} cxxstd=${{matrix.cxxstd}} address-model=${{matrix.addrmd}} variant=debug,release embed-manifest-via=linker
|
||||
|
||||
echo '==================================> INSTALL'
|
||||
posix-cmake-subdir:
|
||||
strategy:
|
||||
fail-fast: false
|
||||
matrix:
|
||||
include:
|
||||
- os: ubuntu-20.04
|
||||
- os: ubuntu-22.04
|
||||
- os: macos-12
|
||||
- os: macos-13
|
||||
|
||||
BOOST_BRANCH=develop && [ "$TRAVIS_BRANCH" == "master" ] && BOOST_BRANCH=master || true
|
||||
runs-on: ${{matrix.os}}
|
||||
|
||||
steps:
|
||||
- uses: actions/checkout@v3
|
||||
|
||||
- name: Install packages
|
||||
if: matrix.install
|
||||
run: sudo apt-get -y install ${{matrix.install}}
|
||||
|
||||
- name: Setup Boost
|
||||
run: |
|
||||
echo GITHUB_REPOSITORY: $GITHUB_REPOSITORY
|
||||
LIBRARY=${GITHUB_REPOSITORY#*/}
|
||||
echo LIBRARY: $LIBRARY
|
||||
echo "LIBRARY=$LIBRARY" >> $GITHUB_ENV
|
||||
echo GITHUB_BASE_REF: $GITHUB_BASE_REF
|
||||
echo GITHUB_REF: $GITHUB_REF
|
||||
REF=${GITHUB_BASE_REF:-$GITHUB_REF}
|
||||
REF=${REF#refs/heads/}
|
||||
echo REF: $REF
|
||||
BOOST_BRANCH=develop && [ "$REF" == "master" ] && BOOST_BRANCH=master || true
|
||||
echo BOOST_BRANCH: $BOOST_BRANCH
|
||||
cd ..
|
||||
git clone -b $BOOST_BRANCH --depth 1 https://github.com/boostorg/boost.git boost-root
|
||||
cd boost-root
|
||||
git submodule update --init tools/build
|
||||
git submodule update --init libs/config
|
||||
cp -r $GITHUB_WORKSPACE/* libs/$LIBRARY
|
||||
git submodule update --init tools/boostdep
|
||||
cp -r $TRAVIS_BUILD_DIR/* libs/optional
|
||||
python tools/boostdep/depinst/depinst.py optional
|
||||
./bootstrap.sh
|
||||
./b2 headers
|
||||
python tools/boostdep/depinst/depinst.py --git_args "--jobs 3" $LIBRARY
|
||||
|
||||
echo '==================================> SCRIPT'
|
||||
- name: Use library with add_subdirectory
|
||||
run: |
|
||||
cd ../boost-root/libs/$LIBRARY/test/cmake_subdir_test
|
||||
mkdir __build__ && cd __build__
|
||||
cmake ..
|
||||
cmake --build .
|
||||
ctest --output-on-failure --no-tests=error
|
||||
|
||||
echo "using $TOOLSET : : $COMPILER ;" > ~/user-config.jam
|
||||
./b2 libs/optional/test toolset=$TOOLSET cxxstd=$CXXSTD
|
||||
posix-cmake-install:
|
||||
strategy:
|
||||
fail-fast: false
|
||||
matrix:
|
||||
include:
|
||||
- os: ubuntu-20.04
|
||||
- os: ubuntu-22.04
|
||||
- os: macos-12
|
||||
- os: macos-13
|
||||
|
||||
fi
|
||||
runs-on: ${{matrix.os}}
|
||||
|
||||
steps:
|
||||
- uses: actions/checkout@v3
|
||||
|
||||
- name: Install packages
|
||||
if: matrix.install
|
||||
run: sudo apt-get -y install ${{matrix.install}}
|
||||
|
||||
- name: Setup Boost
|
||||
run: |
|
||||
echo GITHUB_REPOSITORY: $GITHUB_REPOSITORY
|
||||
LIBRARY=${GITHUB_REPOSITORY#*/}
|
||||
echo LIBRARY: $LIBRARY
|
||||
echo "LIBRARY=$LIBRARY" >> $GITHUB_ENV
|
||||
echo GITHUB_BASE_REF: $GITHUB_BASE_REF
|
||||
echo GITHUB_REF: $GITHUB_REF
|
||||
REF=${GITHUB_BASE_REF:-$GITHUB_REF}
|
||||
REF=${REF#refs/heads/}
|
||||
echo REF: $REF
|
||||
BOOST_BRANCH=develop && [ "$REF" == "master" ] && BOOST_BRANCH=master || true
|
||||
echo BOOST_BRANCH: $BOOST_BRANCH
|
||||
cd ..
|
||||
git clone -b $BOOST_BRANCH --depth 1 https://github.com/boostorg/boost.git boost-root
|
||||
cd boost-root
|
||||
cp -r $GITHUB_WORKSPACE/* libs/$LIBRARY
|
||||
git submodule update --init tools/boostdep
|
||||
python tools/boostdep/depinst/depinst.py --git_args "--jobs 3" $LIBRARY
|
||||
|
||||
- name: Configure
|
||||
run: |
|
||||
cd ../boost-root
|
||||
mkdir __build__ && cd __build__
|
||||
cmake -DBOOST_INCLUDE_LIBRARIES=$LIBRARY -DCMAKE_INSTALL_PREFIX=~/.local ..
|
||||
|
||||
- name: Install
|
||||
run: |
|
||||
cd ../boost-root/__build__
|
||||
cmake --build . --target install
|
||||
|
||||
- name: Use the installed library
|
||||
run: |
|
||||
cd ../boost-root/libs/$LIBRARY/test/cmake_install_test && mkdir __build__ && cd __build__
|
||||
cmake -DCMAKE_INSTALL_PREFIX=~/.local ..
|
||||
cmake --build .
|
||||
ctest --output-on-failure --no-tests=error
|
||||
|
||||
posix-cmake-test:
|
||||
strategy:
|
||||
fail-fast: false
|
||||
matrix:
|
||||
include:
|
||||
- os: ubuntu-20.04
|
||||
- os: ubuntu-22.04
|
||||
- os: macos-12
|
||||
- os: macos-13
|
||||
|
||||
runs-on: ${{matrix.os}}
|
||||
|
||||
steps:
|
||||
- uses: actions/checkout@v3
|
||||
|
||||
- name: Install packages
|
||||
if: matrix.install
|
||||
run: sudo apt-get -y install ${{matrix.install}}
|
||||
|
||||
- name: Setup Boost
|
||||
run: |
|
||||
echo GITHUB_REPOSITORY: $GITHUB_REPOSITORY
|
||||
LIBRARY=${GITHUB_REPOSITORY#*/}
|
||||
echo LIBRARY: $LIBRARY
|
||||
echo "LIBRARY=$LIBRARY" >> $GITHUB_ENV
|
||||
echo GITHUB_BASE_REF: $GITHUB_BASE_REF
|
||||
echo GITHUB_REF: $GITHUB_REF
|
||||
REF=${GITHUB_BASE_REF:-$GITHUB_REF}
|
||||
REF=${REF#refs/heads/}
|
||||
echo REF: $REF
|
||||
BOOST_BRANCH=develop && [ "$REF" == "master" ] && BOOST_BRANCH=master || true
|
||||
echo BOOST_BRANCH: $BOOST_BRANCH
|
||||
cd ..
|
||||
git clone -b $BOOST_BRANCH --depth 1 https://github.com/boostorg/boost.git boost-root
|
||||
cd boost-root
|
||||
cp -r $GITHUB_WORKSPACE/* libs/$LIBRARY
|
||||
git submodule update --init tools/boostdep
|
||||
python tools/boostdep/depinst/depinst.py --git_args "--jobs 3" $LIBRARY
|
||||
|
||||
- name: Configure
|
||||
run: |
|
||||
cd ../boost-root
|
||||
mkdir __build__ && cd __build__
|
||||
cmake -DBOOST_INCLUDE_LIBRARIES=$LIBRARY -DBUILD_TESTING=ON ..
|
||||
|
||||
- name: Build tests
|
||||
run: |
|
||||
cd ../boost-root/__build__
|
||||
cmake --build . --target tests
|
||||
|
||||
- name: Run tests
|
||||
run: |
|
||||
cd ../boost-root/__build__
|
||||
ctest --output-on-failure --no-tests=error
|
||||
|
||||
windows-cmake-subdir:
|
||||
strategy:
|
||||
fail-fast: false
|
||||
matrix:
|
||||
include:
|
||||
- os: windows-2019
|
||||
- os: windows-2022
|
||||
|
||||
runs-on: ${{matrix.os}}
|
||||
|
||||
steps:
|
||||
- uses: actions/checkout@v3
|
||||
|
||||
- name: Setup Boost
|
||||
shell: cmd
|
||||
run: |
|
||||
echo GITHUB_REPOSITORY: %GITHUB_REPOSITORY%
|
||||
for /f %%i in ("%GITHUB_REPOSITORY%") do set LIBRARY=%%~nxi
|
||||
echo LIBRARY: %LIBRARY%
|
||||
echo LIBRARY=%LIBRARY%>>%GITHUB_ENV%
|
||||
echo GITHUB_BASE_REF: %GITHUB_BASE_REF%
|
||||
echo GITHUB_REF: %GITHUB_REF%
|
||||
if "%GITHUB_BASE_REF%" == "" set GITHUB_BASE_REF=%GITHUB_REF%
|
||||
set BOOST_BRANCH=develop
|
||||
for /f %%i in ("%GITHUB_BASE_REF%") do if "%%~nxi" == "master" set BOOST_BRANCH=master
|
||||
echo BOOST_BRANCH: %BOOST_BRANCH%
|
||||
cd ..
|
||||
git clone -b %BOOST_BRANCH% --depth 1 https://github.com/boostorg/boost.git boost-root
|
||||
cd boost-root
|
||||
xcopy /s /e /q %GITHUB_WORKSPACE% libs\%LIBRARY%\
|
||||
git submodule update --init tools/boostdep
|
||||
python tools/boostdep/depinst/depinst.py --git_args "--jobs 3" %LIBRARY%
|
||||
|
||||
- name: Use library with add_subdirectory (Debug)
|
||||
shell: cmd
|
||||
run: |
|
||||
cd ../boost-root/libs/%LIBRARY%/test/cmake_subdir_test
|
||||
mkdir __build__ && cd __build__
|
||||
cmake ..
|
||||
cmake --build . --config Debug
|
||||
ctest --output-on-failure --no-tests=error -C Debug
|
||||
|
||||
- name: Use library with add_subdirectory (Release)
|
||||
shell: cmd
|
||||
run: |
|
||||
cd ../boost-root/libs/%LIBRARY%/test/cmake_subdir_test/__build__
|
||||
cmake --build . --config Release
|
||||
ctest --output-on-failure --no-tests=error -C Release
|
||||
|
||||
windows-cmake-install:
|
||||
strategy:
|
||||
fail-fast: false
|
||||
matrix:
|
||||
include:
|
||||
- os: windows-2019
|
||||
- os: windows-2022
|
||||
|
||||
runs-on: ${{matrix.os}}
|
||||
|
||||
steps:
|
||||
- uses: actions/checkout@v3
|
||||
|
||||
- name: Setup Boost
|
||||
shell: cmd
|
||||
run: |
|
||||
echo GITHUB_REPOSITORY: %GITHUB_REPOSITORY%
|
||||
for /f %%i in ("%GITHUB_REPOSITORY%") do set LIBRARY=%%~nxi
|
||||
echo LIBRARY: %LIBRARY%
|
||||
echo LIBRARY=%LIBRARY%>>%GITHUB_ENV%
|
||||
echo GITHUB_BASE_REF: %GITHUB_BASE_REF%
|
||||
echo GITHUB_REF: %GITHUB_REF%
|
||||
if "%GITHUB_BASE_REF%" == "" set GITHUB_BASE_REF=%GITHUB_REF%
|
||||
set BOOST_BRANCH=develop
|
||||
for /f %%i in ("%GITHUB_BASE_REF%") do if "%%~nxi" == "master" set BOOST_BRANCH=master
|
||||
echo BOOST_BRANCH: %BOOST_BRANCH%
|
||||
cd ..
|
||||
git clone -b %BOOST_BRANCH% --depth 1 https://github.com/boostorg/boost.git boost-root
|
||||
cd boost-root
|
||||
xcopy /s /e /q %GITHUB_WORKSPACE% libs\%LIBRARY%\
|
||||
git submodule update --init tools/boostdep
|
||||
python tools/boostdep/depinst/depinst.py --git_args "--jobs 3" %LIBRARY%
|
||||
|
||||
- name: Configure
|
||||
shell: cmd
|
||||
run: |
|
||||
cd ../boost-root
|
||||
mkdir __build__ && cd __build__
|
||||
cmake -DBOOST_INCLUDE_LIBRARIES=%LIBRARY% -DCMAKE_INSTALL_PREFIX=C:/cmake-prefix ..
|
||||
|
||||
- name: Install (Debug)
|
||||
shell: cmd
|
||||
run: |
|
||||
cd ../boost-root/__build__
|
||||
cmake --build . --target install --config Debug
|
||||
|
||||
- name: Install (Release)
|
||||
shell: cmd
|
||||
run: |
|
||||
cd ../boost-root/__build__
|
||||
cmake --build . --target install --config Release
|
||||
|
||||
- name: Use the installed library (Debug)
|
||||
shell: cmd
|
||||
run: |
|
||||
cd ../boost-root/libs/%LIBRARY%/test/cmake_install_test && mkdir __build__ && cd __build__
|
||||
cmake -DCMAKE_INSTALL_PREFIX=C:/cmake-prefix ..
|
||||
cmake --build . --config Debug
|
||||
ctest --output-on-failure --no-tests=error -C Debug
|
||||
|
||||
- name: Use the installed library (Release)
|
||||
shell: cmd
|
||||
run: |
|
||||
cd ../boost-root/libs/%LIBRARY%/test/cmake_install_test/__build__
|
||||
cmake --build . --config Release
|
||||
ctest --output-on-failure --no-tests=error -C Release
|
||||
|
||||
windows-cmake-test:
|
||||
strategy:
|
||||
fail-fast: false
|
||||
matrix:
|
||||
include:
|
||||
- os: windows-2019
|
||||
- os: windows-2022
|
||||
|
||||
runs-on: ${{matrix.os}}
|
||||
|
||||
steps:
|
||||
- uses: actions/checkout@v3
|
||||
|
||||
- name: Setup Boost
|
||||
shell: cmd
|
||||
run: |
|
||||
echo GITHUB_REPOSITORY: %GITHUB_REPOSITORY%
|
||||
for /f %%i in ("%GITHUB_REPOSITORY%") do set LIBRARY=%%~nxi
|
||||
echo LIBRARY: %LIBRARY%
|
||||
echo LIBRARY=%LIBRARY%>>%GITHUB_ENV%
|
||||
echo GITHUB_BASE_REF: %GITHUB_BASE_REF%
|
||||
echo GITHUB_REF: %GITHUB_REF%
|
||||
if "%GITHUB_BASE_REF%" == "" set GITHUB_BASE_REF=%GITHUB_REF%
|
||||
set BOOST_BRANCH=develop
|
||||
for /f %%i in ("%GITHUB_BASE_REF%") do if "%%~nxi" == "master" set BOOST_BRANCH=master
|
||||
echo BOOST_BRANCH: %BOOST_BRANCH%
|
||||
cd ..
|
||||
git clone -b %BOOST_BRANCH% --depth 1 https://github.com/boostorg/boost.git boost-root
|
||||
cd boost-root
|
||||
xcopy /s /e /q %GITHUB_WORKSPACE% libs\%LIBRARY%\
|
||||
git submodule update --init tools/boostdep
|
||||
python tools/boostdep/depinst/depinst.py --git_args "--jobs 3" %LIBRARY%
|
||||
|
||||
- name: Configure
|
||||
shell: cmd
|
||||
run: |
|
||||
cd ../boost-root
|
||||
mkdir __build__ && cd __build__
|
||||
cmake -DBOOST_INCLUDE_LIBRARIES=%LIBRARY% -DBUILD_TESTING=ON ..
|
||||
|
||||
- name: Build tests (Debug)
|
||||
shell: cmd
|
||||
run: |
|
||||
cd ../boost-root/__build__
|
||||
cmake --build . --target tests --config Debug
|
||||
|
||||
- name: Run tests (Debug)
|
||||
shell: cmd
|
||||
run: |
|
||||
cd ../boost-root/__build__
|
||||
ctest --output-on-failure --no-tests=error -C Debug
|
||||
|
||||
- name: Build tests (Release)
|
||||
shell: cmd
|
||||
run: |
|
||||
cd ../boost-root/__build__
|
||||
cmake --build . --target tests --config Release
|
||||
|
||||
- name: Run tests (Release)
|
||||
shell: cmd
|
||||
run: |
|
||||
cd ../boost-root/__build__
|
||||
ctest --output-on-failure --no-tests=error -C Release
|
||||
|
||||
+25
-19
@@ -1,25 +1,31 @@
|
||||
# Copyright 2019 Mike Dev
|
||||
# Generated by `boostdep --cmake optional`
|
||||
# Copyright 2020, 2021 Peter Dimov
|
||||
# Distributed under the Boost Software License, Version 1.0.
|
||||
# See accompanying file LICENSE_1_0.txt or copy at https://www.boost.org/LICENSE_1_0.txt
|
||||
# https://www.boost.org/LICENSE_1_0.txt
|
||||
|
||||
cmake_minimum_required( VERSION 3.5...3.20 )
|
||||
project( boost_optional VERSION "${BOOST_SUPERPROJECT_VERSION}" LANGUAGES CXX )
|
||||
cmake_minimum_required(VERSION 3.5...3.20)
|
||||
|
||||
add_library( boost_optional INTERFACE )
|
||||
add_library( Boost::optional ALIAS boost_optional )
|
||||
project(boost_optional VERSION "${BOOST_SUPERPROJECT_VERSION}" LANGUAGES CXX)
|
||||
|
||||
target_include_directories( boost_optional INTERFACE include )
|
||||
add_library(boost_optional INTERFACE)
|
||||
add_library(Boost::optional ALIAS boost_optional)
|
||||
|
||||
target_link_libraries( boost_optional
|
||||
INTERFACE
|
||||
Boost::assert
|
||||
Boost::config
|
||||
Boost::core
|
||||
Boost::detail
|
||||
Boost::move
|
||||
Boost::predef
|
||||
Boost::static_assert
|
||||
Boost::throw_exception
|
||||
Boost::type_traits
|
||||
Boost::utility
|
||||
target_include_directories(boost_optional INTERFACE include)
|
||||
|
||||
target_link_libraries(boost_optional
|
||||
INTERFACE
|
||||
Boost::assert
|
||||
Boost::config
|
||||
Boost::core
|
||||
Boost::move
|
||||
Boost::static_assert
|
||||
Boost::throw_exception
|
||||
Boost::type_traits
|
||||
)
|
||||
target_compile_features(boost_optional INTERFACE cxx_std_11)
|
||||
|
||||
if(BUILD_TESTING AND EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/test/CMakeLists.txt")
|
||||
|
||||
add_subdirectory(test)
|
||||
|
||||
endif()
|
||||
|
||||
@@ -0,0 +1,26 @@
|
||||
# Copyright René Ferdinand Rivera Morell 2023-2024
|
||||
# Distributed under the Boost Software License, Version 1.0.
|
||||
# (See accompanying file LICENSE_1_0.txt or copy at
|
||||
# http://www.boost.org/LICENSE_1_0.txt)
|
||||
|
||||
require-b2 5.2 ;
|
||||
|
||||
constant boost_dependencies :
|
||||
/boost/assert//boost_assert
|
||||
/boost/config//boost_config
|
||||
/boost/core//boost_core
|
||||
/boost/move//boost_move
|
||||
/boost/static_assert//boost_static_assert
|
||||
/boost/throw_exception//boost_throw_exception
|
||||
/boost/type_traits//boost_type_traits ;
|
||||
|
||||
project /boost/optional ;
|
||||
|
||||
explicit
|
||||
[ alias boost_optional : : : : <library>$(boost_dependencies) <include>include ]
|
||||
[ alias all : boost_optional test ]
|
||||
;
|
||||
|
||||
call-if : boost-library optional
|
||||
;
|
||||
|
||||
+3
-3
@@ -2,7 +2,7 @@
|
||||
[quickbook 1.4]
|
||||
[authors [Cacciola Carballal, Fernando Luis]]
|
||||
[copyright 2003-2007 Fernando Luis Cacciola Carballal]
|
||||
[copyright 2014-2023 Andrzej Krzemieński]
|
||||
[copyright 2014-2024 Andrzej Krzemieński]
|
||||
[category miscellaneous]
|
||||
[id optional]
|
||||
[dirname optional]
|
||||
@@ -47,14 +47,14 @@ Distributed under the Boost Software License, Version 1.0.
|
||||
[/ Icons ]
|
||||
|
||||
[def __SPACE__ [$images/space.png]]
|
||||
[def __GO_TO__ [$images/callouts/R.png]]
|
||||
[def __GO_TO__ [$images/R.png]]
|
||||
|
||||
|
||||
[section Introduction]
|
||||
Class template `optional` is a wrapper for representing 'optional' (or 'nullable') objects who may not (yet) contain a valid value. Optional objects offer full value semantics; they are good for passing by value and usage inside STL containers. This is a header-only library.
|
||||
|
||||
[heading Problem]
|
||||
Suppose we want to read a parameter form a config file which represents some integral value, let's call it `"MaxValue"`. It is possible that this parameter is not specified; such situation is no error. It is valid to not specify the parameter and in that case the program is supposed to behave slightly differently. Also, suppose that any possible value of type `int` is a valid value for `"MaxValue"`, so we cannot just use `-1` to represent the absence of the parameter in the config file.
|
||||
Suppose we want to read a parameter from a config file which represents some integral value, let's call it `"MaxValue"`. It is possible that this parameter is not specified; such situation is no error. It is valid to not specify the parameter and in that case the program is supposed to behave slightly differently. Also, suppose that any possible value of type `int` is a valid value for `"MaxValue"`, so we cannot just use `-1` to represent the absence of the parameter in the config file.
|
||||
|
||||
[heading Solution]
|
||||
|
||||
|
||||
+15
-10
@@ -41,10 +41,11 @@ template<class CharType, class CharTrait, class T>
|
||||
|
||||
[#reference_operator_ostream]
|
||||
|
||||
|
||||
`template <class CharType, class CharTrait, class T>` [br]
|
||||
std::basic_ostream<CharType, CharTrait>&` [br]
|
||||
operator<<(std::basic_ostream<CharType, CharTrait>& out, optional<T> const& v);`
|
||||
```
|
||||
template <class CharType, class CharTrait, class T>
|
||||
std::basic_ostream<CharType, CharTrait>&
|
||||
operator<<(std::basic_ostream<CharType, CharTrait>& out, optional<T> const& v);
|
||||
```
|
||||
|
||||
* [*Effect:] Outputs an implementation-defined string. The output contains the information about whether the optional object contains a value or not. If `v` contains a value, the output contains result of calling `out << *v`.
|
||||
* [*Returns:] `out`.
|
||||
@@ -52,9 +53,11 @@ template<class CharType, class CharTrait, class T>
|
||||
__SPACE__
|
||||
[#reference_operator_ostream_none]
|
||||
|
||||
`template <class CharType, class CharTrait, class T>` [br]
|
||||
std::basic_ostream<CharType, CharTrait>&` [br]
|
||||
operator<<(std::basic_ostream<CharType, CharTrait>& out, none_t);`
|
||||
```
|
||||
template <class CharType, class CharTrait, class T>
|
||||
std::basic_ostream<CharType, CharTrait>&
|
||||
operator<<(std::basic_ostream<CharType, CharTrait>& out, none_t);
|
||||
```
|
||||
|
||||
* [*Effect:] Outputs an implementation-defined string.
|
||||
* [*Returns:] `out`.
|
||||
@@ -62,9 +65,11 @@ __SPACE__
|
||||
__SPACE__
|
||||
[#reference_operator_istream]
|
||||
|
||||
`template <class CharType, class CharTrait, class T>` [br]
|
||||
std::basic_ostream<CharType, CharTrait>&` [br]
|
||||
operator>>(std::basic_istream<CharType, CharTrait>& in, optional<T>& v);`
|
||||
```
|
||||
template <class CharType, class CharTrait, class T>
|
||||
std::basic_ostream<CharType, CharTrait>&
|
||||
operator>>(std::basic_istream<CharType, CharTrait>& in, optional<T>& v);
|
||||
```
|
||||
|
||||
* [*Requires:] `T` is __STD_DEFAULT_CONSTRUCTIBLE__ and __MOVE_CONSTRUCTIBLE__.
|
||||
* [*Effect:] Reads the value of optional object from `in`. If the string representation indicates that the optional object should contain a value, `v` contains a value and its contained value is obtained as if by default-constructing an object `o` of type `T` and then calling `in >> o`; otherwise `v` does not contain a value, and the previously contained value (if any) has been destroyed.
|
||||
|
||||
@@ -1428,9 +1428,9 @@ struct hash<boost::optional<T&> > ;
|
||||
``
|
||||
|
||||
The specialization `hash<optional<T>>` is enabled if and only if
|
||||
`hash<remove_const_t<T>>` is enabled. When enabled, for an object `o`
|
||||
of type `optional<T>`, if `o.has_value() == true`, then `hash<optional<T>>()(o)`
|
||||
evaluates to the same value as `hash<remove_const_t<T>>()(*o)`; otherwise it
|
||||
`hash<remove_const_t<T>>` is enabled. When enabled, for an object `o`
|
||||
of type `optional<T>`, if `o.has_value() == true`, then `hash<optional<T>>()(o)`
|
||||
evaluates to the same value as `hash<remove_const_t<T>>()(*o)`; otherwise it
|
||||
evaluates to an unspecified value.
|
||||
The member functions are not guaranteed to be `noexcept`.
|
||||
|
||||
|
||||
+12
-15
@@ -17,13 +17,10 @@ The implementation uses the following other Boost modules:
|
||||
# assert
|
||||
# config
|
||||
# core
|
||||
# detail
|
||||
# move
|
||||
# mpl
|
||||
# static_assert
|
||||
# throw_exception
|
||||
# type_traits
|
||||
# utility
|
||||
|
||||
[endsect]
|
||||
|
||||
@@ -34,15 +31,15 @@ Certain constructors and functions in the interface of `optional` perform a 'per
|
||||
template<class... Args> optional(in_place_init_t, Args&&... args);
|
||||
template<class... Args> optional(in_place_init_if_t, bool condition, Args&&... args);
|
||||
template<class... Args> void emplace(Args&&... args);
|
||||
|
||||
|
||||
On compilers that do not support variadic templates, each of these functions is substituted with two overloads, one forwarding a single argument, the other forwarding zero arguments. This forms the following set:
|
||||
|
||||
template<class Arg> optional(in_place_init_t, Arg&& arg);
|
||||
optional(in_place_init_t);
|
||||
|
||||
|
||||
template<class Arg> optional(in_place_init_if_t, bool condition, Arg&& arg);
|
||||
optional(in_place_init_if_t, bool condition);
|
||||
|
||||
|
||||
template<class Arg> void emplace(Arg&& arg);
|
||||
void emplace();
|
||||
|
||||
@@ -51,16 +48,16 @@ On compilers that do not support rvalue references, each of these functions is s
|
||||
template<class Arg> optional(in_place_init_t, const Arg& arg);
|
||||
template<class Arg> optional(in_place_init_t, Arg& arg);
|
||||
optional(in_place_init_t);
|
||||
|
||||
|
||||
template<class Arg> optional(in_place_init_if_t, bool condition, const Arg& arg);
|
||||
template<class Arg> optional(in_place_init_if_t, bool condition, Arg& arg);
|
||||
optional(in_place_init_if_t, bool condition);
|
||||
|
||||
|
||||
template<class Arg> void emplace(const Arg& arg);
|
||||
template<class Arg> void emplace(Arg& arg);
|
||||
void emplace();
|
||||
|
||||
This workaround addresses about 40% of all use cases. If this is insufficient, you need to resort to using [link boost_optional.tutorial.in_place_factories In-Place Factories].
|
||||
This workaround addresses about 40% of all use cases. If this is insufficient, you need to resort to using [link boost_optional.tutorial.in_place_factories In-Place Factories].
|
||||
[endsect]
|
||||
|
||||
[section Optional Reference Binding][#optional_reference_binding]
|
||||
@@ -71,11 +68,11 @@ A number of compilers incorrectly treat const lvalues of integral type as rvalue
|
||||
optional<const int&> or1;
|
||||
optional<const int&> or2 = i; // caution: not portable
|
||||
or1 = i; // caution: not portable
|
||||
|
||||
|
||||
optional<const int&> or3(i); // portable
|
||||
or1 = optional<const int&>(i); // portable
|
||||
|
||||
Compilers known to have these deficiencies include GCC versions 4.2, 4.3, 4.4, 4.5, 5.1, 5.2; QCC 4.4.2; MSVC versions 8.0, 9.0, 10.0, 11.0, 12.0. In order to check if your compiler correctly implements reference binding use this test program.
|
||||
Compilers known to have these deficiencies include GCC versions 4.2, 4.3, 4.4, 4.5, 5.1, 5.2; QCC 4.4.2; MSVC versions 8.0, 9.0, 10.0, 11.0, 12.0. In order to check if your compiler correctly implements reference binding use this test program.
|
||||
|
||||
#include <cassert>
|
||||
|
||||
@@ -88,13 +85,13 @@ Compilers known to have these deficiencies include GCC versions 4.2, 4.3, 4.4, 4
|
||||
assert(&ii == &global_i);
|
||||
}
|
||||
|
||||
void operator=(const int& ii)
|
||||
void operator=(const int& ii)
|
||||
{
|
||||
assert(&ii == &global_i);
|
||||
}
|
||||
|
||||
void operator=(int&&) // remove this if your compiler doesn't have rvalue refs
|
||||
{
|
||||
{
|
||||
assert(false);
|
||||
}
|
||||
};
|
||||
@@ -110,7 +107,7 @@ Compilers known to have these deficiencies include GCC versions 4.2, 4.3, 4.4, 4
|
||||
TestingReferenceBinding ttt2 = iref;
|
||||
ttt2 = iref;
|
||||
}
|
||||
|
||||
|
||||
[endsect]
|
||||
|
||||
[endsect]
|
||||
[endsect]
|
||||
|
||||
+15
-1
@@ -1,7 +1,7 @@
|
||||
[/
|
||||
Boost.Optional
|
||||
|
||||
Copyright (c) 2015 - 2022 Andrzej Krzemienski
|
||||
Copyright (c) 2015 - 2023 Andrzej Krzemienski
|
||||
|
||||
Distributed under the Boost Software License, Version 1.0.
|
||||
(See accompanying file LICENSE_1_0.txt or copy at
|
||||
@@ -11,6 +11,20 @@
|
||||
|
||||
[section:relnotes Release Notes]
|
||||
|
||||
[heading Boost Release 1.87]
|
||||
|
||||
* *Breaking change.* Dropped support for C++03. C++11 is now the required minimum.
|
||||
* Dropped dependency on Boost.Utility.
|
||||
* Dropped dependency on Boost.Predef.
|
||||
* A bit faster implementation of some relational operations.
|
||||
|
||||
|
||||
[heading Boost Release 1.85]
|
||||
|
||||
* Fixed the implementation for trivial types. Now it is slower, because it always initializes the `T`, but it avoids undefined behavior when `optional<T>` is copied. This fixes [@https://github.com/boostorg/optional/issues/108 issue #108].
|
||||
* Fixed some `-Wmaybe-uninitialized` warnings in GCC 12. Thanks to Christian Mazakas for the fix.
|
||||
* Dropped dependency on Boost.Detail.
|
||||
|
||||
[heading Boost Release 1.83]
|
||||
|
||||
* Deprecated support for C++03 and earlier, C++11 will be required in release 1.86.
|
||||
|
||||
+12
-10
@@ -26,23 +26,25 @@ install images
|
||||
images/opt_align2.png
|
||||
images/opt_align3.png
|
||||
images/opt_align4.png
|
||||
images/R.png
|
||||
images/space.png
|
||||
:
|
||||
<location>html/images
|
||||
;
|
||||
|
||||
|
||||
boostbook standalone
|
||||
:
|
||||
optional
|
||||
:
|
||||
<format>html:<xsl:param>boost.root=../../../..
|
||||
<format>html:<xsl:param>boost.libraries=../../../../libs/libraries.htm
|
||||
<xsl:param>chapter.autolabel=0
|
||||
<xsl:param>chunk.section.depth=8
|
||||
<xsl:param>toc.section.depth=2
|
||||
<xsl:param>toc.max.depth=2
|
||||
<xsl:param>generate.section.toc.level=1
|
||||
<format>pdf:<xsl:param>img.src.path=$(images)/
|
||||
<format>pdf:<xsl:param>boost.url.prefix=http://www.boost.org/doc/libs/release/libs/optional/doc/html
|
||||
<format>html:<xsl:param>"boost.root=../../../.."
|
||||
<format>html:<xsl:param>"boost.libraries=../../../../libs/libraries.htm"
|
||||
<xsl:param>"chapter.autolabel=0"
|
||||
<xsl:param>"chunk.section.depth=8"
|
||||
<xsl:param>"toc.section.depth=1"
|
||||
<xsl:param>"toc.max.depth=2"
|
||||
<xsl:param>"generate.section.toc.level=1"
|
||||
<format>pdf:<xsl:param>"img.src.path=$(images)/"
|
||||
<format>pdf:<xsl:param>"boost.url.prefix=http://www.boost.org/doc/libs/release/libs/optional/doc/html"
|
||||
<format>docbook:<auto-index-internal>on
|
||||
;
|
||||
|
||||
|
||||
@@ -1,108 +0,0 @@
|
||||
<html>
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
|
||||
<title>A note about optional<bool></title>
|
||||
<link rel="stylesheet" href="../../../../../doc/src/boostbook.css" type="text/css">
|
||||
<meta name="generator" content="DocBook XSL Stylesheets V1.78.1">
|
||||
<link rel="home" href="../index.html" title="Chapter 1. Boost.Optional">
|
||||
<link rel="up" href="../index.html" title="Chapter 1. Boost.Optional">
|
||||
<link rel="prev" href="detailed_semantics.html" title="Detailed Semantics">
|
||||
<link rel="next" href="exception_safety_guarantees.html" title="Exception Safety Guarantees">
|
||||
</head>
|
||||
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
|
||||
<table cellpadding="2" width="100%"><tr>
|
||||
<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../boost.png"></td>
|
||||
<td align="center"><a href="../../../../../index.html">Home</a></td>
|
||||
<td align="center"><a href="../../../../../libs/libraries.htm">Libraries</a></td>
|
||||
<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
|
||||
<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
|
||||
<td align="center"><a href="../../../../../more/index.htm">More</a></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
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|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
|
||||
<a name="boost_optional.a_note_about_optional_bool_"></a><a class="link" href="a_note_about_optional_bool_.html" title="A note about optional<bool>">A note about
|
||||
optional<bool></a>
|
||||
</h2></div></div></div>
|
||||
<p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span></code> should
|
||||
be used with special caution and consideration.
|
||||
</p>
|
||||
<p>
|
||||
First, it is functionally similar to a tristate boolean (false, maybe, true)
|
||||
—such as <a href="../../../../../doc/html/tribool.html" target="_top">boost::tribool</a>—
|
||||
except that in a tristate boolean, the maybe state <span class="underline">represents
|
||||
a valid value</span>, unlike the corresponding state of an uninitialized
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span></code>. It
|
||||
should be carefully considered if an <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span></code>
|
||||
instead of a <code class="computeroutput"><span class="identifier">tribool</span></code> is really
|
||||
needed.
|
||||
</p>
|
||||
<p>
|
||||
Second, although <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code>
|
||||
provides a contextual conversion to <code class="computeroutput"><span class="keyword">bool</span></code>
|
||||
in C++11, this falls back to an implicit conversion on older compilers. This
|
||||
conversion refers to the initialization state and not to the contained value.
|
||||
Using <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span></code> can
|
||||
lead to subtle errors due to the implicit <code class="computeroutput"><span class="keyword">bool</span></code>
|
||||
conversion:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">foo</span> <span class="special">(</span> <span class="keyword">bool</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="keyword">void</span> <span class="identifier">bar</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">v</span> <span class="special">=</span> <span class="keyword">try</span><span class="special">();</span>
|
||||
|
||||
<span class="comment">// The following intended to pass the value of 'v' to foo():</span>
|
||||
<span class="identifier">foo</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span>
|
||||
<span class="comment">// But instead, the initialization state is passed</span>
|
||||
<span class="comment">// due to a typo: it should have been foo(*v).</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
The only implicit conversion is to <code class="computeroutput"><span class="keyword">bool</span></code>,
|
||||
and it is safe in the sense that typical integral promotions don't apply (i.e.
|
||||
if <code class="computeroutput"><span class="identifier">foo</span><span class="special">()</span></code>
|
||||
takes an <code class="computeroutput"><span class="keyword">int</span></code> instead, it won't
|
||||
compile).
|
||||
</p>
|
||||
<p>
|
||||
Third, mixed comparisons with <code class="computeroutput"><span class="keyword">bool</span></code>
|
||||
work differently than similar mixed comparisons between pointers and <code class="computeroutput"><span class="keyword">bool</span></code>, so the results might surprise you:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">oEmpty</span><span class="special">(</span><span class="identifier">none</span><span class="special">),</span> <span class="identifier">oTrue</span><span class="special">(</span><span class="keyword">true</span><span class="special">),</span> <span class="identifier">oFalse</span><span class="special">(</span><span class="keyword">false</span><span class="special">);</span>
|
||||
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oEmpty</span> <span class="special">==</span> <span class="identifier">none</span><span class="special">);</span> <span class="comment">// renders true</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oEmpty</span> <span class="special">==</span> <span class="keyword">false</span><span class="special">);</span> <span class="comment">// renders false!</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oEmpty</span> <span class="special">==</span> <span class="keyword">true</span><span class="special">);</span> <span class="comment">// renders false!</span>
|
||||
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oFalse</span> <span class="special">==</span> <span class="identifier">none</span><span class="special">);</span> <span class="comment">// renders false</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oFalse</span> <span class="special">==</span> <span class="keyword">false</span><span class="special">);</span> <span class="comment">// renders true!</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oFalse</span> <span class="special">==</span> <span class="keyword">true</span><span class="special">);</span> <span class="comment">// renders false</span>
|
||||
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oTrue</span> <span class="special">==</span> <span class="identifier">none</span><span class="special">);</span> <span class="comment">// renders false</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oTrue</span> <span class="special">==</span> <span class="keyword">false</span><span class="special">);</span> <span class="comment">// renders false</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oTrue</span> <span class="special">==</span> <span class="keyword">true</span><span class="special">);</span> <span class="comment">// renders true</span>
|
||||
</pre>
|
||||
<p>
|
||||
In other words, for <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code>, the following assertion does not hold:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">assert</span><span class="special">((</span><span class="identifier">opt</span> <span class="special">==</span> <span class="keyword">false</span><span class="special">)</span> <span class="special">==</span> <span class="special">(!</span><span class="identifier">opt</span><span class="special">));</span>
|
||||
</pre>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
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<div class="spirit-nav">
|
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<a accesskey="p" href="detailed_semantics.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="exception_safety_guarantees.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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|
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="boost_optional.acknowledgements"></a><a class="link" href="acknowledgements.html" title="Acknowledgements">Acknowledgements</a>
|
||||
</h2></div></div></div>
|
||||
<h4>
|
||||
<a name="boost_optional.acknowledgements.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.acknowledgements.pre_formal_review"></a></span><a class="link" href="acknowledgements.html#boost_optional.acknowledgements.pre_formal_review">Pre-formal
|
||||
review</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
Peter Dimov suggested the name 'optional', and was the first to point out
|
||||
the need for aligned storage.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Douglas Gregor developed 'type_with_alignment', and later Eric Friedman
|
||||
coded 'aligned_storage', which are the core of the optional class implementation.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Andrei Alexandrescu and Brian Parker also worked with aligned storage techniques
|
||||
and their work influenced the current implementation.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Gennadiy Rozental made extensive and important comments which shaped the
|
||||
design.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Vesa Karvonen and Douglas Gregor made quite useful comparisons between
|
||||
optional, variant and any; and made other relevant comments.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Douglas Gregor and Peter Dimov commented on comparisons and evaluation
|
||||
in boolean contexts.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Eric Friedman helped understand the issues involved with aligned storage,
|
||||
move/copy operations and exception safety.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Many others have participated with useful comments: Aleksey Gurotov, Kevlin
|
||||
Henney, David Abrahams, and others I can't recall.
|
||||
</li>
|
||||
</ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.acknowledgements.h1"></a>
|
||||
<span class="phrase"><a name="boost_optional.acknowledgements.post_formal_review"></a></span><a class="link" href="acknowledgements.html#boost_optional.acknowledgements.post_formal_review">Post-formal
|
||||
review</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
William Kempf carefully considered the originally proposed interface and
|
||||
suggested the new interface which is currently used. He also started and
|
||||
fueled the discussion about the analogy optional<>/smart pointer
|
||||
and about relational operators.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Peter Dimov, Joel de Guzman, David Abrahams, Tanton Gibbs and Ian Hanson
|
||||
focused on the relational semantics of optional (originally undefined);
|
||||
concluding with the fact that the pointer-like interface doesn't make it
|
||||
a pointer so it shall have deep relational operators.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Augustus Saunders also explored the different relational semantics between
|
||||
optional<> and a pointer and developed the OptionalPointee concept
|
||||
as an aid against potential conflicts on generic code.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Joel de Guzman noticed that optional<> can be seen as an API on top
|
||||
of variant<T,nil_t>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Dave Gomboc explained the meaning and usage of the Haskell analog to optional<>:
|
||||
the Maybe type constructor (analogy originally pointed out by David Sankel).
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Other comments were posted by Vincent Finn, Anthony Williams, Ed Brey,
|
||||
Rob Stewart, and others.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Joel de Guzman made the case for the support of references and helped with
|
||||
the proper semantics.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Mat Marcus shown the virtues of a value-oriented interface, influencing
|
||||
the current design, and contributed the idea of "none".
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Vladimir Batov's design of Boost.Convert library motivated the development
|
||||
of value accessors for <code class="computeroutput"><span class="identifier">optional</span></code>:
|
||||
functions <code class="computeroutput"><span class="identifier">value</span></code>, <code class="computeroutput"><span class="identifier">value_or</span></code>, <code class="computeroutput"><span class="identifier">value_or_eval</span></code>.
|
||||
</li>
|
||||
</ul></div>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
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</div></td>
|
||||
</tr></table>
|
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<hr>
|
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<div class="spirit-nav">
|
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="boost_optional.acknowledgments"></a><a class="link" href="acknowledgments.html" title="Acknowledgments">Acknowledgments</a>
|
||||
</h2></div></div></div>
|
||||
<h4>
|
||||
<a name="boost_optional.acknowledgments.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.acknowledgments.pre_formal_review"></a></span><a class="link" href="acknowledgments.html#boost_optional.acknowledgments.pre_formal_review">Pre-formal
|
||||
review</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
Peter Dimov suggested the name 'optional', and was the first to point out
|
||||
the need for aligned storage.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Douglas Gregor developed 'type_with_alignment', and later Eric Friedman
|
||||
coded 'aligned_storage', which are the core of the optional class implementation.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Andrei Alexandrescu and Brian Parker also worked with aligned storage techniques
|
||||
and their work influenced the current implementation.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Gennadiy Rozental made extensive and important comments which shaped the
|
||||
design.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Vesa Karvonen and Douglas Gregor made quite useful comparisons between
|
||||
optional, variant and any; and made other relevant comments.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Douglas Gregor and Peter Dimov commented on comparisons and evaluation
|
||||
in boolean contexts.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Eric Friedman helped understand the issues involved with aligned storage,
|
||||
move/copy operations and exception safety.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Many others have participated with useful comments: Aleksey Gurotov, Kevlin
|
||||
Henney, David Abrahams, and others I can't recall.
|
||||
</li>
|
||||
</ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.acknowledgments.h1"></a>
|
||||
<span class="phrase"><a name="boost_optional.acknowledgments.post_formal_review"></a></span><a class="link" href="acknowledgments.html#boost_optional.acknowledgments.post_formal_review">Post-formal
|
||||
review</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
William Kempf carefully considered the originally proposed interface and
|
||||
suggested the new interface which is currently used. He also started and
|
||||
fueled the discussion about the analogy optional<>/smart pointer
|
||||
and about relational operators.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Peter Dimov, Joel de Guzman, David Abrahams, Tanton Gibbs and Ian Hanson
|
||||
focused on the relational semantics of optional (originally undefined);
|
||||
concluding with the fact that the pointer-like interface doesn't make it
|
||||
a pointer so it shall have deep relational operators.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Augustus Saunders also explored the different relational semantics between
|
||||
optional<> and a pointer and developed the OptionalPointee concept
|
||||
as an aid against potential conflicts on generic code.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Joel de Guzman noticed that optional<> can be seen as an API on top
|
||||
of variant<T,nil_t>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Dave Gomboc explained the meaning and usage of the Haskell analog to optional<>:
|
||||
the Maybe type constructor (analogy originally pointed out by David Sankel).
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Other comments were posted by Vincent Finn, Anthony Williams, Ed Brey,
|
||||
Rob Stewart, and others.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Joel de Guzman made the case for the support of references and helped with
|
||||
the proper semantics.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Mat Marcus shown the virtues of a value-oriented interface, influencing
|
||||
the current design, and contributed the idea of "none".
|
||||
</li>
|
||||
</ul></div>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
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|
||||
<div class="spirit-nav">
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|
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and Portability</a>
|
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</h2></div></div></div>
|
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<div class="toc"><dl class="toc">
|
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<dt><span class="section"><a href="dependencies_and_portability.html#boost_optional.dependencies_and_portability.dependencies">Dependencies</a></span></dt>
|
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<dt><span class="section"><a href="dependencies_and_portability/emplace_operations_in_older_compilers.html">Emplace
|
||||
operations in older compilers</a></span></dt>
|
||||
<dt><span class="section"><a href="dependencies_and_portability/optional_reference_binding.html">Optional
|
||||
Reference Binding</a></span></dt>
|
||||
</dl></div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.dependencies_and_portability.dependencies"></a><a class="link" href="dependencies_and_portability.html#boost_optional.dependencies_and_portability.dependencies" title="Dependencies">Dependencies</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
The implementation uses the following other Boost modules:
|
||||
</p>
|
||||
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|
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assert
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|
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detail
|
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|
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move
|
||||
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|
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<li class="listitem">
|
||||
mpl
|
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</li>
|
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<li class="listitem">
|
||||
static_assert
|
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</li>
|
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<li class="listitem">
|
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throw_exception
|
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</li>
|
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<li class="listitem">
|
||||
type_traits
|
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</li>
|
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<li class="listitem">
|
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utility
|
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</li>
|
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</ol></div>
|
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</div>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
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<td align="left"></td>
|
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<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
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|
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|
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<a name="boost_optional.dependencies_and_portability.emplace_operations_in_older_compilers"></a><a class="link" href="emplace_operations_in_older_compilers.html" title="Emplace operations in older compilers">Emplace
|
||||
operations in older compilers</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
<a name="optional_emplace_workaround"></a>Certain constructors and functions
|
||||
in the interface of <code class="computeroutput"><span class="identifier">optional</span></code>
|
||||
perform a 'perfect forwarding' of arguments:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span><span class="special">...</span> <span class="identifier">Args</span><span class="special">></span> <span class="identifier">optional</span><span class="special">(</span><span class="identifier">in_place_init_t</span><span class="special">,</span> <span class="identifier">Args</span><span class="special">&&...</span> <span class="identifier">args</span><span class="special">);</span>
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span><span class="special">...</span> <span class="identifier">Args</span><span class="special">></span> <span class="identifier">optional</span><span class="special">(</span><span class="identifier">in_place_init_if_t</span><span class="special">,</span> <span class="keyword">bool</span> <span class="identifier">condition</span><span class="special">,</span> <span class="identifier">Args</span><span class="special">&&...</span> <span class="identifier">args</span><span class="special">);</span>
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span><span class="special">...</span> <span class="identifier">Args</span><span class="special">></span> <span class="keyword">void</span> <span class="identifier">emplace</span><span class="special">(</span><span class="identifier">Args</span><span class="special">&&...</span> <span class="identifier">args</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
On compilers that do not support variadic templates, each of these functions
|
||||
is substituted with two overloads, one forwarding a single argument, the
|
||||
other forwarding zero arguments. This forms the following set:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">Arg</span><span class="special">></span> <span class="identifier">optional</span><span class="special">(</span><span class="identifier">in_place_init_t</span><span class="special">,</span> <span class="identifier">Arg</span><span class="special">&&</span> <span class="identifier">arg</span><span class="special">);</span>
|
||||
<span class="identifier">optional</span><span class="special">(</span><span class="identifier">in_place_init_t</span><span class="special">);</span>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">Arg</span><span class="special">></span> <span class="identifier">optional</span><span class="special">(</span><span class="identifier">in_place_init_if_t</span><span class="special">,</span> <span class="keyword">bool</span> <span class="identifier">condition</span><span class="special">,</span> <span class="identifier">Arg</span><span class="special">&&</span> <span class="identifier">arg</span><span class="special">);</span>
|
||||
<span class="identifier">optional</span><span class="special">(</span><span class="identifier">in_place_init_if_t</span><span class="special">,</span> <span class="keyword">bool</span> <span class="identifier">condition</span><span class="special">);</span>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">Arg</span><span class="special">></span> <span class="keyword">void</span> <span class="identifier">emplace</span><span class="special">(</span><span class="identifier">Arg</span><span class="special">&&</span> <span class="identifier">arg</span><span class="special">);</span>
|
||||
<span class="keyword">void</span> <span class="identifier">emplace</span><span class="special">();</span>
|
||||
</pre>
|
||||
<p>
|
||||
On compilers that do not support rvalue references, each of these functions
|
||||
is substituted with three overloads: taking <code class="computeroutput"><span class="keyword">const</span></code>
|
||||
and non-<code class="computeroutput"><span class="keyword">const</span></code> lvalue reference,
|
||||
and third forwarding zero arguments. This forms the following set:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">Arg</span><span class="special">></span> <span class="identifier">optional</span><span class="special">(</span><span class="identifier">in_place_init_t</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">Arg</span><span class="special">&</span> <span class="identifier">arg</span><span class="special">);</span>
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">Arg</span><span class="special">></span> <span class="identifier">optional</span><span class="special">(</span><span class="identifier">in_place_init_t</span><span class="special">,</span> <span class="identifier">Arg</span><span class="special">&</span> <span class="identifier">arg</span><span class="special">);</span>
|
||||
<span class="identifier">optional</span><span class="special">(</span><span class="identifier">in_place_init_t</span><span class="special">);</span>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">Arg</span><span class="special">></span> <span class="identifier">optional</span><span class="special">(</span><span class="identifier">in_place_init_if_t</span><span class="special">,</span> <span class="keyword">bool</span> <span class="identifier">condition</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">Arg</span><span class="special">&</span> <span class="identifier">arg</span><span class="special">);</span>
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">Arg</span><span class="special">></span> <span class="identifier">optional</span><span class="special">(</span><span class="identifier">in_place_init_if_t</span><span class="special">,</span> <span class="keyword">bool</span> <span class="identifier">condition</span><span class="special">,</span> <span class="identifier">Arg</span><span class="special">&</span> <span class="identifier">arg</span><span class="special">);</span>
|
||||
<span class="identifier">optional</span><span class="special">(</span><span class="identifier">in_place_init_if_t</span><span class="special">,</span> <span class="keyword">bool</span> <span class="identifier">condition</span><span class="special">);</span>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">Arg</span><span class="special">></span> <span class="keyword">void</span> <span class="identifier">emplace</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">Arg</span><span class="special">&</span> <span class="identifier">arg</span><span class="special">);</span>
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">Arg</span><span class="special">></span> <span class="keyword">void</span> <span class="identifier">emplace</span><span class="special">(</span><span class="identifier">Arg</span><span class="special">&</span> <span class="identifier">arg</span><span class="special">);</span>
|
||||
<span class="keyword">void</span> <span class="identifier">emplace</span><span class="special">();</span>
|
||||
</pre>
|
||||
<p>
|
||||
This workaround addresses about 40% of all use cases. If this is insufficient,
|
||||
you need to resort to using <a class="link" href="../tutorial/in_place_factories.html" title="In-Place Factories">In-Place
|
||||
Factories</a>.
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
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<td align="left"></td>
|
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<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
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</div></td>
|
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</tr></table>
|
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|
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<div class="titlepage"><div><div><h3 class="title">
|
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<a name="boost_optional.dependencies_and_portability.optional_reference_binding"></a><a class="link" href="optional_reference_binding.html" title="Optional Reference Binding">Optional
|
||||
Reference Binding</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
<a name="optional_reference_binding"></a>A number of compilers incorrectly
|
||||
treat const lvalues of integral type as rvalues, and create an illegal temporary
|
||||
when binding to an lvalue reference to const in some expressions. This could
|
||||
result in creating an optional lvalue reference that is in fact bound to
|
||||
an unexpected temporary rather than to the intended object. In order to prevent
|
||||
hard to find run-time bugs, this library performs compile-time checks to
|
||||
prevent expressions that would otherwise bind an optional reference to an
|
||||
unexpected temporary. As a consequence, on certain compilers certain pieces
|
||||
of functionality in optional references are missing. In order to maintain
|
||||
a portability of your code across different compilers, it is recommended that
|
||||
you only stick to the minimum portable interface of optional references:
|
||||
prefer direct-initialization and copy assignment of optional references to
|
||||
copy-initialization and assignment from <code class="computeroutput"><span class="identifier">T</span><span class="special">&</span></code>:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">const</span> <span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&></span> <span class="identifier">or1</span><span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&></span> <span class="identifier">or2</span> <span class="special">=</span> <span class="identifier">i</span><span class="special">;</span> <span class="comment">// caution: not portable</span>
|
||||
<span class="identifier">or1</span> <span class="special">=</span> <span class="identifier">i</span><span class="special">;</span> <span class="comment">// caution: not portable</span>
|
||||
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&></span> <span class="identifier">or3</span><span class="special">(</span><span class="identifier">i</span><span class="special">);</span> <span class="comment">// portable</span>
|
||||
<span class="identifier">or1</span> <span class="special">=</span> <span class="identifier">optional</span><span class="special"><</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&>(</span><span class="identifier">i</span><span class="special">);</span> <span class="comment">// portable</span>
|
||||
</pre>
|
||||
<p>
|
||||
Compilers known to have these deficiencies include GCC versions 4.2, 4.3,
|
||||
4.4, 4.5, 5.1, 5.2; QCC 4.4.2; MSVC versions 8.0, 9.0, 10.0, 11.0, 12.0.
|
||||
In order to check if your compiler correctly implements reference binding
|
||||
use this test program.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">cassert</span><span class="special">></span>
|
||||
|
||||
<span class="keyword">const</span> <span class="keyword">int</span> <span class="identifier">global_i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
|
||||
|
||||
<span class="keyword">struct</span> <span class="identifier">TestingReferenceBinding</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">TestingReferenceBinding</span><span class="special">(</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">ii</span><span class="special">)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">assert</span><span class="special">(&</span><span class="identifier">ii</span> <span class="special">==</span> <span class="special">&</span><span class="identifier">global_i</span><span class="special">);</span>
|
||||
<span class="special">}</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="keyword">operator</span><span class="special">=(</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">ii</span><span class="special">)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">assert</span><span class="special">(&</span><span class="identifier">ii</span> <span class="special">==</span> <span class="special">&</span><span class="identifier">global_i</span><span class="special">);</span>
|
||||
<span class="special">}</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="keyword">operator</span><span class="special">=(</span><span class="keyword">int</span><span class="special">&&)</span> <span class="comment">// remove this if your compiler doesn't have rvalue refs</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="keyword">false</span><span class="special">);</span>
|
||||
<span class="special">}</span>
|
||||
<span class="special">};</span>
|
||||
|
||||
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">iref</span> <span class="special">=</span> <span class="identifier">global_i</span><span class="special">;</span>
|
||||
<span class="identifier">assert</span><span class="special">(&</span><span class="identifier">iref</span> <span class="special">==</span> <span class="special">&</span><span class="identifier">global_i</span><span class="special">);</span>
|
||||
|
||||
<span class="identifier">TestingReferenceBinding</span> <span class="identifier">ttt</span> <span class="special">=</span> <span class="identifier">global_i</span><span class="special">;</span>
|
||||
<span class="identifier">ttt</span> <span class="special">=</span> <span class="identifier">global_i</span><span class="special">;</span>
|
||||
|
||||
<span class="identifier">TestingReferenceBinding</span> <span class="identifier">ttt2</span> <span class="special">=</span> <span class="identifier">iref</span><span class="special">;</span>
|
||||
<span class="identifier">ttt2</span> <span class="special">=</span> <span class="identifier">iref</span><span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
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<a accesskey="p" href="emplace_operations_in_older_compilers.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../dependencies_and_portability.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="../relnotes.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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</div>
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<div class="section">
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||||
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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||||
<a name="boost_optional.development"></a><a class="link" href="development.html" title="Development">Development</a>
|
||||
</h2></div></div></div>
|
||||
<div class="toc"><dl class="toc">
|
||||
<dt><span class="section"><a href="development.html#boost_optional.development.the_models">The models</a></span></dt>
|
||||
<dt><span class="section"><a href="development.html#boost_optional.development.the_semantics">The semantics</a></span></dt>
|
||||
<dt><span class="section"><a href="development.html#boost_optional.development.the_interface">The Interface</a></span></dt>
|
||||
</dl></div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.development.the_models"></a><a class="link" href="development.html#boost_optional.development.the_models" title="The models">The models</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
In C++, we can <span class="emphasis"><em>declare</em></span> an object (a variable) of type
|
||||
<code class="computeroutput"><span class="identifier">T</span></code>, and we can give this variable
|
||||
an <span class="emphasis"><em>initial value</em></span> (through an <span class="emphasis"><em>initializer</em></span>.
|
||||
(cf. 8.5)). When a declaration includes a non-empty initializer (an initial
|
||||
value is given), it is said that the object has been initialized. If the
|
||||
declaration uses an empty initializer (no initial value is given), and neither
|
||||
default nor value initialization applies, it is said that the object is
|
||||
<span class="bold"><strong>uninitialized</strong></span>. Its actual value exist but
|
||||
has an <span class="emphasis"><em>indeterminate initial value</em></span> (cf. 8.5/11). <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> intends
|
||||
to formalize the notion of initialization (or lack of it) allowing a program
|
||||
to test whether an object has been initialized and stating that access to
|
||||
the value of an uninitialized object is undefined behavior. That is, when
|
||||
a variable is declared as <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
and no initial value is given, the variable is <span class="emphasis"><em>formally</em></span>
|
||||
uninitialized. A formally uninitialized optional object has conceptually
|
||||
no value at all and this situation can be tested at runtime. It is formally
|
||||
<span class="emphasis"><em>undefined behavior</em></span> to try to access the value of an
|
||||
uninitialized optional. An uninitialized optional can be assigned a value,
|
||||
in which case its initialization state changes to initialized. Furthermore,
|
||||
given the formal treatment of initialization states in optional objects,
|
||||
it is even possible to reset an optional to <span class="emphasis"><em>uninitialized</em></span>.
|
||||
</p>
|
||||
<p>
|
||||
In C++ there is no formal notion of uninitialized objects, which means that
|
||||
objects always have an initial value even if indeterminate. As discussed
|
||||
on the previous section, this has a drawback because you need additional
|
||||
information to tell if an object has been effectively initialized. One of
|
||||
the typical ways in which this has been historically dealt with is via a
|
||||
special value: <code class="computeroutput"><span class="identifier">EOF</span></code>, <code class="computeroutput"><span class="identifier">npos</span></code>, -1, etc... This is equivalent to
|
||||
adding the special value to the set of possible values of a given type. This
|
||||
super set of <code class="computeroutput"><span class="identifier">T</span></code> plus some
|
||||
<span class="emphasis"><em>nil_t</em></span>—where <code class="computeroutput"><span class="identifier">nil_t</span></code>
|
||||
is some stateless POD—can be modeled in modern languages as a <span class="bold"><strong>discriminated union</strong></span> of T and nil_t. Discriminated
|
||||
unions are often called <span class="emphasis"><em>variants</em></span>. A variant has a <span class="emphasis"><em>current
|
||||
type</em></span>, which in our case is either <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
or <code class="computeroutput"><span class="identifier">nil_t</span></code>. Using the <a href="../../../../variant/index.html" target="_top">Boost.Variant</a> library, this model
|
||||
can be implemented in terms of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">variant</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="identifier">nil_t</span><span class="special">></span></code>.
|
||||
There is precedent for a discriminated union as a model for an optional value:
|
||||
the <a href="http://www.haskell.org/" target="_top">Haskell</a> <span class="bold"><strong>Maybe</strong></span>
|
||||
built-in type constructor. Thus, a discriminated union <code class="computeroutput"><span class="identifier">T</span><span class="special">+</span><span class="identifier">nil_t</span></code>
|
||||
serves as a conceptual foundation.
|
||||
</p>
|
||||
<p>
|
||||
A <code class="computeroutput"><span class="identifier">variant</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="identifier">nil_t</span><span class="special">></span></code> follows naturally from the traditional
|
||||
idiom of extending the range of possible values adding an additional sentinel
|
||||
value with the special meaning of <span class="emphasis"><em>Nothing</em></span>. However,
|
||||
this additional <span class="emphasis"><em>Nothing</em></span> value is largely irrelevant
|
||||
for our purpose since our goal is to formalize the notion of uninitialized
|
||||
objects and, while a special extended value can be used to convey that meaning,
|
||||
it is not strictly necessary in order to do so.
|
||||
</p>
|
||||
<p>
|
||||
The observation made in the last paragraph about the irrelevant nature of
|
||||
the additional <code class="computeroutput"><span class="identifier">nil_t</span></code> with
|
||||
respect to <span class="underline">purpose</span> of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> suggests
|
||||
an alternative model: a <span class="emphasis"><em>container</em></span> that either has a
|
||||
value of <code class="computeroutput"><span class="identifier">T</span></code> or nothing.
|
||||
</p>
|
||||
<p>
|
||||
As of this writing I don't know of any precedent for a variable-size fixed-capacity
|
||||
(of 1) stack-based container model for optional values, yet I believe this
|
||||
is the consequence of the lack of practical implementations of such a container
|
||||
rather than an inherent shortcoming of the container model.
|
||||
</p>
|
||||
<p>
|
||||
In any event, both the discriminated-union or the single-element container
|
||||
models serve as a conceptual ground for a class representing optional—i.e.
|
||||
possibly uninitialized—objects. For instance, these models show the <span class="emphasis"><em>exact</em></span>
|
||||
semantics required for a wrapper of optional values:
|
||||
</p>
|
||||
<p>
|
||||
Discriminated-union:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>deep-copy</strong></span> semantics: copies of the variant
|
||||
implies copies of the value.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>deep-relational</strong></span> semantics: comparisons
|
||||
between variants matches both current types and values
|
||||
</li>
|
||||
<li class="listitem">
|
||||
If the variant's current type is <code class="computeroutput"><span class="identifier">T</span></code>,
|
||||
it is modeling an <span class="emphasis"><em>initialized</em></span> optional.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
If the variant's current type is not <code class="computeroutput"><span class="identifier">T</span></code>,
|
||||
it is modeling an <span class="emphasis"><em>uninitialized</em></span> optional.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Testing if the variant's current type is <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
models testing if the optional is initialized
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Trying to extract a <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
from a variant when its current type is not <code class="computeroutput"><span class="identifier">T</span></code>,
|
||||
models the undefined behavior of trying to access the value of an uninitialized
|
||||
optional
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
Single-element container:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>deep-copy</strong></span> semantics: copies of the container
|
||||
implies copies of the value.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>deep-relational</strong></span> semantics: comparisons
|
||||
between containers compare container size and if match, contained value
|
||||
</li>
|
||||
<li class="listitem">
|
||||
If the container is not empty (contains an object of type <code class="computeroutput"><span class="identifier">T</span></code>), it is modeling an <span class="emphasis"><em>initialized</em></span>
|
||||
optional.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
If the container is empty, it is modeling an <span class="emphasis"><em>uninitialized</em></span>
|
||||
optional.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Testing if the container is empty models testing if the optional is initialized
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Trying to extract a <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
from an empty container models the undefined behavior of trying to access
|
||||
the value of an uninitialized optional
|
||||
</li>
|
||||
</ul></div>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.development.the_semantics"></a><a class="link" href="development.html#boost_optional.development.the_semantics" title="The semantics">The semantics</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
Objects of type <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
are intended to be used in places where objects of type <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
would but which might be uninitialized. Hence, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>'s
|
||||
purpose is to formalize the additional possibly uninitialized state. From
|
||||
the perspective of this role, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
can have the same operational semantics of <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
plus the additional semantics corresponding to this special state. As such,
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> could
|
||||
be thought of as a <span class="emphasis"><em>supertype</em></span> of <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
Of course, we can't do that in C++, so we need to compose the desired semantics
|
||||
using a different mechanism. Doing it the other way around, that is, making
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> a
|
||||
<span class="emphasis"><em>subtype</em></span> of <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
is not only conceptually wrong but also impractical: it is not allowed to
|
||||
derive from a non-class type, such as a built-in type.
|
||||
</p>
|
||||
<p>
|
||||
We can draw from the purpose of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
the required basic semantics:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Default Construction:</strong></span> To introduce a
|
||||
formally uninitialized wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Direct Value Construction via copy:</strong></span>
|
||||
To introduce a formally initialized wrapped object whose value is obtained
|
||||
as a copy of some object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Deep Copy Construction:</strong></span> To obtain a
|
||||
new yet equivalent wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Direct Value Assignment (upon initialized):</strong></span>
|
||||
To assign a value to the wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Direct Value Assignment (upon uninitialized):</strong></span>
|
||||
To initialize the wrapped object with a value obtained as a copy of some
|
||||
object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Assignment (upon initialized):</strong></span> To assign
|
||||
to the wrapped object the value of another wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Assignment (upon uninitialized):</strong></span> To
|
||||
initialize the wrapped object with value of another wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Deep Relational Operations (when supported by the
|
||||
type T):</strong></span> To compare wrapped object values taking into account
|
||||
the presence of uninitialized states.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Value access:</strong></span> To unwrap the wrapped
|
||||
object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Initialization state query:</strong></span> To determine
|
||||
if the object is formally initialized or not.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Swap:</strong></span> To exchange wrapped objects. (with
|
||||
whatever exception safety guarantees are provided by <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
swap).
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>De-initialization:</strong></span> To release the wrapped
|
||||
object (if any) and leave the wrapper in the uninitialized state.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
Additional operations are useful, such as converting constructors and converting
|
||||
assignments, in-place construction and assignment, and safe value access
|
||||
via a pointer to the wrapped object or null.
|
||||
</p>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.development.the_interface"></a><a class="link" href="development.html#boost_optional.development.the_interface" title="The Interface">The Interface</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
Since the purpose of optional is to allow us to use objects with a formal
|
||||
uninitialized additional state, the interface could try to follow the interface
|
||||
of the underlying <code class="computeroutput"><span class="identifier">T</span></code> type
|
||||
as much as possible. In order to choose the proper degree of adoption of
|
||||
the native <code class="computeroutput"><span class="identifier">T</span></code> interface, the
|
||||
following must be noted: Even if all the operations supported by an instance
|
||||
of type <code class="computeroutput"><span class="identifier">T</span></code> are defined for
|
||||
the entire range of values for such a type, an <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
extends such a set of values with a new value for which most (otherwise valid)
|
||||
operations are not defined in terms of <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</p>
|
||||
<p>
|
||||
Furthermore, since <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
itself is merely a <code class="computeroutput"><span class="identifier">T</span></code> wrapper
|
||||
(modeling a <code class="computeroutput"><span class="identifier">T</span></code> supertype),
|
||||
any attempt to define such operations upon uninitialized optionals will be
|
||||
totally artificial w.r.t. <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</p>
|
||||
<p>
|
||||
This library chooses an interface which follows from <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
interface only for those operations which are well defined (w.r.t the type
|
||||
<code class="computeroutput"><span class="identifier">T</span></code>) even if any of the operands
|
||||
are uninitialized. These operations include: construction, copy-construction,
|
||||
assignment, swap and relational operations.
|
||||
</p>
|
||||
<p>
|
||||
For the value access operations, which are undefined (w.r.t the type <code class="computeroutput"><span class="identifier">T</span></code>) when the operand is uninitialized, a
|
||||
different interface is chosen (which will be explained next).
|
||||
</p>
|
||||
<p>
|
||||
Also, the presence of the possibly uninitialized state requires additional
|
||||
operations not provided by <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
itself which are supported by a special interface.
|
||||
</p>
|
||||
<h5>
|
||||
<a name="boost_optional.development.the_interface.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.development.the_interface.lexically_hinted_value_access_in_the_presence_of_possibly_uninitialized_optional_objects__the_operators___and___gt_"></a></span><a class="link" href="development.html#boost_optional.development.the_interface.lexically_hinted_value_access_in_the_presence_of_possibly_uninitialized_optional_objects__the_operators___and___gt_">Lexically-hinted
|
||||
Value Access in the presence of possibly uninitialized optional objects: The
|
||||
operators * and -></a>
|
||||
</h5>
|
||||
<p>
|
||||
A relevant feature of a pointer is that it can have a <span class="bold"><strong>null
|
||||
pointer value</strong></span>. This is a <span class="emphasis"><em>special</em></span> value which
|
||||
is used to indicate that the pointer is not referring to any object at all.
|
||||
In other words, null pointer values convey the notion of nonexistent objects.
|
||||
</p>
|
||||
<p>
|
||||
This meaning of the null pointer value allowed pointers to became a <span class="emphasis"><em>de
|
||||
facto</em></span> standard for handling optional objects because all you have
|
||||
to do to refer to a value which you don't really have is to use a null pointer
|
||||
value of the appropriate type. Pointers have been used for decades—from
|
||||
the days of C APIs to modern C++ libraries—to <span class="emphasis"><em>refer</em></span>
|
||||
to optional (that is, possibly nonexistent) objects; particularly as optional
|
||||
arguments to a function, but also quite often as optional data members.
|
||||
</p>
|
||||
<p>
|
||||
The possible presence of a null pointer value makes the operations that access
|
||||
the pointee's value possibly undefined, therefore, expressions which use
|
||||
dereference and access operators, such as: <code class="computeroutput"><span class="special">(</span>
|
||||
<span class="special">*</span><span class="identifier">p</span> <span class="special">=</span> <span class="number">2</span> <span class="special">)</span></code>
|
||||
and <code class="computeroutput"><span class="special">(</span> <span class="identifier">p</span><span class="special">-></span><span class="identifier">foo</span><span class="special">()</span> <span class="special">)</span></code>, implicitly
|
||||
convey the notion of optionality, and this information is tied to the <span class="emphasis"><em>syntax</em></span>
|
||||
of the expressions. That is, the presence of operators <code class="computeroutput"><span class="special">*</span></code>
|
||||
and <code class="computeroutput"><span class="special">-></span></code> tell by themselves
|
||||
—without any additional context— that the expression will be undefined
|
||||
unless the implied pointee actually exist.
|
||||
</p>
|
||||
<p>
|
||||
Such a <span class="emphasis"><em>de facto</em></span> idiom for referring to optional objects
|
||||
can be formalized in the form of a concept: the <a href="../../../../utility/OptionalPointee.html" target="_top">OptionalPointee</a>
|
||||
concept. This concept captures the syntactic usage of operators <code class="computeroutput"><span class="special">*</span></code>, <code class="computeroutput"><span class="special">-></span></code>
|
||||
and contextual conversion to <code class="computeroutput"><span class="keyword">bool</span></code>
|
||||
to convey the notion of optionality.
|
||||
</p>
|
||||
<p>
|
||||
However, pointers are good to <span class="underline">refer</span>
|
||||
to optional objects, but not particularly good to handle the optional objects
|
||||
in all other respects, such as initializing or moving/copying them. The problem
|
||||
resides in the shallow-copy of pointer semantics: if you need to effectively
|
||||
move or copy the object, pointers alone are not enough. The problem is that
|
||||
copies of pointers do not imply copies of pointees. For example, as was discussed
|
||||
in the motivation, pointers alone cannot be used to return optional objects
|
||||
from a function because the object must move outside from the function and
|
||||
into the caller's context.
|
||||
</p>
|
||||
<p>
|
||||
A solution to the shallow-copy problem that is often used is to resort to
|
||||
dynamic allocation and use a smart pointer to automatically handle the details
|
||||
of this. For example, if a function is to optionally return an object <code class="computeroutput"><span class="identifier">X</span></code>, it can use <code class="computeroutput"><span class="identifier">shared_ptr</span><span class="special"><</span><span class="identifier">X</span><span class="special">></span></code>
|
||||
as the return value. However, this requires dynamic allocation of <code class="computeroutput"><span class="identifier">X</span></code>. If <code class="computeroutput"><span class="identifier">X</span></code>
|
||||
is a built-in or small POD, this technique is very poor in terms of required
|
||||
resources. Optional objects are essentially values so it is very convenient
|
||||
to be able to use automatic storage and deep-copy semantics to manipulate
|
||||
optional values just as we do with ordinary values. Pointers do not have
|
||||
this semantics, so are inappropriate for the initialization and transport
|
||||
of optional values, yet are quite convenient for handling the access to the
|
||||
possible undefined value because of the idiomatic aid present in the <a href="../../../../utility/OptionalPointee.html" target="_top">OptionalPointee</a> concept
|
||||
incarnated by pointers.
|
||||
</p>
|
||||
<h5>
|
||||
<a name="boost_optional.development.the_interface.h1"></a>
|
||||
<span class="phrase"><a name="boost_optional.development.the_interface.optional_lt_t_gt__as_a_model_of_optionalpointee"></a></span><a class="link" href="development.html#boost_optional.development.the_interface.optional_lt_t_gt__as_a_model_of_optionalpointee">Optional<T>
|
||||
as a model of OptionalPointee</a>
|
||||
</h5>
|
||||
<p>
|
||||
For value access operations <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code> uses operators <code class="computeroutput"><span class="special">*</span></code>
|
||||
and <code class="computeroutput"><span class="special">-></span></code> to lexically warn
|
||||
about the possibly uninitialized state appealing to the familiar pointer
|
||||
semantics w.r.t. to null pointers.
|
||||
</p>
|
||||
<div class="warning"><table border="0" summary="Warning">
|
||||
<tr>
|
||||
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../doc/src/images/warning.png"></td>
|
||||
<th align="left">Warning</th>
|
||||
</tr>
|
||||
<tr><td align="left" valign="top"><p>
|
||||
However, it is particularly important to note that <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code> objects are not pointers. <span class="underline"><code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code> is not, and does not model, a pointer</span>.
|
||||
</p></td></tr>
|
||||
</table></div>
|
||||
<p>
|
||||
For instance, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code>
|
||||
does not have shallow-copy so does not alias: two different optionals never
|
||||
refer to the <span class="emphasis"><em>same</em></span> value unless <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
itself is a reference (but may have <span class="emphasis"><em>equivalent</em></span> values).
|
||||
The difference between an <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
and a pointer must be kept in mind, particularly because the semantics of
|
||||
relational operators are different: since <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
is a value-wrapper, relational operators are deep: they compare optional
|
||||
values; but relational operators for pointers are shallow: they do not compare
|
||||
pointee values. As a result, you might be able to replace <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
by <code class="computeroutput"><span class="identifier">T</span><span class="special">*</span></code>
|
||||
on some situations but not always. Specifically, on generic code written
|
||||
for both, you cannot use relational operators directly, and must use the
|
||||
template functions <a href="../../../../utility/OptionalPointee.html#equal" target="_top"><code class="computeroutput"><span class="identifier">equal_pointees</span><span class="special">()</span></code></a>
|
||||
and <a href="../../../../utility/OptionalPointee.html#less" target="_top"><code class="computeroutput"><span class="identifier">less_pointees</span><span class="special">()</span></code></a>
|
||||
instead.
|
||||
</p>
|
||||
</div>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="tutorial.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="synopsis.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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</div>
|
||||
</body>
|
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</html>
|
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@@ -1,128 +0,0 @@
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||||
<html>
|
||||
<head>
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<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
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<meta name="generator" content="DocBook XSL Stylesheets V1.78.1">
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<link rel="home" href="../index.html" title="Chapter 1. Boost.Optional">
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<link rel="up" href="../index.html" title="Chapter 1. Boost.Optional">
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<link rel="prev" href="quick_start.html" title="Quick Start">
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<link rel="next" href="development.html" title="Development">
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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<table cellpadding="2" width="100%"><tr>
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<td align="center"><a href="../../../../../index.html">Home</a></td>
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<td align="center"><a href="../../../../../libs/libraries.htm">Libraries</a></td>
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<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
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<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
|
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<td align="center"><a href="../../../../../more/index.htm">More</a></td>
|
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<a accesskey="p" href="quick_start.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="development.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
|
||||
<a name="boost_optional.discussion"></a><a class="link" href="discussion.html" title="Discussion">Discussion</a>
|
||||
</h2></div></div></div>
|
||||
<p>
|
||||
Consider these functions which should return a value but which might not have
|
||||
a value to return:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
(A) <code class="computeroutput"><span class="keyword">double</span> <span class="identifier">sqrt</span><span class="special">(</span><span class="keyword">double</span> <span class="identifier">n</span> <span class="special">);</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
(B) <code class="computeroutput"><span class="keyword">char</span> <span class="identifier">get_async_input</span><span class="special">();</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
(C) <code class="computeroutput"><span class="identifier">point</span> <span class="identifier">polygon</span><span class="special">::</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span></code>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
There are different approaches to the issue of not having a value to return.
|
||||
</p>
|
||||
<p>
|
||||
A typical approach is to consider the existence of a valid return value as
|
||||
a postcondition, so that if the function cannot compute the value to return,
|
||||
it has either undefined behavior (and can use assert in a debug build) or uses
|
||||
a runtime check and throws an exception if the postcondition is violated. This
|
||||
is a reasonable choice for example, for function (A), because the lack of a
|
||||
proper return value is directly related to an invalid parameter (out of domain
|
||||
argument), so it is appropriate to require the callee to supply only parameters
|
||||
in a valid domain for execution to continue normally.
|
||||
</p>
|
||||
<p>
|
||||
However, function (B), because of its asynchronous nature, does not fail just
|
||||
because it can't find a value to return; so it is incorrect to consider such
|
||||
a situation an error and assert or throw an exception. This function must return,
|
||||
and somehow, must tell the callee that it is not returning a meaningful value.
|
||||
</p>
|
||||
<p>
|
||||
A similar situation occurs with function (C): it is conceptually an error to
|
||||
ask a <span class="emphasis"><em>null-area</em></span> polygon to return a point inside itself,
|
||||
but in many applications, it is just impractical for performance reasons to
|
||||
treat this as an error (because detecting that the polygon has no area might
|
||||
be too expensive to be required to be tested previously), and either an arbitrary
|
||||
point (typically at infinity) is returned, or some efficient way to tell the
|
||||
callee that there is no such point is used.
|
||||
</p>
|
||||
<p>
|
||||
There are various mechanisms to let functions communicate that the returned
|
||||
value is not valid. One such mechanism, which is quite common since it has
|
||||
zero or negligible overhead, is to use a special value which is reserved to
|
||||
communicate this. Classical examples of such special values are <code class="computeroutput"><span class="identifier">EOF</span></code>, <code class="computeroutput"><span class="identifier">string</span><span class="special">::</span><span class="identifier">npos</span></code>, points
|
||||
at infinity, etc...
|
||||
</p>
|
||||
<p>
|
||||
When those values exist, i.e. the return type can hold all meaningful values
|
||||
<span class="emphasis"><em>plus</em></span> the <span class="emphasis"><em>signal</em></span> value, this mechanism
|
||||
is quite appropriate and well known. Unfortunately, there are cases when such
|
||||
values do not exist. In these cases, the usual alternative is either to use
|
||||
a wider type, such as <code class="computeroutput"><span class="keyword">int</span></code> in place
|
||||
of <code class="computeroutput"><span class="keyword">char</span></code>; or a compound type, such
|
||||
as <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span></code>.
|
||||
</p>
|
||||
<p>
|
||||
Returning a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span></code>, thus attaching a boolean flag to the result
|
||||
which indicates if the result is meaningful, has the advantage that can be
|
||||
turned into a consistent idiom since the first element of the pair can be whatever
|
||||
the function would conceptually return. For example, the last two functions
|
||||
could have the following interface:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="keyword">char</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">get_async_input</span><span class="special">();</span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">polygon</span><span class="special">::</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span>
|
||||
</pre>
|
||||
<p>
|
||||
These functions use a consistent interface for dealing with possibly nonexistent
|
||||
results:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">poly</span><span class="special">.</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">second</span> <span class="special">)</span>
|
||||
<span class="identifier">flood_fill</span><span class="special">(</span><span class="identifier">p</span><span class="special">.</span><span class="identifier">first</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
However, not only is this quite a burden syntactically, it is also error prone
|
||||
since the user can easily use the function result (first element of the pair)
|
||||
without ever checking if it has a valid value.
|
||||
</p>
|
||||
<p>
|
||||
Clearly, we need a better idiom.
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
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|
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|
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<div class="toc"><dl class="toc">
|
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<dt><span class="section"><a href="examples.html#boost_optional.examples.optional_return_values">Optional
|
||||
return values</a></span></dt>
|
||||
<dt><span class="section"><a href="examples.html#boost_optional.examples.optional_local_variables">Optional
|
||||
local variables</a></span></dt>
|
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<dt><span class="section"><a href="examples.html#boost_optional.examples.optional_data_members">Optional
|
||||
data members</a></span></dt>
|
||||
<dt><span class="section"><a href="examples.html#boost_optional.examples.bypassing_expensive_unnecessary_default_construction">Bypassing
|
||||
expensive unnecessary default construction</a></span></dt>
|
||||
</dl></div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.examples.optional_return_values"></a><a class="link" href="examples.html#boost_optional.examples.optional_return_values" title="Optional return values">Optional
|
||||
return values</a>
|
||||
</h3></div></div></div>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">char</span><span class="special">></span> <span class="identifier">get_async_input</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span> <span class="special">!</span><span class="identifier">queue</span><span class="special">.</span><span class="identifier">empty</span><span class="special">()</span> <span class="special">)</span>
|
||||
<span class="keyword">return</span> <span class="identifier">optional</span><span class="special"><</span><span class="keyword">char</span><span class="special">>(</span><span class="identifier">queue</span><span class="special">.</span><span class="identifier">top</span><span class="special">());</span>
|
||||
<span class="keyword">else</span> <span class="keyword">return</span> <span class="identifier">optional</span><span class="special"><</span><span class="keyword">char</span><span class="special">>();</span> <span class="comment">// uninitialized</span>
|
||||
<span class="special">}</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">receive_async_message</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">char</span><span class="special">></span> <span class="identifier">rcv</span> <span class="special">;</span>
|
||||
<span class="comment">// The safe boolean conversion from 'rcv' is used here.</span>
|
||||
<span class="keyword">while</span> <span class="special">(</span> <span class="special">(</span><span class="identifier">rcv</span> <span class="special">=</span> <span class="identifier">get_async_input</span><span class="special">())</span> <span class="special">&&</span> <span class="special">!</span><span class="identifier">timeout</span><span class="special">()</span> <span class="special">)</span>
|
||||
<span class="identifier">output</span><span class="special">(*</span><span class="identifier">rcv</span><span class="special">);</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.examples.optional_local_variables"></a><a class="link" href="examples.html#boost_optional.examples.optional_local_variables" title="Optional local variables">Optional
|
||||
local variables</a>
|
||||
</h3></div></div></div>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">string</span><span class="special">></span> <span class="identifier">name</span> <span class="special">;</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span> <span class="identifier">database</span><span class="special">.</span><span class="identifier">open</span><span class="special">()</span> <span class="special">)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">name</span> <span class="special">=</span> <span class="identifier">database</span><span class="special">.</span><span class="identifier">lookup</span><span class="special">(</span><span class="identifier">employer_name</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
<span class="keyword">else</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span> <span class="identifier">can_ask_user</span> <span class="special">)</span>
|
||||
<span class="identifier">name</span> <span class="special">=</span> <span class="identifier">user</span><span class="special">.</span><span class="identifier">ask</span><span class="special">(</span><span class="identifier">employer_name</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
|
||||
<span class="keyword">if</span> <span class="special">(</span> <span class="identifier">name</span> <span class="special">)</span>
|
||||
<span class="identifier">print</span><span class="special">(*</span><span class="identifier">name</span><span class="special">);</span>
|
||||
<span class="keyword">else</span> <span class="identifier">print</span><span class="special">(</span><span class="string">"employer's name not found!"</span><span class="special">);</span>
|
||||
</pre>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.examples.optional_data_members"></a><a class="link" href="examples.html#boost_optional.examples.optional_data_members" title="Optional data members">Optional
|
||||
data members</a>
|
||||
</h3></div></div></div>
|
||||
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">figure</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">public</span><span class="special">:</span>
|
||||
|
||||
<span class="identifier">figure</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="comment">// data member 'm_clipping_rect' is uninitialized at this point.</span>
|
||||
<span class="special">}</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">clip_in_rect</span> <span class="special">(</span> <span class="identifier">rect</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">rect</span> <span class="special">)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="special">....</span>
|
||||
<span class="identifier">m_clipping_rect</span> <span class="special">=</span> <span class="identifier">rect</span> <span class="special">;</span> <span class="comment">// initialized here.</span>
|
||||
<span class="special">}</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">draw</span> <span class="special">(</span> <span class="identifier">canvas</span><span class="special">&</span> <span class="identifier">cvs</span> <span class="special">)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span> <span class="identifier">m_clipping_rect</span> <span class="special">)</span>
|
||||
<span class="identifier">do_clipping</span><span class="special">(*</span><span class="identifier">m_clipping_rect</span><span class="special">);</span>
|
||||
|
||||
<span class="identifier">cvs</span><span class="special">.</span><span class="identifier">drawXXX</span><span class="special">(..);</span>
|
||||
<span class="special">}</span>
|
||||
|
||||
<span class="comment">// this can return NULL.</span>
|
||||
<span class="identifier">rect</span> <span class="keyword">const</span><span class="special">*</span> <span class="identifier">get_clipping_rect</span><span class="special">()</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">get_pointer</span><span class="special">(</span><span class="identifier">m_clipping_rect</span><span class="special">);</span> <span class="special">}</span>
|
||||
|
||||
<span class="keyword">private</span> <span class="special">:</span>
|
||||
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">rect</span><span class="special">></span> <span class="identifier">m_clipping_rect</span> <span class="special">;</span>
|
||||
|
||||
<span class="special">};</span>
|
||||
</pre>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.examples.bypassing_expensive_unnecessary_default_construction"></a><a class="link" href="examples.html#boost_optional.examples.bypassing_expensive_unnecessary_default_construction" title="Bypassing expensive unnecessary default construction">Bypassing
|
||||
expensive unnecessary default construction</a>
|
||||
</h3></div></div></div>
|
||||
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">ExpensiveCtor</span> <span class="special">{</span> <span class="special">...</span> <span class="special">}</span> <span class="special">;</span>
|
||||
<span class="keyword">class</span> <span class="identifier">Fred</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">Fred</span><span class="special">()</span> <span class="special">:</span> <span class="identifier">mLargeVector</span><span class="special">(</span><span class="number">10000</span><span class="special">)</span> <span class="special">{}</span>
|
||||
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">ExpensiveCtor</span><span class="special">></span> <span class="special">></span> <span class="identifier">mLargeVector</span> <span class="special">;</span>
|
||||
<span class="special">}</span> <span class="special">;</span>
|
||||
</pre>
|
||||
</div>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
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|
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<div class="spirit-nav">
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<a accesskey="p" href="detailed_semantics.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="optional_references.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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<a name="boost_optional.exception_safety_guarantees"></a><a class="link" href="exception_safety_guarantees.html" title="Exception Safety Guarantees">Exception Safety
|
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Guarantees</a>
|
||||
</h2></div></div></div>
|
||||
<p>
|
||||
This library assumes that <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
destructor does not throw exceptions. If it does, the behaviour of many operations
|
||||
on <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is
|
||||
undefined.
|
||||
</p>
|
||||
<p>
|
||||
The following mutating operations never throw exceptions:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">none_t</span> <span class="special">)</span> <span class="keyword">noexcept</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">reset</span><span class="special">()</span> <span class="keyword">noexcept</span></code>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
In addition, the following constructors and the destructor never throw exceptions:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">optional</span><span class="special">()</span>
|
||||
<span class="keyword">noexcept</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">optional</span><span class="special">(</span> <span class="identifier">none_t</span> <span class="special">)</span> <span class="keyword">noexcept</span></code>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
Regarding the following assignment functions:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">InPlaceFactory</span><span class="special">></span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">InPlaceFactory</span>
|
||||
<span class="keyword">const</span><span class="special">&</span>
|
||||
<span class="special">)</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">TypedInPlaceFactory</span><span class="special">></span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">TypedInPlaceFactory</span>
|
||||
<span class="keyword">const</span><span class="special">&</span>
|
||||
<span class="special">)</span> </code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">reset</span><span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span></code>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
They forward calls to the corresponding <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
constructors or assignments (depending on whether the optional object is initialized
|
||||
or not); so if both <code class="computeroutput"><span class="identifier">T</span></code>'s constructor
|
||||
and the assignment provide strong exception safety guarantee, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>'s assignment
|
||||
also provides strong exception safety guarantee; otherwise we only get the
|
||||
basic guarantee. Additionally, if both involved <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
constructor and the assignment never throw, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>'s
|
||||
assignment also never throws.
|
||||
</p>
|
||||
<p>
|
||||
Unless <code class="computeroutput"><span class="identifier">T</span></code>'s constructor or assignment
|
||||
throws, assignments to <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
do not throw anything else on its own. A throw during assignment never changes
|
||||
the initialization state of any optional object involved:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">opt1</span><span class="special">(</span><span class="identifier">val1</span><span class="special">);</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">opt2</span><span class="special">(</span><span class="identifier">val2</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">opt1</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">opt2</span><span class="special">);</span>
|
||||
|
||||
<span class="keyword">try</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">opt1</span> <span class="special">=</span> <span class="identifier">opt2</span><span class="special">;</span> <span class="comment">// throws</span>
|
||||
<span class="special">}</span>
|
||||
<span class="keyword">catch</span><span class="special">(...)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">opt1</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">opt2</span><span class="special">);</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
This also applies to move assignments/constructors. However, move operations
|
||||
are made no-throw more often.
|
||||
</p>
|
||||
<p>
|
||||
Operation <code class="computeroutput"><span class="identifier">emplace</span></code> provides
|
||||
basic exception safety guarantee. If it throws, the optional object becomes
|
||||
uninitialized regardless of its initial state, and its previous contained value
|
||||
(if any) is destroyed. It doesn't call any assignment or move/copy constructor
|
||||
on <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</p>
|
||||
<h4>
|
||||
<a name="boost_optional.exception_safety_guarantees.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.exception_safety_guarantees.swap"></a></span><a class="link" href="exception_safety_guarantees.html#boost_optional.exception_safety_guarantees.swap">Swap</a>
|
||||
</h4>
|
||||
<p>
|
||||
Unless <code class="computeroutput"><span class="identifier">swap</span></code> on optional is
|
||||
customized, its primary implementation forwards calls to <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
<code class="computeroutput"><span class="identifier">swap</span></code> or move constructor (depending
|
||||
on the initialization state of the optional objects). Thus, if both <code class="computeroutput"><span class="identifier">T</span></code>'s <code class="computeroutput"><span class="identifier">swap</span></code>
|
||||
and move constructor never throw, <code class="computeroutput"><span class="identifier">swap</span></code>
|
||||
on <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> never
|
||||
throws. similarly, if both <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
<code class="computeroutput"><span class="identifier">swap</span></code> and move constructor offer
|
||||
strong guarantee, <code class="computeroutput"><span class="identifier">swap</span></code> on
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> also
|
||||
offers a strong guarantee.
|
||||
</p>
|
||||
<p>
|
||||
In case <code class="computeroutput"><span class="identifier">swap</span></code> on optional is
|
||||
customized, the call to <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
move constructor are replaced with the calls to <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
default constructor followed by <code class="computeroutput"><span class="identifier">swap</span></code>.
|
||||
(This is more useful on older compilers that do not support move semantics,
|
||||
when one wants to achieve stronger exception safety guarantees.) In this case
|
||||
the exception safety guarantees for <code class="computeroutput"><span class="identifier">swap</span></code>
|
||||
are reliant on the guarantees of <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
<code class="computeroutput"><span class="identifier">swap</span></code> and default constructor
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
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<a accesskey="p" href="a_note_about_optional_bool_.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="type_requirements.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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|
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<html>
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<head>
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="boost_optional.implementation_notes"></a><a class="link" href="implementation_notes.html" title="Implementation Notes">Implementation Notes</a>
|
||||
</h2></div></div></div>
|
||||
<p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is
|
||||
currently implemented using a custom aligned storage facility built from <code class="computeroutput"><span class="identifier">alignment_of</span></code> and <code class="computeroutput"><span class="identifier">type_with_alignment</span></code>
|
||||
(both from Type Traits). It uses a separate boolean flag to indicate the initialization
|
||||
state. Placement new with <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
copy/move constructor and <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
destructor are explicitly used to initialize, copy, move and destroy optional
|
||||
values. As a result, <code class="computeroutput"><span class="identifier">T</span></code>'s default
|
||||
constructor is effectively by-passed, but the exception guarantees are basic.
|
||||
It is planned to replace the current implementation with another with stronger
|
||||
exception safety, such as a future <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">variant</span></code>.
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
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<hr>
|
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<div class="spirit-nav">
|
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<a accesskey="p" href="type_requirements.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="dependencies_and_portability.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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@@ -1,196 +0,0 @@
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<html>
|
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<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
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<title>In-Place Factories</title>
|
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<link rel="stylesheet" href="../../../../../doc/src/boostbook.css" type="text/css">
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<link rel="prev" href="detailed_semantics.html" title="Detailed Semantics">
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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<table cellpadding="2" width="100%"><tr>
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<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../boost.png"></td>
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<td align="center"><a href="../../../../../index.html">Home</a></td>
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<td align="center"><a href="../../../../../libs/libraries.htm">Libraries</a></td>
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<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
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<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
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<td align="center"><a href="../../../../../more/index.htm">More</a></td>
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<a accesskey="p" href="detailed_semantics.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="a_note_about_optional_bool_.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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</div>
|
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<div class="section">
|
||||
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
|
||||
<a name="boost_optional.in_place_factories"></a><a class="link" href="in_place_factories.html" title="In-Place Factories">In-Place Factories</a>
|
||||
</h2></div></div></div>
|
||||
<p>
|
||||
One of the typical problems with wrappers and containers is that their interfaces
|
||||
usually provide an operation to initialize or assign the contained object as
|
||||
a copy of some other object. This not only requires the underlying type to
|
||||
be <a href="../../../../utility/CopyConstructible.html" target="_top">Copy Constructible</a>,
|
||||
but also requires the existence of a fully constructed object, often temporary,
|
||||
just to follow the copy from:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">X</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">X</span> <span class="special">(</span> <span class="keyword">int</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">}</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">class</span> <span class="identifier">W</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">public</span><span class="special">:</span>
|
||||
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">X</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span> <span class="special">)</span> <span class="special">:</span> <span class="identifier">wrapped_</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">{}</span>
|
||||
<span class="special">}</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="comment">// Temporary object created.</span>
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">X</span><span class="special">(</span><span class="number">123</span><span class="special">,</span><span class="string">"hello"</span><span class="special">)</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
A solution to this problem is to support direct construction of the contained
|
||||
object right in the container's storage. In this scheme, the user only needs
|
||||
to supply the arguments to the constructor to use in the wrapped object construction.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">W</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">public</span><span class="special">:</span>
|
||||
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">X</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span> <span class="special">)</span> <span class="special">:</span> <span class="identifier">wrapped_</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">{}</span>
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="keyword">int</span> <span class="identifier">a0</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">a1</span><span class="special">)</span> <span class="special">:</span> <span class="identifier">wrapped_</span><span class="special">(</span><span class="identifier">a0</span><span class="special">,</span><span class="identifier">a1</span><span class="special">)</span> <span class="special">{}</span>
|
||||
<span class="special">}</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="comment">// Wrapped object constructed in-place</span>
|
||||
<span class="comment">// No temporary created.</span>
|
||||
<span class="identifier">W</span> <span class="special">(</span><span class="number">123</span><span class="special">,</span><span class="string">"hello"</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
A limitation of this method is that it doesn't scale well to wrapped objects
|
||||
with multiple constructors nor to generic code were the constructor overloads
|
||||
are unknown.
|
||||
</p>
|
||||
<p>
|
||||
The solution presented in this library is the family of <span class="bold"><strong>InPlaceFactories</strong></span>
|
||||
and <span class="bold"><strong>TypedInPlaceFactories</strong></span>. These factories
|
||||
are a family of classes which encapsulate an increasing number of arbitrary
|
||||
constructor parameters and supply a method to construct an object of a given
|
||||
type using those parameters at an address specified by the user via placement
|
||||
new.
|
||||
</p>
|
||||
<p>
|
||||
For example, one member of this family looks like:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span><span class="keyword">class</span> <span class="identifier">A0</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">A1</span><span class="special">></span>
|
||||
<span class="keyword">class</span> <span class="identifier">TypedInPlaceFactory2</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">A0</span> <span class="identifier">m_a0</span> <span class="special">;</span> <span class="identifier">A1</span> <span class="identifier">m_a1</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">public</span><span class="special">:</span>
|
||||
|
||||
<span class="identifier">TypedInPlaceFactory2</span><span class="special">(</span> <span class="identifier">A0</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">a0</span><span class="special">,</span> <span class="identifier">A1</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">a1</span> <span class="special">)</span> <span class="special">:</span> <span class="identifier">m_a0</span><span class="special">(</span><span class="identifier">a0</span><span class="special">),</span> <span class="identifier">m_a1</span><span class="special">(</span><span class="identifier">a1</span><span class="special">)</span> <span class="special">{}</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">construct</span> <span class="special">(</span> <span class="keyword">void</span><span class="special">*</span> <span class="identifier">p</span> <span class="special">)</span> <span class="special">{</span> <span class="keyword">new</span> <span class="special">(</span><span class="identifier">p</span><span class="special">)</span> <span class="identifier">T</span><span class="special">(</span><span class="identifier">m_a0</span><span class="special">,</span><span class="identifier">m_a1</span><span class="special">)</span> <span class="special">;</span> <span class="special">}</span>
|
||||
<span class="special">}</span> <span class="special">;</span>
|
||||
</pre>
|
||||
<p>
|
||||
A wrapper class aware of this can use it as:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">W</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">public</span><span class="special">:</span>
|
||||
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">X</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span> <span class="special">)</span> <span class="special">:</span> <span class="identifier">wrapped_</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">{}</span>
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">TypedInPlaceFactory2</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">fac</span> <span class="special">)</span> <span class="special">{</span> <span class="identifier">fac</span><span class="special">.</span><span class="identifier">construct</span><span class="special">(&</span><span class="identifier">wrapped_</span><span class="special">)</span> <span class="special">;</span> <span class="special">}</span>
|
||||
<span class="special">}</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="comment">// Wrapped object constructed in-place via a TypedInPlaceFactory.</span>
|
||||
<span class="comment">// No temporary created.</span>
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">TypedInPlaceFactory2</span><span class="special"><</span><span class="identifier">X</span><span class="special">,</span><span class="keyword">int</span><span class="special">,</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">>(</span><span class="number">123</span><span class="special">,</span><span class="string">"hello"</span><span class="special">))</span> <span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
The factories are divided in two groups:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="underline">TypedInPlaceFactories</span>: those which
|
||||
take the target type as a primary template parameter.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="underline">InPlaceFactories</span>: those with a template
|
||||
<code class="computeroutput"><span class="identifier">construct</span><span class="special">(</span><span class="keyword">void</span><span class="special">*)</span></code> member
|
||||
function taking the target type.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
Within each group, all the family members differ only in the number of parameters
|
||||
allowed.
|
||||
</p>
|
||||
<p>
|
||||
This library provides an overloaded set of helper template functions to construct
|
||||
these factories without requiring unnecessary template parameters:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">A0</span><span class="special">,...,</span><span class="keyword">class</span> <span class="identifier">AN</span><span class="special">></span>
|
||||
<span class="identifier">InPlaceFactoryN</span> <span class="special"><</span><span class="identifier">A0</span><span class="special">,...,</span><span class="identifier">AN</span><span class="special">></span> <span class="identifier">in_place</span> <span class="special">(</span> <span class="identifier">A0</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">a0</span><span class="special">,</span> <span class="special">...,</span> <span class="identifier">AN</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">aN</span><span class="special">)</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span><span class="keyword">class</span> <span class="identifier">A0</span><span class="special">,...,</span><span class="keyword">class</span> <span class="identifier">AN</span><span class="special">></span>
|
||||
<span class="identifier">TypedInPlaceFactoryN</span> <span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="identifier">A0</span><span class="special">,...,</span><span class="identifier">AN</span><span class="special">></span> <span class="identifier">in_place</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">a0</span><span class="special">,</span> <span class="identifier">A0</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">a0</span><span class="special">,</span> <span class="special">...,</span> <span class="identifier">AN</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">aN</span><span class="special">)</span> <span class="special">;</span>
|
||||
</pre>
|
||||
<p>
|
||||
In-place factories can be used generically by the wrapper and user as follows:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">W</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">public</span><span class="special">:</span>
|
||||
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">X</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span> <span class="special">)</span> <span class="special">:</span> <span class="identifier">wrapped_</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">{}</span>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span> <span class="keyword">class</span> <span class="identifier">InPlaceFactory</span> <span class="special">></span>
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">InPlaceFactory</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">fac</span> <span class="special">)</span> <span class="special">{</span> <span class="identifier">fac</span><span class="special">.</span><span class="keyword">template</span> <span class="special"><</span><span class="identifier">X</span><span class="special">></span><span class="identifier">construct</span><span class="special">(&</span><span class="identifier">wrapped_</span><span class="special">)</span> <span class="special">;</span> <span class="special">}</span>
|
||||
|
||||
<span class="special">}</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="comment">// Wrapped object constructed in-place via a InPlaceFactory.</span>
|
||||
<span class="comment">// No temporary created.</span>
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">in_place</span><span class="special">(</span><span class="number">123</span><span class="special">,</span><span class="string">"hello"</span><span class="special">)</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
The factories are implemented in the headers: <a href="../../../../../boost/utility/in_place_factory.hpp" target="_top">in_place_factory.hpp</a>
|
||||
and <a href="../../../../../boost/utility/typed_in_place_factory.hpp" target="_top">typed_in_place_factory.hpp</a>
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
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|
||||
</tr></table>
|
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|
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<div class="spirit-nav">
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<a accesskey="p" href="detailed_semantics.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="a_note_about_optional_bool_.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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|
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<html>
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<head>
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|
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<a accesskey="p" href="../index.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="development.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
|
||||
<a name="boost_optional.motivation"></a><a class="link" href="motivation.html" title="Motivation">Motivation</a>
|
||||
</h2></div></div></div>
|
||||
<p>
|
||||
Consider these functions which should return a value but which might not have
|
||||
a value to return:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
(A) <code class="computeroutput"><span class="keyword">double</span> <span class="identifier">sqrt</span><span class="special">(</span><span class="keyword">double</span> <span class="identifier">n</span> <span class="special">);</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
(B) <code class="computeroutput"><span class="keyword">char</span> <span class="identifier">get_async_input</span><span class="special">();</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
(C) <code class="computeroutput"><span class="identifier">point</span> <span class="identifier">polygon</span><span class="special">::</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span></code>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
There are different approaches to the issue of not having a value to return.
|
||||
</p>
|
||||
<p>
|
||||
A typical approach is to consider the existence of a valid return value as
|
||||
a postcondition, so that if the function cannot compute the value to return,
|
||||
it has either undefined behavior (and can use assert in a debug build) or uses
|
||||
a runtime check and throws an exception if the postcondition is violated. This
|
||||
is a reasonable choice for example, for function (A), because the lack of a
|
||||
proper return value is directly related to an invalid parameter (out of domain
|
||||
argument), so it is appropriate to require the callee to supply only parameters
|
||||
in a valid domain for execution to continue normally.
|
||||
</p>
|
||||
<p>
|
||||
However, function (B), because of its asynchronous nature, does not fail just
|
||||
because it can't find a value to return; so it is incorrect to consider such
|
||||
a situation an error and assert or throw an exception. This function must return,
|
||||
and somehow, must tell the callee that it is not returning a meaningful value.
|
||||
</p>
|
||||
<p>
|
||||
A similar situation occurs with function (C): it is conceptually an error to
|
||||
ask a <span class="emphasis"><em>null-area</em></span> polygon to return a point inside itself,
|
||||
but in many applications, it is just impractical for performance reasons to
|
||||
treat this as an error (because detecting that the polygon has no area might
|
||||
be too expensive to be required to be tested previously), and either an arbitrary
|
||||
point (typically at infinity) is returned, or some efficient way to tell the
|
||||
callee that there is no such point is used.
|
||||
</p>
|
||||
<p>
|
||||
There are various mechanisms to let functions communicate that the returned
|
||||
value is not valid. One such mechanism, which is quite common since it has
|
||||
zero or negligible overhead, is to use a special value which is reserved to
|
||||
communicate this. Classical examples of such special values are <code class="computeroutput"><span class="identifier">EOF</span></code>, <code class="computeroutput"><span class="identifier">string</span><span class="special">::</span><span class="identifier">npos</span></code>, points
|
||||
at infinity, etc...
|
||||
</p>
|
||||
<p>
|
||||
When those values exist, i.e. the return type can hold all meaningful values
|
||||
<span class="emphasis"><em>plus</em></span> the <span class="emphasis"><em>signal</em></span> value, this mechanism
|
||||
is quite appropriate and well known. Unfortunately, there are cases when such
|
||||
values do not exist. In these cases, the usual alternative is either to use
|
||||
a wider type, such as <code class="computeroutput"><span class="keyword">int</span></code> in place
|
||||
of <code class="computeroutput"><span class="keyword">char</span></code>; or a compound type, such
|
||||
as <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span></code>.
|
||||
</p>
|
||||
<p>
|
||||
Returning a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span></code>, thus attaching a boolean flag to the result
|
||||
which indicates if the result is meaningful, has the advantage that can be
|
||||
turned into a consistent idiom since the first element of the pair can be whatever
|
||||
the function would conceptually return. For example, the last two functions
|
||||
could have the following interface:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="keyword">char</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">get_async_input</span><span class="special">();</span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">polygon</span><span class="special">::</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span>
|
||||
</pre>
|
||||
<p>
|
||||
These functions use a consistent interface for dealing with possibly nonexistent
|
||||
results:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">poly</span><span class="special">.</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">second</span> <span class="special">)</span>
|
||||
<span class="identifier">flood_fill</span><span class="special">(</span><span class="identifier">p</span><span class="special">.</span><span class="identifier">first</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
However, not only is this quite a burden syntactically, it is also error prone
|
||||
since the user can easily use the function result (first element of the pair)
|
||||
without ever checking if it has a valid value.
|
||||
</p>
|
||||
<p>
|
||||
Clearly, we need a better idiom.
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
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|
||||
<div class="spirit-nav">
|
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@@ -1,114 +0,0 @@
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||||
<html>
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||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
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<title>Optional references</title>
|
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||||
<div class="spirit-nav">
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<a accesskey="p" href="detailed_semantics.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="rebinding_semantics_for_assignment_of_optional_references.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
|
||||
<a name="boost_optional.optional_references"></a><a class="link" href="optional_references.html" title="Optional references">Optional references</a>
|
||||
</h2></div></div></div>
|
||||
<p>
|
||||
This library allows the template parameter <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
to be of reference type: <code class="computeroutput"><span class="identifier">T</span><span class="special">&</span></code>, and to some extent, <code class="computeroutput"><span class="identifier">T</span>
|
||||
<span class="keyword">const</span><span class="special">&</span></code>.
|
||||
</p>
|
||||
<p>
|
||||
However, since references are not real objects some restrictions apply and
|
||||
some operations are not available in this case:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
Converting constructors
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Converting assignment
|
||||
</li>
|
||||
<li class="listitem">
|
||||
InPlace construction
|
||||
</li>
|
||||
<li class="listitem">
|
||||
InPlace assignment
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Value-access via pointer
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
Also, even though <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>
|
||||
treats it wrapped pseudo-object much as a real value, a true real reference
|
||||
is stored so aliasing will occur:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
Copies of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>
|
||||
will copy the references but all these references will nonetheless refer
|
||||
to the same object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Value-access will actually provide access to the referenced object rather
|
||||
than the reference itself.
|
||||
</li>
|
||||
</ul></div>
|
||||
<div class="warning"><table border="0" summary="Warning">
|
||||
<tr>
|
||||
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../doc/src/images/warning.png"></td>
|
||||
<th align="left">Warning</th>
|
||||
</tr>
|
||||
<tr><td align="left" valign="top"><p>
|
||||
On compilers that do not conform to Standard C++ rules of reference binding,
|
||||
operations on optional references might give adverse results: rather than
|
||||
binding a reference to a designated object they may create an unexpected
|
||||
temporary and bind to it. For more details see <a class="link" href="dependencies_and_portability/optional_reference_binding.html" title="Optional Reference Binding">Dependencies
|
||||
and Portability section</a>.
|
||||
</p></td></tr>
|
||||
</table></div>
|
||||
<h4>
|
||||
<a name="boost_optional.optional_references.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.optional_references.rvalue_references"></a></span><a class="link" href="optional_references.html#boost_optional.optional_references.rvalue_references">Rvalue
|
||||
references</a>
|
||||
</h4>
|
||||
<p>
|
||||
Rvalue references and lvalue references to const have the ability in C++ to
|
||||
extend the life time of a temporary they bind to. Optional references do not
|
||||
have this capability, therefore to avoid surprising effects it is not possible
|
||||
to initialize an optional references from a temporary. Optional rvalue references
|
||||
are disabled altogether. Also, the initialization and assignment of an optional
|
||||
reference to const from rvalue reference is disabled.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">1</span><span class="special">;</span> <span class="comment">// legal</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&></span> <span class="identifier">oi</span> <span class="special">=</span> <span class="number">1</span><span class="special">;</span> <span class="comment">// illegal</span>
|
||||
</pre>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="detailed_semantics.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="rebinding_semantics_for_assignment_of_optional_references.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
|
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</div>
|
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</body>
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</html>
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@@ -1,169 +0,0 @@
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||||
<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
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<title>Quick Start</title>
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<link rel="stylesheet" href="../../../../../doc/src/boostbook.css" type="text/css">
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<meta name="generator" content="DocBook XSL Stylesheets Vsnapshot">
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<a accesskey="p" href="../index.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="quick_start/optional_automatic_variables.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="boost_optional.quick_start"></a><a class="link" href="quick_start.html" title="Quick Start">Quick Start</a>
|
||||
</h2></div></div></div>
|
||||
<div class="toc"><dl class="toc">
|
||||
<dt><span class="section"><a href="quick_start.html#boost_optional.quick_start.optional_return_values">Optional
|
||||
return values</a></span></dt>
|
||||
<dt><span class="section"><a href="quick_start/optional_automatic_variables.html">Optional
|
||||
automatic variables</a></span></dt>
|
||||
<dt><span class="section"><a href="quick_start/optional_data_members.html">Optional
|
||||
data members</a></span></dt>
|
||||
<dt><span class="section"><a href="quick_start/bypassing_unnecessary_default_construction.html">Bypassing
|
||||
unnecessary default construction</a></span></dt>
|
||||
<dt><span class="section"><a href="quick_start/storage_in_containers.html">Storage
|
||||
in containers</a></span></dt>
|
||||
</dl></div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.quick_start.optional_return_values"></a><a class="link" href="quick_start.html#boost_optional.quick_start.optional_return_values" title="Optional return values">Optional
|
||||
return values</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
Let's write and use a converter function that converts a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>
|
||||
to an <code class="computeroutput"><span class="keyword">int</span></code>. It is possible that
|
||||
for a given string (e.g. <code class="computeroutput"><span class="string">"cat"</span></code>)
|
||||
there exists no value of type <code class="computeroutput"><span class="keyword">int</span></code>
|
||||
capable of representing the conversion result. We do not consider such situation
|
||||
an error. We expect that the converter can be used only to check if the conversion
|
||||
is possible. A natural signature for this function can be:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">optional</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">convert</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&</span> <span class="identifier">text</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
All necessary functionality can be included with one header <code class="computeroutput"><span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">optional</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code>.
|
||||
The above function signature means that the function can either return a
|
||||
value of type <code class="computeroutput"><span class="keyword">int</span></code> or a flag
|
||||
indicating that no value of <code class="computeroutput"><span class="keyword">int</span></code>
|
||||
is available. This does not indicate an error. It is like one additional
|
||||
value of <code class="computeroutput"><span class="keyword">int</span></code>. This is how we
|
||||
can use our function:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&</span> <span class="identifier">text</span> <span class="special">=</span> <span class="comment">/*... */</span><span class="special">;</span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">oi</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">text</span><span class="special">);</span> <span class="comment">// move-construct</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oi</span><span class="special">)</span> <span class="comment">// contextual conversion to bool</span>
|
||||
<span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">*</span><span class="identifier">oi</span><span class="special">;</span> <span class="comment">// operator*</span>
|
||||
</pre>
|
||||
<p>
|
||||
In order to test if <code class="computeroutput"><span class="identifier">optional</span></code>
|
||||
contains a value, we use the contextual conversion to type <code class="computeroutput"><span class="keyword">bool</span></code>. Because of this we can combine the initialization
|
||||
of the optional object and the test into one instruction:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">if</span> <span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">oi</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">text</span><span class="special">))</span>
|
||||
<span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">*</span><span class="identifier">oi</span><span class="special">;</span>
|
||||
</pre>
|
||||
<p>
|
||||
We extract the contained value with <code class="computeroutput"><span class="keyword">operator</span><span class="special">*</span></code> (and with <code class="computeroutput"><span class="keyword">operator</span><span class="special">-></span></code> where it makes sense). An attempt to
|
||||
extract the contained value of an uninitialized optional object is an <span class="emphasis"><em>undefined
|
||||
behaviour</em></span> (UB). This implementation guards the call with <code class="computeroutput"><span class="identifier">BOOST_ASSERT</span></code>. Therefore you should be sure
|
||||
that the contained value is there before extracting. For instance, the following
|
||||
code is reasonably UB-safe:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">*</span><span class="identifier">convert</span><span class="special">(</span><span class="string">"100"</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
This is because we know that string value <code class="computeroutput"><span class="string">"100"</span></code>
|
||||
converts to a valid value of <code class="computeroutput"><span class="keyword">int</span></code>.
|
||||
If you do not like this potential UB, you can use an alternative way of extracting
|
||||
the contained value:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">try</span> <span class="special">{</span>
|
||||
<span class="keyword">int</span> <span class="identifier">j</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">text</span><span class="special">).</span><span class="identifier">value</span><span class="special">();</span>
|
||||
<span class="special">}</span>
|
||||
<span class="keyword">catch</span> <span class="special">(</span><span class="keyword">const</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">bad_optional_access</span><span class="special">&)</span> <span class="special">{</span>
|
||||
<span class="comment">// deal with it</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
This version throws an exception upon an attempt to access a nonexistent
|
||||
contained value. If your way of dealing with the missing value is to use
|
||||
some default, like <code class="computeroutput"><span class="number">0</span></code>, there exists
|
||||
a yet another alternative:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">k</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">text</span><span class="special">).</span><span class="identifier">value_or</span><span class="special">(</span><span class="number">0</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
This uses the <code class="computeroutput"><span class="identifier">atoi</span></code>-like approach
|
||||
to conversions: if <code class="computeroutput"><span class="identifier">text</span></code> does
|
||||
not represent an integral number just return <code class="computeroutput"><span class="number">0</span></code>.
|
||||
Finally, you can provide a callback to be called when trying to access the
|
||||
contained value fails:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">fallback_to_default</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">cerr</span> <span class="special"><<</span> <span class="string">"could not convert; using -1 instead"</span> <span class="special"><<</span> <span class="identifier">endl</span><span class="special">;</span>
|
||||
<span class="keyword">return</span> <span class="special">-</span><span class="number">1</span><span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
|
||||
<span class="keyword">int</span> <span class="identifier">l</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">text</span><span class="special">).</span><span class="identifier">value_or_eval</span><span class="special">(</span><span class="identifier">fallback_to_default</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
This will call the provided callback and return whatever the callback returns.
|
||||
The callback can have side effects: they will only be observed when the optional
|
||||
object does not contain a value.
|
||||
</p>
|
||||
<p>
|
||||
Now, let's consider how function <code class="computeroutput"><span class="identifier">convert</span></code>
|
||||
can be implemented.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">convert</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&</span> <span class="identifier">text</span><span class="special">)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">stringstream</span> <span class="identifier">s</span><span class="special">(</span><span class="identifier">text</span><span class="special">);</span>
|
||||
<span class="keyword">int</span> <span class="identifier">i</span><span class="special">;</span>
|
||||
<span class="keyword">if</span> <span class="special">((</span><span class="identifier">s</span> <span class="special">>></span> <span class="identifier">i</span><span class="special">)</span> <span class="special">&&</span> <span class="identifier">s</span><span class="special">.</span><span class="identifier">get</span><span class="special">()</span> <span class="special">==</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">char_traits</span><span class="special"><</span><span class="keyword">char</span><span class="special">>::</span><span class="identifier">eof</span><span class="special">())</span>
|
||||
<span class="keyword">return</span> <span class="identifier">i</span><span class="special">;</span>
|
||||
<span class="keyword">else</span>
|
||||
<span class="keyword">return</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span><span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
Observe the two return statements. <code class="computeroutput"><span class="keyword">return</span>
|
||||
<span class="identifier">i</span></code> uses the converting constructor
|
||||
that can create <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
from <code class="computeroutput"><span class="identifier">T</span></code>. Thus constructed
|
||||
optional object is initialized and its value is a copy of <code class="computeroutput"><span class="identifier">i</span></code>.
|
||||
The other return statement uses another converting constructor from a special
|
||||
tag <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code>. It is used to indicate that we want
|
||||
to create an uninitialized optional object.
|
||||
</p>
|
||||
</div>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
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<a name="boost_optional.quick_start.bypassing_unnecessary_default_construction"></a><a class="link" href="bypassing_unnecessary_default_construction.html" title="Bypassing unnecessary default construction">Bypassing
|
||||
unnecessary default construction</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
Suppose we have class <code class="computeroutput"><span class="identifier">Date</span></code>,
|
||||
which does not have a default constructor: there is no good candidate for
|
||||
a default date. We have a function that returns two dates in form of a <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">tuple</span></code>:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">tuple</span><span class="special"><</span><span class="identifier">Date</span><span class="special">,</span> <span class="identifier">Date</span><span class="special">></span> <span class="identifier">getPeriod</span><span class="special">();</span>
|
||||
</pre>
|
||||
<p>
|
||||
In other place we want to use the result of <code class="computeroutput"><span class="identifier">getPeriod</span></code>,
|
||||
but want the two dates to be named: <code class="computeroutput"><span class="identifier">begin</span></code>
|
||||
and <code class="computeroutput"><span class="identifier">end</span></code>. We want to implement
|
||||
something like 'multiple return values':
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">Date</span> <span class="identifier">begin</span><span class="special">,</span> <span class="identifier">end</span><span class="special">;</span> <span class="comment">// Error: no default ctor!</span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">tie</span><span class="special">(</span><span class="identifier">begin</span><span class="special">,</span> <span class="identifier">end</span><span class="special">)</span> <span class="special">=</span> <span class="identifier">getPeriod</span><span class="special">();</span>
|
||||
</pre>
|
||||
<p>
|
||||
The second line works already, this is the capability of <a href="../../../../../tuple/index.html" target="_top">Boost.Tuple</a>
|
||||
library, but the first line won't work. We could set some invented initial
|
||||
dates, but it is confusing and may be an unacceptable cost, given that these
|
||||
values will be overwritten in the next line anyway. This is where <code class="computeroutput"><span class="identifier">optional</span></code> can help:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="identifier">Date</span><span class="special">></span> <span class="identifier">begin</span><span class="special">,</span> <span class="identifier">end</span><span class="special">;</span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">tie</span><span class="special">(</span><span class="identifier">begin</span><span class="special">,</span> <span class="identifier">end</span><span class="special">)</span> <span class="special">=</span> <span class="identifier">getPeriod</span><span class="special">();</span>
|
||||
</pre>
|
||||
<p>
|
||||
It works because inside <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">tie</span></code> a
|
||||
move-assignment from <code class="computeroutput"><span class="identifier">T</span></code> is
|
||||
invoked on <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>,
|
||||
which internally calls a move-constructor of <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
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<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
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<a name="boost_optional.quick_start.optional_automatic_variables"></a><a class="link" href="optional_automatic_variables.html" title="Optional automatic variables">Optional
|
||||
automatic variables</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
We could write function <code class="computeroutput"><span class="identifier">convert</span></code>
|
||||
in a slightly different manner, so that it has a single <code class="computeroutput"><span class="keyword">return</span></code>-statement:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">convert</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&</span> <span class="identifier">text</span><span class="special">)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">ans</span><span class="special">;</span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">stringstream</span> <span class="identifier">s</span><span class="special">(</span><span class="identifier">text</span><span class="special">);</span>
|
||||
<span class="keyword">int</span> <span class="identifier">i</span><span class="special">;</span>
|
||||
<span class="keyword">if</span> <span class="special">((</span><span class="identifier">s</span> <span class="special">>></span> <span class="identifier">i</span><span class="special">)</span> <span class="special">&&</span> <span class="identifier">s</span><span class="special">.</span><span class="identifier">get</span><span class="special">()</span> <span class="special">==</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">char_traits</span><span class="special"><</span><span class="keyword">char</span><span class="special">>::</span><span class="identifier">eof</span><span class="special">())</span>
|
||||
<span class="identifier">ans</span> <span class="special">=</span> <span class="identifier">i</span><span class="special">;</span>
|
||||
|
||||
<span class="keyword">return</span> <span class="identifier">ans</span><span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
The default constructor of <code class="computeroutput"><span class="identifier">optional</span></code>
|
||||
creates an uninitialized optional object. Unlike with <code class="computeroutput"><span class="keyword">int</span></code>s
|
||||
you cannot have an <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code>
|
||||
in an indeterminate state. Its state is always well defined. Instruction
|
||||
<code class="computeroutput"><span class="identifier">ans</span> <span class="special">=</span>
|
||||
<span class="identifier">i</span></code> initializes the optional object.
|
||||
It uses the 'mixed' assignment from <code class="computeroutput"><span class="keyword">int</span></code>.
|
||||
In general, for <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>,
|
||||
when an assignment from <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
is invoked, it can do two things. If the optional object is not initialized
|
||||
(our case here), it initializes the contained value using <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
copy constructor. If the optional object is already initialized, it assigns
|
||||
the new value to it using <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
copy assignment.
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
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</div></td>
|
||||
</tr></table>
|
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<div class="spirit-nav">
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<a accesskey="p" href="../quick_start.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../quick_start.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="optional_data_members.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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<div class="titlepage"><div><div><h3 class="title">
|
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<a name="boost_optional.quick_start.optional_data_members"></a><a class="link" href="optional_data_members.html" title="Optional data members">Optional
|
||||
data members</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
Suppose we want to implement a <span class="emphasis"><em>lazy load</em></span> optimization.
|
||||
This is because we do not want to perform an expensive initialization of
|
||||
our <code class="computeroutput"><span class="identifier">Resource</span></code> until (if at
|
||||
all) it is really used. We can do it this way:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">Widget</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">mutable</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">const</span> <span class="identifier">Resource</span><span class="special">></span> <span class="identifier">resource_</span><span class="special">;</span>
|
||||
|
||||
<span class="keyword">public</span><span class="special">:</span>
|
||||
<span class="identifier">Widget</span><span class="special">()</span> <span class="special">{}</span>
|
||||
|
||||
<span class="keyword">const</span> <span class="identifier">Resource</span><span class="special">&</span> <span class="identifier">getResource</span><span class="special">()</span> <span class="keyword">const</span> <span class="comment">// not thread-safe</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">resource_</span> <span class="special">==</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span><span class="special">)</span>
|
||||
<span class="identifier">resource_</span><span class="special">.</span><span class="identifier">emplace</span><span class="special">(</span><span class="string">"resource"</span><span class="special">,</span> <span class="string">"arguments"</span><span class="special">);</span>
|
||||
|
||||
<span class="keyword">return</span> <span class="special">*</span><span class="identifier">resource_</span><span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
<span class="special">};</span>
|
||||
</pre>
|
||||
<p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span></code>'s default constructor
|
||||
creates an uninitialized optional. No call to <code class="computeroutput"><span class="identifier">Resource</span></code>'s
|
||||
default constructor is attempted. <code class="computeroutput"><span class="identifier">Resource</span></code>
|
||||
doesn't have to be <a href="http://www.sgi.com/tech/stl/DefaultConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">DefaultConstructible</span></code></a>. In function
|
||||
<code class="computeroutput"><span class="identifier">getResource</span></code> we first check
|
||||
if <code class="computeroutput"><span class="identifier">resource_</span></code> is initialized.
|
||||
This time we do not use the contextual conversion to <code class="computeroutput"><span class="keyword">bool</span></code>,
|
||||
but a comparison with <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code>.
|
||||
These two ways are equivalent. Function <code class="computeroutput"><span class="identifier">emplace</span></code>
|
||||
initializes the optional in-place by perfect-forwarding the arguments to
|
||||
the constructor of <code class="computeroutput"><span class="identifier">Resource</span></code>.
|
||||
No copy- or move-construction is involved here. <code class="computeroutput"><span class="identifier">Resource</span></code>
|
||||
doesn't even have to be <code class="computeroutput"><span class="identifier">MoveConstructible</span></code>.
|
||||
</p>
|
||||
<div class="note"><table border="0" summary="Note">
|
||||
<tr>
|
||||
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../doc/src/images/note.png"></td>
|
||||
<th align="left">Note</th>
|
||||
</tr>
|
||||
<tr><td align="left" valign="top"><p>
|
||||
Function <code class="computeroutput"><span class="identifier">emplace</span></code> is only
|
||||
available on compilers that support rvalue references and variadic templates.
|
||||
If your compiler does not support these features and you still need to
|
||||
avoid any move-constructions, use <a class="link" href="../tutorial/in_place_factories.html" title="In-Place Factories">In-Place
|
||||
Factories</a>.
|
||||
</p></td></tr>
|
||||
</table></div>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="boost_optional.quick_start.optional_return_values"></a><a class="link" href="optional_return_values.html" title="Optional return values">Optional
|
||||
return values</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
Let's write and use a converter function that converts an a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>
|
||||
to an <code class="computeroutput"><span class="keyword">int</span></code>. It is possible that
|
||||
for a given string (e.g. <code class="computeroutput"><span class="string">"cat"</span></code>)
|
||||
there exist no value of type <code class="computeroutput"><span class="keyword">int</span></code>
|
||||
capable of representing the conversion result. We do not consider such situation
|
||||
an error. We expect that the converter can be used only to check if the conversion
|
||||
is possible. A natural signature for this function can be:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">optional</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">convert</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&</span> <span class="identifier">text</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
All necessary functionality can be included with one header <code class="computeroutput"><span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">optional</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code>.
|
||||
The above function signature means that the function can either return a
|
||||
value of type <code class="computeroutput"><span class="keyword">int</span></code> or a flag
|
||||
indicating that no value of <code class="computeroutput"><span class="keyword">int</span></code>
|
||||
is available. This does not indicate an error. It is like one additional
|
||||
value of <code class="computeroutput"><span class="keyword">int</span></code>. This is how we
|
||||
can use our function:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&</span> <span class="identifier">text</span> <span class="special">=</span> <span class="comment">/*... */</span><span class="special">;</span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">oi</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">text</span><span class="special">);</span> <span class="comment">// move-construct</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oi</span><span class="special">)</span> <span class="comment">// contextual conversion to bool</span>
|
||||
<span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">*</span><span class="identifier">oi</span><span class="special">;</span> <span class="comment">// operator*</span>
|
||||
</pre>
|
||||
<p>
|
||||
In order to test if <code class="computeroutput"><span class="identifier">optional</span></code>
|
||||
contains a value, we use the contextual conversion to type <code class="computeroutput"><span class="keyword">bool</span></code>. Because of this we can combine the initialization
|
||||
of the optional object and the test into one instruction:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">if</span> <span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">oi</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">text</span><span class="special">))</span>
|
||||
<span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">*</span><span class="identifier">oi</span><span class="special">;</span>
|
||||
</pre>
|
||||
<p>
|
||||
We extract the contained value with <code class="computeroutput"><span class="keyword">operator</span><span class="special">*</span></code> (and with <code class="computeroutput"><span class="keyword">operator</span><span class="special">-></span></code> where it makes sense). An attempt to
|
||||
extract the contained value of an uninitialized optional object is an <span class="emphasis"><em>undefined
|
||||
behaviour</em></span> (UB). This implementation guards the call with <code class="computeroutput"><span class="identifier">BOOST_ASSERT</span></code>. Therefore you should be sure
|
||||
that the contained value is there before extracting. For instance, the following
|
||||
code is reasonably UB-safe:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">*</span><span class="identifier">convert</span><span class="special">(</span><span class="string">"100"</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
This is because we know that string value <code class="computeroutput"><span class="string">"100"</span></code>
|
||||
converts to a valid value of <code class="computeroutput"><span class="keyword">int</span></code>.
|
||||
If you do not like this potential UB, you can use an alternative way of extracting
|
||||
the contained value:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">try</span> <span class="special">{</span>
|
||||
<span class="keyword">int</span> <span class="identifier">j</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">text</span><span class="special">).</span><span class="identifier">value</span><span class="special">();</span>
|
||||
<span class="special">}</span>
|
||||
<span class="keyword">catch</span> <span class="special">(</span><span class="keyword">const</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">bad_optional_access</span><span class="special">&)</span> <span class="special">{</span>
|
||||
<span class="comment">// deal with it</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
This version throws an exception upon an attempt to access a nonexistent
|
||||
contained value. If your way of dealing with the missing value is to use
|
||||
some default, like <code class="computeroutput"><span class="number">0</span></code>, there exists
|
||||
a yet another alternative:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">k</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">text</span><span class="special">).</span><span class="identifier">value_or</span><span class="special">(</span><span class="number">0</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
This uses the <code class="computeroutput"><span class="identifier">atoi</span></code>-like approach
|
||||
to conversions: if <code class="computeroutput"><span class="identifier">text</span></code> does
|
||||
not represent an integral number just return <code class="computeroutput"><span class="number">0</span></code>.
|
||||
Now, let's consider how function <code class="computeroutput"><span class="identifier">convert</span></code>
|
||||
can be implemented.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">convert</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&</span> <span class="identifier">text</span><span class="special">)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">stringstream</span> <span class="identifier">s</span><span class="special">(</span><span class="identifier">text</span><span class="special">);</span>
|
||||
<span class="keyword">int</span> <span class="identifier">i</span><span class="special">;</span>
|
||||
<span class="keyword">if</span> <span class="special">((</span><span class="identifier">s</span> <span class="special">>></span> <span class="identifier">i</span><span class="special">)</span> <span class="special">&&</span> <span class="identifier">s</span><span class="special">.</span><span class="identifier">get</span><span class="special">()</span> <span class="special">==</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">char_traits</span><span class="special"><</span><span class="keyword">char</span><span class="special">>::</span><span class="identifier">eof</span><span class="special">())</span>
|
||||
<span class="keyword">return</span> <span class="identifier">i</span><span class="special">;</span>
|
||||
<span class="keyword">else</span>
|
||||
<span class="keyword">return</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span><span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
Observe the two return statements. <code class="computeroutput"><span class="keyword">return</span>
|
||||
<span class="identifier">i</span></code> uses the converting constructor
|
||||
that can create <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
from <code class="computeroutput"><span class="identifier">T</span></code>. Thus constructed
|
||||
optional object is initialized and its value is a copy of <code class="computeroutput"><span class="identifier">i</span></code>.
|
||||
The other return statement uses another converting constructor from a special
|
||||
tag <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code>. It is used to indicate that we want
|
||||
to create an uninitialized optional object.
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
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<div class="spirit-nav">
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
|
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<a name="boost_optional.quick_start.storage_in_containers"></a><a class="link" href="storage_in_containers.html" title="Storage in containers">Storage
|
||||
in containers</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
Suppose you want to ask users to choose some number (an <code class="computeroutput"><span class="keyword">int</span></code>).
|
||||
One of the valid responses is to choose nothing, which is represented by
|
||||
an uninitialized <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code>.
|
||||
You want to make a histogram showing how many times each choice was made.
|
||||
You can use an <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span></code>:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special"><</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">>,</span> <span class="keyword">int</span><span class="special">></span> <span class="identifier">choices</span><span class="special">;</span>
|
||||
|
||||
<span class="keyword">for</span> <span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special"><</span> <span class="identifier">LIMIT</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span> <span class="special">{</span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">choice</span> <span class="special">=</span> <span class="identifier">readChoice</span><span class="special">();</span>
|
||||
<span class="special">++</span><span class="identifier">choices</span><span class="special">[</span><span class="identifier">choice</span><span class="special">];</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
This works because <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
is <a href="http://www.sgi.com/tech/stl/LessThanComparable.html" target="_top"><code class="computeroutput"><span class="identifier">LessThanComparable</span></code></a> whenever <code class="computeroutput"><span class="identifier">T</span></code> is <a href="http://www.sgi.com/tech/stl/LessThanComparable.html" target="_top"><code class="computeroutput"><span class="identifier">LessThanComparable</span></code></a>. In this case
|
||||
the state of being uninitialized is treated as a yet another value of <code class="computeroutput"><span class="identifier">T</span></code>, which is compared less than any value
|
||||
of <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
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</div></td>
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</tr></table>
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<html>
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<head>
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<a accesskey="p" href="optional_references.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="in_place_factories.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
|
||||
<a name="boost_optional.rebinding_semantics_for_assignment_of_optional_references"></a><a class="link" href="rebinding_semantics_for_assignment_of_optional_references.html" title="Rebinding semantics for assignment of optional references">Rebinding
|
||||
semantics for assignment of optional references</a>
|
||||
</h2></div></div></div>
|
||||
<p>
|
||||
If you assign to an <span class="emphasis"><em>uninitialized </em></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>
|
||||
the effect is to bind (for the first time) to the object. Clearly, there is
|
||||
no other choice.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">rx</span> <span class="special">=</span> <span class="identifier">x</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">ora</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">orb</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">ora</span> <span class="special">=</span> <span class="identifier">orb</span> <span class="special">;</span> <span class="comment">// now 'ora' is bound to 'x' through 'rx'</span>
|
||||
<span class="special">*</span><span class="identifier">ora</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span> <span class="comment">// Changes value of 'x' through 'ora'</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">x</span><span class="special">==</span><span class="number">2</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
If you assign to a bare C++ reference, the assignment is forwarded to the referenced
|
||||
object; its value changes but the reference is never rebound.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">a</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">ra</span> <span class="special">=</span> <span class="identifier">a</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span> <span class="identifier">b</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">rb</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">;</span>
|
||||
<span class="identifier">ra</span> <span class="special">=</span> <span class="identifier">rb</span> <span class="special">;</span> <span class="comment">// Changes the value of 'a' to 'b'</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">a</span><span class="special">==</span><span class="identifier">b</span><span class="special">);</span>
|
||||
<span class="identifier">b</span> <span class="special">=</span> <span class="number">3</span> <span class="special">;</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">ra</span><span class="special">!=</span><span class="identifier">b</span><span class="special">);</span> <span class="comment">// 'ra' is not rebound to 'b'</span>
|
||||
</pre>
|
||||
<p>
|
||||
Now, if you assign to an <span class="emphasis"><em>initialized </em></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>,
|
||||
the effect is to <span class="bold"><strong>rebind</strong></span> to the new object
|
||||
instead of assigning the referee. This is unlike bare C++ references.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">a</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span> <span class="identifier">b</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">ra</span> <span class="special">=</span> <span class="identifier">a</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">rb</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">ora</span><span class="special">(</span><span class="identifier">ra</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">orb</span><span class="special">(</span><span class="identifier">rb</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">ora</span> <span class="special">=</span> <span class="identifier">orb</span> <span class="special">;</span> <span class="comment">// 'ora' is rebound to 'b'</span>
|
||||
<span class="special">*</span><span class="identifier">ora</span> <span class="special">=</span> <span class="number">3</span> <span class="special">;</span> <span class="comment">// Changes value of 'b' (not 'a')</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">a</span><span class="special">==</span><span class="number">1</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">b</span><span class="special">==</span><span class="number">3</span><span class="special">);</span>
|
||||
</pre>
|
||||
<h4>
|
||||
<a name="boost_optional.rebinding_semantics_for_assignment_of_optional_references.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.rebinding_semantics_for_assignment_of_optional_references.rationale"></a></span><a class="link" href="rebinding_semantics_for_assignment_of_optional_references.html#boost_optional.rebinding_semantics_for_assignment_of_optional_references.rationale">Rationale</a>
|
||||
</h4>
|
||||
<p>
|
||||
Rebinding semantics for the assignment of <span class="emphasis"><em>initialized </em></span>
|
||||
<code class="computeroutput"><span class="identifier">optional</span></code> references has been
|
||||
chosen to provide <span class="bold"><strong>consistency among initialization states</strong></span>
|
||||
even at the expense of lack of consistency with the semantics of bare C++ references.
|
||||
It is true that <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span></code> strives
|
||||
to behave as much as possible as <code class="computeroutput"><span class="identifier">U</span></code>
|
||||
does whenever it is initialized; but in the case when <code class="computeroutput"><span class="identifier">U</span></code>
|
||||
is <code class="computeroutput"><span class="identifier">T</span><span class="special">&</span></code>,
|
||||
doing so would result in inconsistent behavior w.r.t to the lvalue initialization
|
||||
state.
|
||||
</p>
|
||||
<p>
|
||||
Imagine <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>
|
||||
forwarding assignment to the referenced object (thus changing the referenced
|
||||
object value but not rebinding), and consider the following code:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">a</span> <span class="special">=</span> <span class="identifier">get</span><span class="special">();</span>
|
||||
<span class="keyword">int</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">rx</span> <span class="special">=</span> <span class="identifier">x</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">b</span><span class="special">(</span><span class="identifier">rx</span><span class="special">);</span>
|
||||
<span class="identifier">a</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">;</span>
|
||||
</pre>
|
||||
<p>
|
||||
What does the assignment do?
|
||||
</p>
|
||||
<p>
|
||||
If <code class="computeroutput"><span class="identifier">a</span></code> is <span class="emphasis"><em>uninitialized</em></span>,
|
||||
the answer is clear: it binds to <code class="computeroutput"><span class="identifier">x</span></code>
|
||||
(we now have another reference to <code class="computeroutput"><span class="identifier">x</span></code>).
|
||||
But what if <code class="computeroutput"><span class="identifier">a</span></code> is already <span class="emphasis"><em>initialized</em></span>?
|
||||
it would change the value of the referenced object (whatever that is); which
|
||||
is inconsistent with the other possible case.
|
||||
</p>
|
||||
<p>
|
||||
If <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>
|
||||
would assign just like <code class="computeroutput"><span class="identifier">T</span><span class="special">&</span></code>
|
||||
does, you would never be able to use Optional's assignment without explicitly
|
||||
handling the previous initialization state unless your code is capable of functioning
|
||||
whether after the assignment, <code class="computeroutput"><span class="identifier">a</span></code>
|
||||
aliases the same object as <code class="computeroutput"><span class="identifier">b</span></code>
|
||||
or not.
|
||||
</p>
|
||||
<p>
|
||||
That is, you would have to discriminate in order to be consistent.
|
||||
</p>
|
||||
<p>
|
||||
If in your code rebinding to another object is not an option, then it is very
|
||||
likely that binding for the first time isn't either. In such case, assignment
|
||||
to an <span class="emphasis"><em>uninitialized </em></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>
|
||||
shall be prohibited. It is quite possible that in such a scenario it is a precondition
|
||||
that the lvalue must be already initialized. If it isn't, then binding for
|
||||
the first time is OK while rebinding is not which is IMO very unlikely. In
|
||||
such a scenario, you can assign the value itself directly, as in:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">assert</span><span class="special">(!!</span><span class="identifier">opt</span><span class="special">);</span>
|
||||
<span class="special">*</span><span class="identifier">opt</span><span class="special">=</span><span class="identifier">value</span><span class="special">;</span>
|
||||
</pre>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="optional_references.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="in_place_factories.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
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<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.reference.acknowledgements"></a><a class="link" href="acknowledgements.html" title="Acknowledgements">Acknowledgements</a>
|
||||
</h3></div></div></div>
|
||||
<h5>
|
||||
<a name="boost_optional.reference.acknowledgements.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.reference.acknowledgements.pre_formal_review"></a></span><a class="link" href="acknowledgements.html#boost_optional.reference.acknowledgements.pre_formal_review">Pre-formal
|
||||
review</a>
|
||||
</h5>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
Peter Dimov suggested the name 'optional', and was the first to point
|
||||
out the need for aligned storage.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Douglas Gregor developed 'type_with_alignment', and later Eric Friedman
|
||||
coded 'aligned_storage', which are the core of the optional class implementation.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Andrei Alexandrescu and Brian Parker also worked with aligned storage
|
||||
techniques and their work influenced the current implementation.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Gennadiy Rozental made extensive and important comments which shaped
|
||||
the design.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Vesa Karvonen and Douglas Gregor made quite useful comparisons between
|
||||
optional, variant and any; and made other relevant comments.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Douglas Gregor and Peter Dimov commented on comparisons and evaluation
|
||||
in boolean contexts.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Eric Friedman helped understand the issues involved with aligned storage,
|
||||
move/copy operations and exception safety.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Many others have participated with useful comments: Aleksey Gurotov,
|
||||
Kevlin Henney, David Abrahams, and others I can't recall.
|
||||
</li>
|
||||
</ul></div>
|
||||
<h5>
|
||||
<a name="boost_optional.reference.acknowledgements.h1"></a>
|
||||
<span class="phrase"><a name="boost_optional.reference.acknowledgements.post_formal_review"></a></span><a class="link" href="acknowledgements.html#boost_optional.reference.acknowledgements.post_formal_review">Post-formal
|
||||
review</a>
|
||||
</h5>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
William Kempf carefully considered the originally proposed interface
|
||||
and suggested the new interface which is currently used. He also started
|
||||
and fueled the discussion about the analogy optional<>/smart pointer
|
||||
and about relational operators.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Peter Dimov, Joel de Guzman, David Abrahams, Tanton Gibbs and Ian Hanson
|
||||
focused on the relational semantics of optional (originally undefined);
|
||||
concluding with the fact that the pointer-like interface doesn't make
|
||||
it a pointer so it shall have deep relational operators.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Augustus Saunders also explored the different relational semantics between
|
||||
optional<> and a pointer and developed the OptionalPointee concept
|
||||
as an aid against potential conflicts on generic code.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Joel de Guzman noticed that optional<> can be seen as an API on
|
||||
top of variant<T,nil_t>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Dave Gomboc explained the meaning and usage of the Haskell analog to
|
||||
optional<>: the Maybe type constructor (analogy originally pointed
|
||||
out by David Sankel).
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Other comments were posted by Vincent Finn, Anthony Williams, Ed Brey,
|
||||
Rob Stewart, and others.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Joel de Guzman made the case for the support of references and helped
|
||||
with the proper semantics.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Mat Marcus shown the virtues of a value-oriented interface, influencing
|
||||
the current design, and contributed the idea of "none".
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Vladimir Batov's design of Boost.Convert library motivated the development
|
||||
of value accessors for <code class="computeroutput"><span class="identifier">optional</span></code>:
|
||||
functions <code class="computeroutput"><span class="identifier">value</span></code>, <code class="computeroutput"><span class="identifier">value_or</span></code>, <code class="computeroutput"><span class="identifier">value_or_eval</span></code>.
|
||||
</li>
|
||||
</ul></div>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2016 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
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Reference Binding</a>
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</h4></div></div></div>
|
||||
<p>
|
||||
On compilers that do not conform to Standard C++ rules of reference binding,
|
||||
operations on optional references might give adverse results: rather than
|
||||
binding a reference to a designated object they may create an unexpected
|
||||
temporary and bind to it. Compilers known to have these deficiencies include
|
||||
GCC versions 4.2, 4.3, 4.4, 4.5; QCC 4.4.2; MSVC versions 8.0, 9.0, 10.0,
|
||||
11.0, 12.0. On these compilers prefer using direct-initialization and copy
|
||||
assignment of optional references to copy-initialization and assignment
|
||||
from <code class="computeroutput"><span class="identifier">T</span><span class="special">&</span></code>:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">const</span> <span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&></span> <span class="identifier">or1</span><span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&></span> <span class="identifier">or2</span> <span class="special">=</span> <span class="identifier">i</span><span class="special">;</span> <span class="comment">// not portable</span>
|
||||
<span class="identifier">or1</span> <span class="special">=</span> <span class="identifier">i</span><span class="special">;</span> <span class="comment">// not portable</span>
|
||||
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&></span> <span class="identifier">or3</span><span class="special">(</span><span class="identifier">i</span><span class="special">);</span> <span class="comment">// portable</span>
|
||||
<span class="identifier">or1</span> <span class="special">=</span> <span class="identifier">optional</span><span class="special"><</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&>(</span><span class="identifier">i</span><span class="special">);</span> <span class="comment">// portable</span>
|
||||
</pre>
|
||||
<p>
|
||||
In order to check if your compiler correctly implements reference binding
|
||||
use this test program.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">cassert</span><span class="special">></span>
|
||||
|
||||
<span class="keyword">const</span> <span class="keyword">int</span> <span class="identifier">global_i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
|
||||
|
||||
<span class="keyword">struct</span> <span class="identifier">TestingReferenceBinding</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">TestingReferenceBinding</span><span class="special">(</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">ii</span><span class="special">)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">assert</span><span class="special">(&</span><span class="identifier">ii</span> <span class="special">==</span> <span class="special">&</span><span class="identifier">global_i</span><span class="special">);</span>
|
||||
<span class="special">}</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="keyword">operator</span><span class="special">=(</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">ii</span><span class="special">)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">assert</span><span class="special">(&</span><span class="identifier">ii</span> <span class="special">==</span> <span class="special">&</span><span class="identifier">global_i</span><span class="special">);</span>
|
||||
<span class="special">}</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="keyword">operator</span><span class="special">=(</span><span class="keyword">int</span><span class="special">&&)</span> <span class="comment">// remove this if your compiler doesn't have rvalue refs</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="keyword">false</span><span class="special">);</span>
|
||||
<span class="special">}</span>
|
||||
<span class="special">};</span>
|
||||
|
||||
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">iref</span> <span class="special">=</span> <span class="identifier">global_i</span><span class="special">;</span>
|
||||
<span class="identifier">assert</span><span class="special">(&</span><span class="identifier">iref</span> <span class="special">==</span> <span class="special">&</span><span class="identifier">global_i</span><span class="special">);</span>
|
||||
|
||||
<span class="identifier">TestingReferenceBinding</span> <span class="identifier">ttt</span> <span class="special">=</span> <span class="identifier">global_i</span><span class="special">;</span>
|
||||
<span class="identifier">ttt</span> <span class="special">=</span> <span class="identifier">global_i</span><span class="special">;</span>
|
||||
|
||||
<span class="identifier">TestingReferenceBinding</span> <span class="identifier">ttt2</span> <span class="special">=</span> <span class="identifier">iref</span><span class="special">;</span>
|
||||
<span class="identifier">ttt2</span> <span class="special">=</span> <span class="identifier">iref</span><span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
</div>
|
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2016 Andrzej Krzemieński<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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|
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|
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</p>
|
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<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span>
|
||||
|
||||
<span class="keyword">class</span> <span class="identifier">bad_optional_access</span> <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">logic_error</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">public</span><span class="special">:</span>
|
||||
<span class="identifier">bad_optional_access</span><span class="special">();</span> <a class="link" href="header__boost_optional_bad_optional_access_hpp_/detailed_semantics.html#reference_bad_optional_access_constructor"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="special">};</span>
|
||||
|
||||
<span class="special">}</span> <span class="comment">// namespace boost</span>
|
||||
</pre>
|
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|
||||
</p>
|
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</div>
|
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<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
|
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Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
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<a name="reference_bad_optional_access_constructor"></a><code class="computeroutput"><span class="identifier">bad_optional_access</span><span class="special">();</span></code>
|
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</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effect:</strong></span> Constructs an object of class
|
||||
<code class="computeroutput"><span class="identifier">bad_optional_access</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="identifier">what</span><span class="special">()</span></code> returns an implementation-defined
|
||||
NTBS.
|
||||
</li>
|
||||
</ul></div>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
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This is an alias for header <a class="link" href="../../optional/reference/header__boost_optional_optional_hpp_.html#boost_optional.reference.header__boost_optional_optional_hpp_.header_optional_optional" title="Synopsis"><code class="computeroutput"><span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">optional</span><span class="special">/</span><span class="identifier">optional</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code></a>.
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|
||||
</h4></div></div></div>
|
||||
<p>
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span>
|
||||
|
||||
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">class</span> <span class="identifier">optional</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">void</span> <span class="identifier">swap</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="special">);</span>
|
||||
|
||||
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">optional_swap_should_use_default_constructor</span> <span class="special">;</span>
|
||||
|
||||
<span class="special">}</span> <span class="comment">// namespace boost</span>
|
||||
</pre>
|
||||
<p>
|
||||
</p>
|
||||
<p>
|
||||
This header only contains declarations.
|
||||
</p>
|
||||
</div>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
|
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Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
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<a name="boost_optional.reference.header__boost_optional_optional_hpp_.detailed_semantics____free_functions"></a><a class="link" href="detailed_semantics____free_functions.html" title="Detailed Semantics -- Free Functions">Detailed
|
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|
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<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
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</p>
|
||||
<a name="reference_make_optional_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">make_optional</span><span class="special">(</span>
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>(</span><span class="identifier">v</span><span class="special">)</span></code> for the <span class="emphasis"><em>deduced</em></span>
|
||||
type <code class="computeroutput"><span class="identifier">T</span></code> of <code class="computeroutput"><span class="identifier">v</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Example:</strong></span>
|
||||
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">void</span> <span class="identifier">foo</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">;</span>
|
||||
|
||||
<span class="identifier">foo</span> <span class="special">(</span> <span class="identifier">make_optional</span><span class="special">(</span><span class="number">1</span><span class="special">+</span><span class="number">1</span><span class="special">)</span> <span class="special">)</span> <span class="special">;</span> <span class="comment">// Creates an optional<int></span>
|
||||
</pre>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_make_optional_bool_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">></span> <span class="identifier">make_optional</span><span class="special">(</span> <span class="keyword">bool</span> <span class="identifier">condition</span><span class="special">,</span>
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>(</span><span class="identifier">condition</span><span class="special">,</span><span class="identifier">v</span><span class="special">)</span></code> for the <span class="emphasis"><em>deduced</em></span>
|
||||
type <code class="computeroutput"><span class="identifier">T</span></code> of <code class="computeroutput"><span class="identifier">v</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Example:</strong></span>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span> <span class="identifier">calculate_foo</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">double</span> <span class="identifier">val</span> <span class="special">=</span> <span class="identifier">compute_foo</span><span class="special">();</span>
|
||||
<span class="keyword">return</span> <span class="identifier">make_optional</span><span class="special">(</span><span class="identifier">is_not_nan_and_finite</span><span class="special">(</span><span class="identifier">val</span><span class="special">),</span><span class="identifier">val</span><span class="special">);</span>
|
||||
<span class="special">}</span>
|
||||
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span> <span class="identifier">v</span> <span class="special">=</span> <span class="identifier">calculate_foo</span><span class="special">();</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span> <span class="special">!</span><span class="identifier">v</span> <span class="special">)</span>
|
||||
<span class="identifier">error</span><span class="special">(</span><span class="string">"foo wasn't computed"</span><span class="special">);</span>
|
||||
</pre>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_operator_compare_equal_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special">==</span> <span class="special">(</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">);</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
shall meet requirements of <a href="http://www.sgi.com/tech/stl/EqualityComparable.html" target="_top"><code class="computeroutput"><span class="identifier">EqualityComparable</span></code></a>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Returns:</strong></span> If both <code class="computeroutput"><span class="identifier">x</span></code>
|
||||
and <code class="computeroutput"><span class="identifier">y</span></code> are initialized,
|
||||
<code class="computeroutput"><span class="special">(*</span><span class="identifier">x</span>
|
||||
<span class="special">==</span> <span class="special">*</span><span class="identifier">y</span><span class="special">)</span></code>.
|
||||
If only <code class="computeroutput"><span class="identifier">x</span></code> or <code class="computeroutput"><span class="identifier">y</span></code> is initialized, <code class="computeroutput"><span class="keyword">false</span></code>.
|
||||
If both are uninitialized, <code class="computeroutput"><span class="keyword">true</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Notes:</strong></span> This definition guarantees
|
||||
that <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
not containing a value is compared unequal to any <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> containing any value, and equal
|
||||
to any other <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> not containing a value. Pointers
|
||||
have shallow relational operators while <code class="computeroutput"><span class="identifier">optional</span></code>
|
||||
has deep relational operators. Do not use <code class="computeroutput"><span class="keyword">operator</span><span class="special">==</span></code> directly in generic code which expect
|
||||
to be given either an <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> or a pointer; use <a href="../../../../../../utility/OptionalPointee.html#equal" target="_top"><code class="computeroutput"><span class="identifier">equal_pointees</span><span class="special">()</span></code></a>
|
||||
instead
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Example:</strong></span>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">oN</span><span class="special">,</span> <span class="identifier">oN_</span><span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">o1</span><span class="special">(</span><span class="identifier">T</span><span class="special">(</span><span class="number">1</span><span class="special">)),</span> <span class="identifier">o1_</span><span class="special">(</span><span class="identifier">T</span><span class="special">(</span><span class="number">1</span><span class="special">));</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">o2</span><span class="special">(</span><span class="identifier">T</span><span class="special">(</span><span class="number">2</span><span class="special">));</span>
|
||||
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">oN</span> <span class="special">==</span> <span class="identifier">oN</span> <span class="special">);</span> <span class="comment">// Identity implies equality</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">o1</span> <span class="special">==</span> <span class="identifier">o1</span> <span class="special">);</span> <span class="comment">//</span>
|
||||
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">oN</span> <span class="special">==</span> <span class="identifier">oN_</span> <span class="special">);</span> <span class="comment">// Both uninitialized compare equal</span>
|
||||
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">oN</span> <span class="special">!=</span> <span class="identifier">o1</span> <span class="special">);</span> <span class="comment">// Initialized unequal to initialized.</span>
|
||||
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">o1</span> <span class="special">==</span> <span class="identifier">o1_</span> <span class="special">);</span> <span class="comment">// Both initialized compare as (*lhs == *rhs)</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">o1</span> <span class="special">!=</span> <span class="identifier">o2</span> <span class="special">);</span> <span class="comment">//</span>
|
||||
</pre>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_operator_compare_less_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special"><</span> <span class="special">(</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">);</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Requires:</strong></span> Expression <code class="computeroutput"><span class="special">*</span><span class="identifier">x</span> <span class="special"><</span> <span class="special">*</span><span class="identifier">y</span></code> shall be well-formed and its result
|
||||
shall be convertible to <code class="computeroutput"><span class="keyword">bool</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="special">(!</span><span class="identifier">y</span><span class="special">)</span> <span class="special">?</span> <span class="keyword">false</span> <span class="special">:</span> <span class="special">(!</span><span class="identifier">x</span><span class="special">)</span> <span class="special">?</span> <span class="keyword">true</span> <span class="special">:</span> <span class="special">*</span><span class="identifier">x</span> <span class="special"><</span>
|
||||
<span class="special">*</span><span class="identifier">y</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Notes:</strong></span> This definition guarantees
|
||||
that <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
not containing a value is ordered as less than any <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> containing any value, and equivalent
|
||||
to any other <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> not containing a value. Pointers
|
||||
have shallow relational operators while <code class="computeroutput"><span class="identifier">optional</span></code>
|
||||
has deep relational operators. Do not use <code class="computeroutput"><span class="keyword">operator</span><span class="special"><</span></code> directly in generic code which
|
||||
expect to be given either an <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> or a pointer; use <a href="../../../../../../utility/OptionalPointee.html#less" target="_top"><code class="computeroutput"><span class="identifier">less_pointees</span><span class="special">()</span></code></a>
|
||||
instead. <code class="computeroutput"><span class="identifier">T</span></code> need not
|
||||
be <a href="http://www.sgi.com/tech/stl/LessThanComparable.html" target="_top"><code class="computeroutput"><span class="identifier">LessThanComparable</span></code></a>. Only
|
||||
single <code class="computeroutput"><span class="keyword">operator</span><span class="special"><</span></code>
|
||||
is required. Other relational operations are defined in terms of this
|
||||
one. If <code class="computeroutput"><span class="identifier">T</span></code>'s <code class="computeroutput"><span class="keyword">operator</span><span class="special"><</span></code>
|
||||
satisfies the axioms of <a href="http://www.sgi.com/tech/stl/LessThanComparable.html" target="_top"><code class="computeroutput"><span class="identifier">LessThanComparable</span></code></a> (transitivity,
|
||||
antisymmetry and irreflexivity), <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is <a href="http://www.sgi.com/tech/stl/LessThanComparable.html" target="_top"><code class="computeroutput"><span class="identifier">LessThanComparable</span></code></a>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Example:</strong></span>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">oN</span><span class="special">,</span> <span class="identifier">oN_</span><span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">o0</span><span class="special">(</span><span class="identifier">T</span><span class="special">(</span><span class="number">0</span><span class="special">));</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">o1</span><span class="special">(</span><span class="identifier">T</span><span class="special">(</span><span class="number">1</span><span class="special">));</span>
|
||||
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">!(</span><span class="identifier">oN</span> <span class="special"><</span> <span class="identifier">oN</span><span class="special">)</span> <span class="special">);</span> <span class="comment">// Identity implies equivalence</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">!(</span><span class="identifier">o1</span> <span class="special"><</span> <span class="identifier">o1</span><span class="special">)</span> <span class="special">);</span>
|
||||
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">!(</span><span class="identifier">oN</span> <span class="special"><</span> <span class="identifier">oN_</span><span class="special">)</span> <span class="special">);</span> <span class="comment">// Two uninitialized are equivalent</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">!(</span><span class="identifier">oN_</span> <span class="special"><</span> <span class="identifier">oN</span><span class="special">)</span> <span class="special">);</span>
|
||||
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">oN</span> <span class="special"><</span> <span class="identifier">o0</span> <span class="special">);</span> <span class="comment">// Uninitialized is less than initialized</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">!(</span><span class="identifier">o0</span> <span class="special"><</span> <span class="identifier">oN</span><span class="special">)</span> <span class="special">);</span>
|
||||
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">o1</span> <span class="special"><</span> <span class="identifier">o2</span> <span class="special">)</span> <span class="special">;</span> <span class="comment">// Two initialized compare as (*lhs < *rhs)</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">!(</span><span class="identifier">o2</span> <span class="special"><</span> <span class="identifier">o1</span><span class="special">)</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">!(</span><span class="identifier">o2</span> <span class="special"><</span> <span class="identifier">o2</span><span class="special">)</span> <span class="special">)</span> <span class="special">;</span>
|
||||
</pre>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_operator_compare_not_equal_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special">!=</span> <span class="special">(</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">);</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="special">!(</span>
|
||||
<span class="identifier">x</span> <span class="special">==</span>
|
||||
<span class="identifier">y</span> <span class="special">);</span></code>
|
||||
</li></ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_operator_compare_greater_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special">></span> <span class="special">(</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">);</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="special">(</span>
|
||||
<span class="identifier">y</span> <span class="special"><</span>
|
||||
<span class="identifier">x</span> <span class="special">);</span></code>
|
||||
</li></ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_operator_compare_less_or_equal_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special"><=</span> <span class="special">(</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">);</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="special">!(</span>
|
||||
<span class="identifier">y</span> <span class="special"><</span>
|
||||
<span class="identifier">x</span> <span class="special">);</span></code>
|
||||
</li></ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_operator_compare_greater_or_equal_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special">>=</span> <span class="special">(</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">);</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="special">!(</span>
|
||||
<span class="identifier">x</span> <span class="special"><</span>
|
||||
<span class="identifier">y</span> <span class="special">);</span></code>
|
||||
</li></ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_operator_compare_equal_optional_none"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special">==</span> <span class="special">(</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">none_t</span> <span class="special">)</span>
|
||||
<span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special">==</span> <span class="special">(</span>
|
||||
<span class="identifier">none_t</span><span class="special">,</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span> <span class="special">)</span> <span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="special">!</span><span class="identifier">x</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Notes:</strong></span> <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
need not meet requirements of <a href="http://www.sgi.com/tech/stl/EqualityComparable.html" target="_top"><code class="computeroutput"><span class="identifier">EqualityComparable</span></code></a>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_operator_compare_not_equal_optional_none"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special">!=</span> <span class="special">(</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">none_t</span> <span class="special">)</span>
|
||||
<span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special">!=</span> <span class="special">(</span>
|
||||
<span class="identifier">none_t</span><span class="special">,</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span> <span class="special">)</span> <span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="special">!(</span>
|
||||
<span class="identifier">x</span> <span class="special">==</span>
|
||||
<span class="identifier">y</span> <span class="special">);</span></code>
|
||||
</li></ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_swap_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">void</span> <span class="identifier">swap</span>
|
||||
<span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="identifier">y</span>
|
||||
<span class="special">)</span> <span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Requires:</strong></span> Lvalues of type <code class="computeroutput"><span class="identifier">T</span></code> shall be swappable and <code class="computeroutput"><span class="identifier">T</span></code> shall be <code class="computeroutput"><span class="identifier">MoveConstructible</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<p class="simpara">
|
||||
<span class="bold"><strong>Effects:</strong></span>
|
||||
</p>
|
||||
<div class="informaltable"><table class="table">
|
||||
<colgroup>
|
||||
<col>
|
||||
<col>
|
||||
<col>
|
||||
</colgroup>
|
||||
<thead><tr></tr></thead>
|
||||
<tbody>
|
||||
<tr>
|
||||
<td>
|
||||
</td>
|
||||
<td>
|
||||
<p>
|
||||
<span class="bold"><strong><code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> contains a value</strong></span>
|
||||
</p>
|
||||
</td>
|
||||
<td>
|
||||
<p>
|
||||
<span class="bold"><strong><code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> does not contain a value</strong></span>
|
||||
</p>
|
||||
</td>
|
||||
</tr>
|
||||
<tr>
|
||||
<td>
|
||||
<p>
|
||||
<span class="bold"><strong><code class="computeroutput"><span class="identifier">rhs</span></code>
|
||||
contains a value</strong></span>
|
||||
</p>
|
||||
</td>
|
||||
<td>
|
||||
<p>
|
||||
calls <code class="computeroutput"><span class="identifier">swap</span><span class="special">(*(*</span><span class="keyword">this</span><span class="special">),</span> <span class="special">*</span><span class="identifier">rhs</span><span class="special">)</span></code>
|
||||
</p>
|
||||
</td>
|
||||
<td>
|
||||
<p>
|
||||
initializes the contained value of <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> as if direct-initializing
|
||||
an object of type <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
with the expression <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(*</span><span class="identifier">rhs</span><span class="special">)</span></code>, followed by <code class="computeroutput"><span class="identifier">rhs</span><span class="special">.</span><span class="identifier">val</span><span class="special">-></span><span class="identifier">T</span><span class="special">::~</span><span class="identifier">T</span><span class="special">()</span></code>,
|
||||
<code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
|
||||
contains a value and <code class="computeroutput"><span class="identifier">rhs</span></code>
|
||||
does not contain a value
|
||||
</p>
|
||||
</td>
|
||||
</tr>
|
||||
<tr>
|
||||
<td>
|
||||
<p>
|
||||
<span class="bold"><strong><code class="computeroutput"><span class="identifier">rhs</span></code>
|
||||
does not contain a value</strong></span>
|
||||
</p>
|
||||
</td>
|
||||
<td>
|
||||
<p>
|
||||
initializes the contained value of <code class="computeroutput"><span class="identifier">rhs</span></code>
|
||||
as if direct-initializing an object of type <code class="computeroutput"><span class="identifier">T</span></code> with the expression
|
||||
<code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(*(*</span><span class="keyword">this</span><span class="special">))</span></code>,
|
||||
followed by <code class="computeroutput"><span class="identifier">val</span><span class="special">-></span><span class="identifier">T</span><span class="special">::~</span><span class="identifier">T</span><span class="special">()</span></code>, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> does not contain a value
|
||||
and <code class="computeroutput"><span class="identifier">rhs</span></code> contains
|
||||
a value
|
||||
</p>
|
||||
</td>
|
||||
<td>
|
||||
<p>
|
||||
no effect
|
||||
</p>
|
||||
</td>
|
||||
</tr>
|
||||
</tbody>
|
||||
</table></div>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Postconditions:</strong></span> The states of <code class="computeroutput"><span class="identifier">x</span></code> and <code class="computeroutput"><span class="identifier">y</span></code>
|
||||
interchanged.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Throws:</strong></span> If both are initialized, whatever
|
||||
<code class="computeroutput"><span class="identifier">swap</span><span class="special">(</span><span class="identifier">T</span><span class="special">&,</span><span class="identifier">T</span><span class="special">&)</span></code>
|
||||
throws. If only one is initialized, whatever <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="identifier">T</span> <span class="special">(</span> <span class="identifier">T</span><span class="special">&&</span> <span class="special">)</span></code>
|
||||
throws.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Example:</strong></span>
|
||||
<pre class="programlisting"><span class="identifier">T</span> <span class="identifier">x</span><span class="special">(</span><span class="number">12</span><span class="special">);</span>
|
||||
<span class="identifier">T</span> <span class="identifier">y</span><span class="special">(</span><span class="number">21</span><span class="special">);</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">def0</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">def1</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">optX</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">optY</span><span class="special">(</span><span class="identifier">y</span><span class="special">);</span>
|
||||
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span><span class="special">(</span><span class="identifier">def0</span><span class="special">,</span><span class="identifier">def1</span><span class="special">);</span> <span class="comment">// no-op</span>
|
||||
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span><span class="special">(</span><span class="identifier">def0</span><span class="special">,</span><span class="identifier">optX</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">def0</span> <span class="special">==</span> <span class="identifier">x</span> <span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">!</span><span class="identifier">optX</span> <span class="special">);</span>
|
||||
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span><span class="special">(</span><span class="identifier">def0</span><span class="special">,</span><span class="identifier">optX</span><span class="special">);</span> <span class="comment">// Get back to original values</span>
|
||||
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span><span class="special">(</span><span class="identifier">optX</span><span class="special">,</span><span class="identifier">optY</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">optX</span> <span class="special">==</span> <span class="identifier">y</span> <span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">optY</span> <span class="special">==</span> <span class="identifier">x</span> <span class="special">);</span>
|
||||
</pre>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_swap_optional_reference"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">void</span> <span class="identifier">swap</span>
|
||||
<span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>&</span> <span class="identifier">y</span>
|
||||
<span class="special">)</span> <span class="keyword">noexcept</span>
|
||||
<span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="identifier">x</span></code>
|
||||
refers to what <code class="computeroutput"><span class="identifier">y</span></code> referred
|
||||
to before the swap (if anything). <code class="computeroutput"><span class="identifier">y</span></code>
|
||||
refers to whatever <code class="computeroutput"><span class="identifier">x</span></code>
|
||||
referred to before the swap.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Example:</strong></span>
|
||||
<pre class="programlisting"><span class="identifier">T</span> <span class="identifier">x</span><span class="special">(</span><span class="number">12</span><span class="special">);</span>
|
||||
<span class="identifier">T</span> <span class="identifier">y</span><span class="special">(</span><span class="number">21</span><span class="special">);</span>
|
||||
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span> <span class="identifier">opt0</span><span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span> <span class="identifier">optX</span> <span class="special">(</span><span class="identifier">x</span><span class="special">);</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span> <span class="identifier">optY</span> <span class="special">(</span><span class="identifier">y</span><span class="special">);</span>
|
||||
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span><span class="special">(</span><span class="identifier">optX</span><span class="special">,</span> <span class="identifier">optY</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span><span class="identifier">addressof</span><span class="special">(*</span><span class="identifier">optX</span><span class="special">)</span> <span class="special">==</span> <span class="identifier">addressof</span><span class="special">(</span><span class="identifier">y</span><span class="special">));</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span><span class="identifier">addressof</span><span class="special">(*</span><span class="identifier">optY</span><span class="special">)</span> <span class="special">==</span> <span class="identifier">addressof</span><span class="special">(</span><span class="identifier">x</span><span class="special">));</span>
|
||||
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span><span class="special">(</span><span class="identifier">opt0</span><span class="special">,</span> <span class="identifier">optX</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">opt0</span> <span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">!</span><span class="identifier">optX</span> <span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span><span class="identifier">addressof</span><span class="special">(*</span><span class="identifier">opt0</span><span class="special">)</span> <span class="special">==</span> <span class="identifier">addressof</span><span class="special">(</span><span class="identifier">y</span><span class="special">));</span>
|
||||
</pre>
|
||||
[endsect]
|
||||
</li>
|
||||
</ul></div>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2016 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="detailed_semantics____optional_references.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../../optional/reference/header__boost_optional_optional_hpp_.html"><img src="../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="header__boost_optional_hpp_.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a>
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@@ -1,57 +0,0 @@
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<div class="section">
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||||
<div class="titlepage"><div><div><h4 class="title">
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||||
<a name="boost_optional.reference.header__boost_optional_optional_hpp_.detailed_semantics____optional_references"></a><a class="link" href="detailed_semantics____optional_references.html" title="Detailed Semantics -- Optional References">Detailed
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Semantics -- Optional References</a>
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<p>
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||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_default_ctor"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><&>::</span><span class="identifier">optional</span><span class="special">()</span>
|
||||
<span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><&>::</span><span class="identifier">optional</span><span class="special">(</span><span class="identifier">none_t</span><span class="special">)</span> <span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> refers to nothing.
|
||||
</li></ul></div>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
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<td align="left"></td>
|
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<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2016 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
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</p>
|
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</div></td>
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<div class="titlepage"><div><div><h4 class="title">
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<a name="boost_optional.reference.header__boost_optional_optional_hpp_.detailed_semantics___free_functions"></a><a class="link" href="detailed_semantics___free_functions.html" title="Detailed Semantics - Free Functions">Detailed
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Semantics - Free Functions</a>
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</h4></div></div></div>
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<p>
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<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
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</p>
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<a name="reference_make_optional_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">make_optional</span><span class="special">(</span>
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>(</span><span class="identifier">v</span><span class="special">)</span></code> for the <span class="emphasis"><em>deduced</em></span>
|
||||
type <code class="computeroutput"><span class="identifier">T</span></code> of <code class="computeroutput"><span class="identifier">v</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Example:</strong></span>
|
||||
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">void</span> <span class="identifier">foo</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">;</span>
|
||||
|
||||
<span class="identifier">foo</span> <span class="special">(</span> <span class="identifier">make_optional</span><span class="special">(</span><span class="number">1</span><span class="special">+</span><span class="number">1</span><span class="special">)</span> <span class="special">)</span> <span class="special">;</span> <span class="comment">// Creates an optional<int></span>
|
||||
</pre>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_make_optional_rvalue"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">decay_t</span><span class="special"><</span><span class="identifier">T</span><span class="special">>></span>
|
||||
<span class="identifier">make_optional</span><span class="special">(</span>
|
||||
<span class="identifier">T</span> <span class="special">&&</span>
|
||||
<span class="identifier">v</span> <span class="special">)</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">decay_t</span><span class="special"><</span><span class="identifier">T</span><span class="special">>>(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(</span><span class="identifier">v</span><span class="special">))</span></code> for the <span class="emphasis"><em>deduced</em></span>
|
||||
type <code class="computeroutput"><span class="identifier">T</span></code> of <code class="computeroutput"><span class="identifier">v</span></code>.
|
||||
</li></ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_make_optional_bool_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">make_optional</span><span class="special">(</span>
|
||||
<span class="keyword">bool</span> <span class="identifier">condition</span><span class="special">,</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>(</span><span class="identifier">condition</span><span class="special">,</span> <span class="identifier">v</span><span class="special">)</span></code> for the <span class="emphasis"><em>deduced</em></span>
|
||||
type <code class="computeroutput"><span class="identifier">T</span></code> of <code class="computeroutput"><span class="identifier">v</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Example:</strong></span>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span> <span class="identifier">calculate_foo</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">double</span> <span class="identifier">val</span> <span class="special">=</span> <span class="identifier">compute_foo</span><span class="special">();</span>
|
||||
<span class="keyword">return</span> <span class="identifier">make_optional</span><span class="special">(</span><span class="identifier">is_not_nan_and_finite</span><span class="special">(</span><span class="identifier">val</span><span class="special">),</span><span class="identifier">val</span><span class="special">);</span>
|
||||
<span class="special">}</span>
|
||||
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span> <span class="identifier">v</span> <span class="special">=</span> <span class="identifier">calculate_foo</span><span class="special">();</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span> <span class="special">!</span><span class="identifier">v</span> <span class="special">)</span>
|
||||
<span class="identifier">error</span><span class="special">(</span><span class="string">"foo wasn't computed"</span><span class="special">);</span>
|
||||
</pre>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_make_optional_bool_rvalue"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">decay_t</span><span class="special"><</span><span class="identifier">T</span><span class="special">>></span>
|
||||
<span class="identifier">make_optional</span><span class="special">(</span>
|
||||
<span class="keyword">bool</span> <span class="identifier">condition</span><span class="special">,</span> <span class="identifier">T</span> <span class="special">&&</span> <span class="identifier">v</span>
|
||||
<span class="special">)</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">decay_t</span><span class="special"><</span><span class="identifier">T</span><span class="special">>>(</span><span class="identifier">condition</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(</span><span class="identifier">v</span><span class="special">))</span></code> for the <span class="emphasis"><em>deduced</em></span>
|
||||
type <code class="computeroutput"><span class="identifier">T</span></code> of <code class="computeroutput"><span class="identifier">v</span></code>.
|
||||
</li></ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_operator_compare_equal_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special">==</span> <span class="special">(</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">);</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
shall meet requirements of <a href="http://www.sgi.com/tech/stl/EqualityComparable.html" target="_top"><code class="computeroutput"><span class="identifier">EqualityComparable</span></code></a>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Returns:</strong></span> If both <code class="computeroutput"><span class="identifier">x</span></code>
|
||||
and <code class="computeroutput"><span class="identifier">y</span></code> are initialized,
|
||||
<code class="computeroutput"><span class="special">(*</span><span class="identifier">x</span>
|
||||
<span class="special">==</span> <span class="special">*</span><span class="identifier">y</span><span class="special">)</span></code>.
|
||||
If only <code class="computeroutput"><span class="identifier">x</span></code> or <code class="computeroutput"><span class="identifier">y</span></code> is initialized, <code class="computeroutput"><span class="keyword">false</span></code>.
|
||||
If both are uninitialized, <code class="computeroutput"><span class="keyword">true</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Notes:</strong></span> This definition guarantees
|
||||
that <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
not containing a value is compared unequal to any <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> containing any value, and equal
|
||||
to any other <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> not containing a value. Pointers
|
||||
have shallow relational operators while <code class="computeroutput"><span class="identifier">optional</span></code>
|
||||
has deep relational operators. Do not use <code class="computeroutput"><span class="keyword">operator</span><span class="special">==</span></code> directly in generic code which expect
|
||||
to be given either an <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> or a pointer; use <a href="../../../../../../utility/OptionalPointee.html#equal" target="_top"><code class="computeroutput"><span class="identifier">equal_pointees</span><span class="special">()</span></code></a>
|
||||
instead
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Example:</strong></span>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">oN</span><span class="special">,</span> <span class="identifier">oN_</span><span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">o1</span><span class="special">(</span><span class="identifier">T</span><span class="special">(</span><span class="number">1</span><span class="special">)),</span> <span class="identifier">o1_</span><span class="special">(</span><span class="identifier">T</span><span class="special">(</span><span class="number">1</span><span class="special">));</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">o2</span><span class="special">(</span><span class="identifier">T</span><span class="special">(</span><span class="number">2</span><span class="special">));</span>
|
||||
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">oN</span> <span class="special">==</span> <span class="identifier">oN</span> <span class="special">);</span> <span class="comment">// Identity implies equality</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">o1</span> <span class="special">==</span> <span class="identifier">o1</span> <span class="special">);</span> <span class="comment">//</span>
|
||||
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">oN</span> <span class="special">==</span> <span class="identifier">oN_</span> <span class="special">);</span> <span class="comment">// Both uninitialized compare equal</span>
|
||||
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">oN</span> <span class="special">!=</span> <span class="identifier">o1</span> <span class="special">);</span> <span class="comment">// Initialized unequal to initialized.</span>
|
||||
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">o1</span> <span class="special">==</span> <span class="identifier">o1_</span> <span class="special">);</span> <span class="comment">// Both initialized compare as (*lhs == *rhs)</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">o1</span> <span class="special">!=</span> <span class="identifier">o2</span> <span class="special">);</span> <span class="comment">//</span>
|
||||
</pre>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_operator_compare_less_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special"><</span> <span class="special">(</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">);</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Requires:</strong></span> Expression <code class="computeroutput"><span class="special">*</span><span class="identifier">x</span> <span class="special"><</span> <span class="special">*</span><span class="identifier">y</span></code> shall be well-formed and its result
|
||||
shall be convertible to <code class="computeroutput"><span class="keyword">bool</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="special">(!</span><span class="identifier">y</span><span class="special">)</span> <span class="special">?</span> <span class="keyword">false</span> <span class="special">:</span> <span class="special">(!</span><span class="identifier">x</span><span class="special">)</span> <span class="special">?</span> <span class="keyword">true</span> <span class="special">:</span> <span class="special">*</span><span class="identifier">x</span> <span class="special"><</span>
|
||||
<span class="special">*</span><span class="identifier">y</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Notes:</strong></span> This definition guarantees
|
||||
that <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
not containing a value is ordered as less than any <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> containing any value, and equivalent
|
||||
to any other <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> not containing a value. Pointers
|
||||
have shallow relational operators while <code class="computeroutput"><span class="identifier">optional</span></code>
|
||||
has deep relational operators. Do not use <code class="computeroutput"><span class="keyword">operator</span><span class="special"><</span></code> directly in generic code which
|
||||
expect to be given either an <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> or a pointer; use <a href="../../../../../../utility/OptionalPointee.html#less" target="_top"><code class="computeroutput"><span class="identifier">less_pointees</span><span class="special">()</span></code></a>
|
||||
instead. <code class="computeroutput"><span class="identifier">T</span></code> need not
|
||||
be <a href="http://www.sgi.com/tech/stl/LessThanComparable.html" target="_top"><code class="computeroutput"><span class="identifier">LessThanComparable</span></code></a>. Only
|
||||
single <code class="computeroutput"><span class="keyword">operator</span><span class="special"><</span></code>
|
||||
is required. Other relational operations are defined in terms of this
|
||||
one. If <code class="computeroutput"><span class="identifier">T</span></code>'s <code class="computeroutput"><span class="keyword">operator</span><span class="special"><</span></code>
|
||||
satisfies the axioms of <a href="http://www.sgi.com/tech/stl/LessThanComparable.html" target="_top"><code class="computeroutput"><span class="identifier">LessThanComparable</span></code></a> (transitivity,
|
||||
antisymmetry and irreflexivity), <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is <a href="http://www.sgi.com/tech/stl/LessThanComparable.html" target="_top"><code class="computeroutput"><span class="identifier">LessThanComparable</span></code></a>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Example:</strong></span>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">oN</span><span class="special">,</span> <span class="identifier">oN_</span><span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">o0</span><span class="special">(</span><span class="identifier">T</span><span class="special">(</span><span class="number">0</span><span class="special">));</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">o1</span><span class="special">(</span><span class="identifier">T</span><span class="special">(</span><span class="number">1</span><span class="special">));</span>
|
||||
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">!(</span><span class="identifier">oN</span> <span class="special"><</span> <span class="identifier">oN</span><span class="special">)</span> <span class="special">);</span> <span class="comment">// Identity implies equivalence</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">!(</span><span class="identifier">o1</span> <span class="special"><</span> <span class="identifier">o1</span><span class="special">)</span> <span class="special">);</span>
|
||||
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">!(</span><span class="identifier">oN</span> <span class="special"><</span> <span class="identifier">oN_</span><span class="special">)</span> <span class="special">);</span> <span class="comment">// Two uninitialized are equivalent</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">!(</span><span class="identifier">oN_</span> <span class="special"><</span> <span class="identifier">oN</span><span class="special">)</span> <span class="special">);</span>
|
||||
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">oN</span> <span class="special"><</span> <span class="identifier">o0</span> <span class="special">);</span> <span class="comment">// Uninitialized is less than initialized</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">!(</span><span class="identifier">o0</span> <span class="special"><</span> <span class="identifier">oN</span><span class="special">)</span> <span class="special">);</span>
|
||||
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">o1</span> <span class="special"><</span> <span class="identifier">o2</span> <span class="special">)</span> <span class="special">;</span> <span class="comment">// Two initialized compare as (*lhs < *rhs)</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">!(</span><span class="identifier">o2</span> <span class="special"><</span> <span class="identifier">o1</span><span class="special">)</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">!(</span><span class="identifier">o2</span> <span class="special"><</span> <span class="identifier">o2</span><span class="special">)</span> <span class="special">)</span> <span class="special">;</span>
|
||||
</pre>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_operator_compare_not_equal_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special">!=</span> <span class="special">(</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">);</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="special">!(</span>
|
||||
<span class="identifier">x</span> <span class="special">==</span>
|
||||
<span class="identifier">y</span> <span class="special">);</span></code>
|
||||
</li></ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_operator_compare_greater_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special">></span> <span class="special">(</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">);</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="special">(</span>
|
||||
<span class="identifier">y</span> <span class="special"><</span>
|
||||
<span class="identifier">x</span> <span class="special">);</span></code>
|
||||
</li></ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_operator_compare_less_or_equal_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special"><=</span> <span class="special">(</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">);</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="special">!(</span>
|
||||
<span class="identifier">y</span> <span class="special"><</span>
|
||||
<span class="identifier">x</span> <span class="special">);</span></code>
|
||||
</li></ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_operator_compare_greater_or_equal_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special">>=</span> <span class="special">(</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">);</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="special">!(</span>
|
||||
<span class="identifier">x</span> <span class="special"><</span>
|
||||
<span class="identifier">y</span> <span class="special">);</span></code>
|
||||
</li></ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_operator_compare_equal_optional_none"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special">==</span> <span class="special">(</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">none_t</span> <span class="special">)</span>
|
||||
<span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special">==</span> <span class="special">(</span>
|
||||
<span class="identifier">none_t</span><span class="special">,</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span> <span class="special">)</span> <span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="special">!</span><span class="identifier">x</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Notes:</strong></span> <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
need not meet requirements of <a href="http://www.sgi.com/tech/stl/EqualityComparable.html" target="_top"><code class="computeroutput"><span class="identifier">EqualityComparable</span></code></a>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_operator_compare_not_equal_optional_none"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special">!=</span> <span class="special">(</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">none_t</span> <span class="special">)</span>
|
||||
<span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special">!=</span> <span class="special">(</span>
|
||||
<span class="identifier">none_t</span><span class="special">,</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span> <span class="special">)</span> <span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span></code>
|
||||
</li></ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_free_get_pointer"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">auto</span> <span class="identifier">get_pointer</span>
|
||||
<span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="identifier">o</span>
|
||||
<span class="special">)</span> <span class="special">-></span>
|
||||
<span class="keyword">typename</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">pointer_type</span>
|
||||
<span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">auto</span> <span class="identifier">get_pointer</span>
|
||||
<span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">o</span> <span class="special">)</span> <span class="special">-></span> <span class="keyword">typename</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">pointer_const_type</span>
|
||||
<span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="identifier">o</span><span class="special">.</span><span class="identifier">get_ptr</span><span class="special">()</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Throws:</strong></span> Nothing.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_free_get_value_or"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">auto</span> <span class="identifier">get_optional_value_or</span>
|
||||
<span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="identifier">o</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">reference_type</span> <span class="identifier">def</span>
|
||||
<span class="special">)</span> <span class="special">-></span>
|
||||
<span class="keyword">typename</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">reference_type</span>
|
||||
<span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">auto</span> <span class="identifier">get_optional_value_or</span>
|
||||
<span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">o</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">reference_const_type</span> <span class="identifier">def</span>
|
||||
<span class="special">)</span> <span class="special">-></span>
|
||||
<span class="keyword">typename</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">reference_const_type</span>
|
||||
<span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="identifier">o</span><span class="special">.</span><span class="identifier">get_value_or</span><span class="special">(</span><span class="identifier">def</span><span class="special">)</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Throws:</strong></span> Nothing.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Remarks:</strong></span> This function is deprecated.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_swap_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">void</span> <span class="identifier">swap</span>
|
||||
<span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="identifier">y</span>
|
||||
<span class="special">)</span> <span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Requires:</strong></span> Lvalues of type <code class="computeroutput"><span class="identifier">T</span></code> shall be swappable and <code class="computeroutput"><span class="identifier">T</span></code> shall be <code class="computeroutput"><span class="identifier">MoveConstructible</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<p class="simpara">
|
||||
<span class="bold"><strong>Effects:</strong></span>
|
||||
</p>
|
||||
<div class="informaltable"><table class="table">
|
||||
<colgroup>
|
||||
<col>
|
||||
<col>
|
||||
<col>
|
||||
</colgroup>
|
||||
<thead><tr></tr></thead>
|
||||
<tbody>
|
||||
<tr>
|
||||
<td>
|
||||
</td>
|
||||
<td>
|
||||
<p>
|
||||
<span class="bold"><strong><code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> contains a value</strong></span>
|
||||
</p>
|
||||
</td>
|
||||
<td>
|
||||
<p>
|
||||
<span class="bold"><strong><code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> does not contain a value</strong></span>
|
||||
</p>
|
||||
</td>
|
||||
</tr>
|
||||
<tr>
|
||||
<td>
|
||||
<p>
|
||||
<span class="bold"><strong><code class="computeroutput"><span class="identifier">rhs</span></code>
|
||||
contains a value</strong></span>
|
||||
</p>
|
||||
</td>
|
||||
<td>
|
||||
<p>
|
||||
calls <code class="computeroutput"><span class="identifier">swap</span><span class="special">(*(*</span><span class="keyword">this</span><span class="special">),</span> <span class="special">*</span><span class="identifier">rhs</span><span class="special">)</span></code>
|
||||
</p>
|
||||
</td>
|
||||
<td>
|
||||
<p>
|
||||
initializes the contained value of <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> as if direct-initializing
|
||||
an object of type <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
with the expression <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(*</span><span class="identifier">rhs</span><span class="special">)</span></code>, followed by <code class="computeroutput"><span class="identifier">rhs</span><span class="special">.</span><span class="identifier">val</span><span class="special">-></span><span class="identifier">T</span><span class="special">::~</span><span class="identifier">T</span><span class="special">()</span></code>,
|
||||
<code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
|
||||
contains a value and <code class="computeroutput"><span class="identifier">rhs</span></code>
|
||||
does not contain a value
|
||||
</p>
|
||||
</td>
|
||||
</tr>
|
||||
<tr>
|
||||
<td>
|
||||
<p>
|
||||
<span class="bold"><strong><code class="computeroutput"><span class="identifier">rhs</span></code>
|
||||
does not contain a value</strong></span>
|
||||
</p>
|
||||
</td>
|
||||
<td>
|
||||
<p>
|
||||
initializes the contained value of <code class="computeroutput"><span class="identifier">rhs</span></code>
|
||||
as if direct-initializing an object of type <code class="computeroutput"><span class="identifier">T</span></code> with the expression
|
||||
<code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(*(*</span><span class="keyword">this</span><span class="special">))</span></code>,
|
||||
followed by <code class="computeroutput"><span class="identifier">val</span><span class="special">-></span><span class="identifier">T</span><span class="special">::~</span><span class="identifier">T</span><span class="special">()</span></code>, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> does not contain a value
|
||||
and <code class="computeroutput"><span class="identifier">rhs</span></code> contains
|
||||
a value
|
||||
</p>
|
||||
</td>
|
||||
<td>
|
||||
<p>
|
||||
no effect
|
||||
</p>
|
||||
</td>
|
||||
</tr>
|
||||
</tbody>
|
||||
</table></div>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Postconditions:</strong></span> The states of <code class="computeroutput"><span class="identifier">x</span></code> and <code class="computeroutput"><span class="identifier">y</span></code>
|
||||
interchanged.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Throws:</strong></span> If both are initialized, whatever
|
||||
<code class="computeroutput"><span class="identifier">swap</span><span class="special">(</span><span class="identifier">T</span><span class="special">&,</span><span class="identifier">T</span><span class="special">&)</span></code>
|
||||
throws. If only one is initialized, whatever <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="identifier">T</span> <span class="special">(</span> <span class="identifier">T</span><span class="special">&&</span> <span class="special">)</span></code>
|
||||
throws.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Example:</strong></span>
|
||||
<pre class="programlisting"><span class="identifier">T</span> <span class="identifier">x</span><span class="special">(</span><span class="number">12</span><span class="special">);</span>
|
||||
<span class="identifier">T</span> <span class="identifier">y</span><span class="special">(</span><span class="number">21</span><span class="special">);</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">def0</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">def1</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">optX</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">optY</span><span class="special">(</span><span class="identifier">y</span><span class="special">);</span>
|
||||
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span><span class="special">(</span><span class="identifier">def0</span><span class="special">,</span><span class="identifier">def1</span><span class="special">);</span> <span class="comment">// no-op</span>
|
||||
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span><span class="special">(</span><span class="identifier">def0</span><span class="special">,</span><span class="identifier">optX</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">def0</span> <span class="special">==</span> <span class="identifier">x</span> <span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">!</span><span class="identifier">optX</span> <span class="special">);</span>
|
||||
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span><span class="special">(</span><span class="identifier">def0</span><span class="special">,</span><span class="identifier">optX</span><span class="special">);</span> <span class="comment">// Get back to original values</span>
|
||||
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span><span class="special">(</span><span class="identifier">optX</span><span class="special">,</span><span class="identifier">optY</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">optX</span> <span class="special">==</span> <span class="identifier">y</span> <span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">optY</span> <span class="special">==</span> <span class="identifier">x</span> <span class="special">);</span>
|
||||
</pre>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_swap_optional_reference"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">void</span> <span class="identifier">swap</span>
|
||||
<span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>&</span> <span class="identifier">y</span>
|
||||
<span class="special">)</span> <span class="keyword">noexcept</span>
|
||||
<span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="identifier">x</span></code>
|
||||
refers to what <code class="computeroutput"><span class="identifier">y</span></code> referred
|
||||
to before the swap (if anything). <code class="computeroutput"><span class="identifier">y</span></code>
|
||||
refers to whatever <code class="computeroutput"><span class="identifier">x</span></code>
|
||||
referred to before the swap.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Example:</strong></span>
|
||||
<pre class="programlisting"><span class="identifier">T</span> <span class="identifier">x</span><span class="special">(</span><span class="number">12</span><span class="special">);</span>
|
||||
<span class="identifier">T</span> <span class="identifier">y</span><span class="special">(</span><span class="number">21</span><span class="special">);</span>
|
||||
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span> <span class="identifier">opt0</span><span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span> <span class="identifier">optX</span> <span class="special">(</span><span class="identifier">x</span><span class="special">);</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span> <span class="identifier">optY</span> <span class="special">(</span><span class="identifier">y</span><span class="special">);</span>
|
||||
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span><span class="special">(</span><span class="identifier">optX</span><span class="special">,</span> <span class="identifier">optY</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span><span class="identifier">addressof</span><span class="special">(*</span><span class="identifier">optX</span><span class="special">)</span> <span class="special">==</span> <span class="identifier">addressof</span><span class="special">(</span><span class="identifier">y</span><span class="special">));</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span><span class="identifier">addressof</span><span class="special">(*</span><span class="identifier">optY</span><span class="special">)</span> <span class="special">==</span> <span class="identifier">addressof</span><span class="special">(</span><span class="identifier">x</span><span class="special">));</span>
|
||||
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span><span class="special">(</span><span class="identifier">opt0</span><span class="special">,</span> <span class="identifier">optX</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">opt0</span> <span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">!</span><span class="identifier">optX</span> <span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span><span class="identifier">addressof</span><span class="special">(*</span><span class="identifier">opt0</span><span class="special">)</span> <span class="special">==</span> <span class="identifier">addressof</span><span class="special">(</span><span class="identifier">y</span><span class="special">));</span>
|
||||
</pre>
|
||||
</li>
|
||||
</ul></div>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="detailed_semantics___optional_references.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../../optional/reference/header__boost_optional_optional_hpp_.html"><img src="../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="../header__boost_optional_hpp_.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a>
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</body>
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</html>
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-552
@@ -1,552 +0,0 @@
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
|
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<title>Detailed Semantics - Optional References</title>
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<link rel="stylesheet" href="../../../../../../../doc/src/boostbook.css" type="text/css">
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<meta name="generator" content="DocBook XSL Stylesheets Vsnapshot">
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<link rel="home" href="../../../index.html" title="Boost.Optional">
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<link rel="up" href="../../../optional/reference/header__boost_optional_optional_hpp_.html" title="Header <boost/optional/optional.hpp>">
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<link rel="prev" href="detailed_semantics___optional_values.html" title="Detailed Semantics - Optional Values">
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<link rel="next" href="detailed_semantics___free_functions.html" title="Detailed Semantics - Free Functions">
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</head>
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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<table cellpadding="2" width="100%"><tr>
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<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../../boost.png"></td>
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<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
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<div class="spirit-nav">
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<a accesskey="p" href="detailed_semantics___optional_values.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../../optional/reference/header__boost_optional_optional_hpp_.html"><img src="../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="detailed_semantics___free_functions.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h4 class="title">
|
||||
<a name="boost_optional.reference.header__boost_optional_optional_hpp_.detailed_semantics___optional_references"></a><a class="link" href="detailed_semantics___optional_references.html" title="Detailed Semantics - Optional References">Detailed
|
||||
Semantics - Optional References</a>
|
||||
</h4></div></div></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_default_ctor"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="identifier">optional</span><span class="special">()</span>
|
||||
<span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="identifier">optional</span><span class="special">(</span><span class="identifier">none_t</span><span class="special">)</span> <span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="special">==</span> <span class="keyword">false</span></code>; <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> refers to nothing.
|
||||
</li></ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_value_ctor"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">R</span><span class="special">></span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="identifier">optional</span><span class="special">(</span><span class="identifier">R</span><span class="special">&&</span> <span class="identifier">r</span><span class="special">)</span> <span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="special">==</span> <span class="keyword">true</span></code>; <code class="computeroutput"><span class="identifier">addressof</span><span class="special">(**</span><span class="keyword">this</span><span class="special">)</span> <span class="special">==</span> <span class="identifier">addressof</span><span class="special">(</span><span class="identifier">r</span><span class="special">)</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Remarks:</strong></span> Unless <code class="computeroutput"><span class="identifier">R</span></code>
|
||||
is an lvalue reference, the program is ill-formed. This constructor
|
||||
does not participate in overload resolution if <code class="computeroutput"><span class="identifier">decay</span><span class="special"><</span><span class="identifier">R</span><span class="special">></span></code> is an instance of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Notes:</strong></span> This constructor is declared
|
||||
<code class="computeroutput"><span class="keyword">explicit</span></code> on compilers
|
||||
that do not correctly support binding to const lvalues of integral types.
|
||||
For more details <a class="link" href="../../dependencies_and_portability/optional_reference_binding.html#optional_reference_binding">see here</a>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Example:</strong></span>
|
||||
<pre class="programlisting"><span class="identifier">T</span> <span class="identifier">v</span><span class="special">;</span>
|
||||
<span class="identifier">T</span><span class="special">&</span> <span class="identifier">vref</span> <span class="special">=</span> <span class="identifier">v</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span> <span class="identifier">opt</span><span class="special">(</span><span class="identifier">vref</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">opt</span> <span class="special">==</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">++</span> <span class="identifier">v</span> <span class="special">;</span> <span class="comment">// mutate referee</span>
|
||||
<span class="identifier">assert</span> <span class="special">(*</span><span class="identifier">opt</span> <span class="special">==</span> <span class="identifier">v</span><span class="special">);</span>
|
||||
</pre>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_cond_value_ctor"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">R</span><span class="special">></span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="identifier">optional</span><span class="special">(</span><span class="keyword">bool</span> <span class="identifier">cond</span><span class="special">,</span> <span class="identifier">R</span><span class="special">&&</span>
|
||||
<span class="identifier">r</span><span class="special">)</span>
|
||||
<span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effects: </strong></span> Initializes <code class="computeroutput"><span class="identifier">ref</span></code> with expression <code class="computeroutput"><span class="identifier">cond</span> <span class="special">?</span>
|
||||
<span class="identifier">addressof</span><span class="special">(</span><span class="identifier">r</span><span class="special">)</span> <span class="special">:</span> <span class="keyword">nullptr</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="special">==</span> <span class="identifier">cond</span></code>; If <code class="computeroutput"><span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span></code>, <code class="computeroutput"><span class="identifier">addressof</span><span class="special">(**</span><span class="keyword">this</span><span class="special">)</span> <span class="special">==</span> <span class="identifier">addressof</span><span class="special">(</span><span class="identifier">r</span><span class="special">)</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Remarks:</strong></span> Unless <code class="computeroutput"><span class="identifier">R</span></code>
|
||||
is an lvalue reference, the program is ill-formed. This constructor
|
||||
does not participate in overload resolution if <code class="computeroutput"><span class="identifier">decay</span><span class="special"><</span><span class="identifier">R</span><span class="special">></span></code> is an instance of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span></code>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_copy_ctor"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="identifier">optional</span> <span class="special">(</span>
|
||||
<span class="identifier">optional</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">rhs</span>
|
||||
<span class="special">)</span> <span class="keyword">noexcept</span>
|
||||
<span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effects: </strong></span> Initializes <code class="computeroutput"><span class="identifier">ref</span></code> with expression <code class="computeroutput"><span class="identifier">rhs</span><span class="special">.</span><span class="identifier">ref</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="special">==</span> <span class="keyword">bool</span><span class="special">(</span><span class="identifier">rhs</span><span class="special">)</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Example:</strong></span>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span> <span class="identifier">uninit</span> <span class="special">;</span>
|
||||
<span class="identifier">assert</span> <span class="special">(!</span><span class="identifier">uninit</span><span class="special">);</span>
|
||||
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span> <span class="identifier">uinit2</span> <span class="special">(</span> <span class="identifier">uninit</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">uninit2</span> <span class="special">==</span> <span class="identifier">uninit</span> <span class="special">);</span>
|
||||
|
||||
<span class="identifier">T</span> <span class="identifier">v</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">ref</span> <span class="special">=</span> <span class="identifier">v</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">init</span><span class="special">(</span><span class="identifier">ref</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">init</span> <span class="special">==</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span>
|
||||
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">init2</span> <span class="special">(</span> <span class="identifier">init</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">init2</span> <span class="special">==</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span>
|
||||
|
||||
<span class="identifier">v</span> <span class="special">=</span> <span class="number">3</span> <span class="special">;</span>
|
||||
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">init</span> <span class="special">==</span> <span class="number">3</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">init2</span> <span class="special">==</span> <span class="number">3</span> <span class="special">)</span> <span class="special">;</span>
|
||||
</pre>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_ctor_from_opt_U"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="keyword">explicit</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="identifier">optional</span> <span class="special">(</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">&></span>
|
||||
<span class="keyword">const</span><span class="special">&</span>
|
||||
<span class="identifier">rhs</span> <span class="special">)</span>
|
||||
<span class="keyword">noexcept</span> <span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="identifier">is_convertible</span><span class="special"><</span><span class="identifier">U</span><span class="special">&,</span> <span class="identifier">T</span><span class="special">&>::</span><span class="identifier">value</span></code>
|
||||
is <code class="computeroutput"><span class="keyword">true</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effects: </strong></span> Initializes <code class="computeroutput"><span class="identifier">ref</span></code> with expression <code class="computeroutput"><span class="identifier">rhs</span><span class="special">.</span><span class="identifier">ref</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="special">==</span> <span class="keyword">bool</span><span class="special">(</span><span class="identifier">rhs</span><span class="special">)</span></code>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_assign_none_t"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">none_t</span>
|
||||
<span class="special">)</span> <span class="keyword">noexcept</span>
|
||||
<span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effects: </strong></span> Assigns <code class="computeroutput"><span class="identifier">ref</span></code>
|
||||
with expression <code class="computeroutput"><span class="keyword">nullptr</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>returns:</strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="special">==</span> <span class="keyword">false</span></code>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<a name="reference_optional_ref_copy_assign"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special">&</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span>
|
||||
<span class="keyword">const</span><span class="special">&</span>
|
||||
<span class="identifier">rhs</span> <span class="special">)</span>
|
||||
<span class="keyword">noexcept</span> <span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effects: </strong></span> Assigns <code class="computeroutput"><span class="identifier">ref</span></code>
|
||||
with expression <code class="computeroutput"><span class="identifier">rhs</span><span class="special">.</span><span class="identifier">ref</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>returns:</strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="special">==</span> <span class="keyword">bool</span><span class="special">(</span><span class="identifier">rhs</span><span class="special">)</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Notes:</strong></span> This behaviour is called <span class="emphasis"><em>rebinding
|
||||
semantics</em></span>. See <a class="link" href="../../tutorial/optional_references/rebinding_semantics_for_assignment_of_optional_references.html" title="Rebinding semantics for assignment of optional references">here</a>
|
||||
for details.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Example:</strong></span>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">a</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span> <span class="identifier">b</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span>
|
||||
<span class="identifier">T</span><span class="special">&</span> <span class="identifier">ra</span> <span class="special">=</span> <span class="identifier">a</span> <span class="special">;</span>
|
||||
<span class="identifier">T</span><span class="special">&</span> <span class="identifier">rb</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">def</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">ora</span><span class="special">(</span><span class="identifier">ra</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">orb</span><span class="special">(</span><span class="identifier">rb</span><span class="special">)</span> <span class="special">;</span>
|
||||
|
||||
<span class="identifier">def</span> <span class="special">=</span> <span class="identifier">orb</span> <span class="special">;</span> <span class="comment">// binds 'def' to 'b' through 'rb' wrapped within 'orb'</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">def</span> <span class="special">==</span> <span class="identifier">b</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">*</span><span class="identifier">def</span> <span class="special">=</span> <span class="identifier">ora</span> <span class="special">;</span> <span class="comment">// changes the value of 'b' to a copy of the value of 'a'</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">b</span> <span class="special">==</span> <span class="identifier">a</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span> <span class="identifier">c</span> <span class="special">=</span> <span class="number">3</span><span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">rc</span> <span class="special">=</span> <span class="identifier">c</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">orc</span><span class="special">(</span><span class="identifier">rc</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">ora</span> <span class="special">=</span> <span class="identifier">orc</span> <span class="special">;</span> <span class="comment">// REBINDS ora to 'c' through 'rc'</span>
|
||||
<span class="identifier">c</span> <span class="special">=</span> <span class="number">4</span> <span class="special">;</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">ora</span> <span class="special">==</span> <span class="number">4</span> <span class="special">)</span> <span class="special">;</span>
|
||||
</pre>
|
||||
</li>
|
||||
</ul></div>
|
||||
<a name="reference_optional_ref_assign_optional_U"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="identifier">optional</span><span class="special">&</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">&></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">rhs</span>
|
||||
<span class="special">)</span> <span class="keyword">noexcept</span>
|
||||
<span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="identifier">is_convertible</span><span class="special"><</span><span class="identifier">U</span><span class="special">&,</span> <span class="identifier">T</span><span class="special">&>::</span><span class="identifier">value</span></code>
|
||||
is <code class="computeroutput"><span class="keyword">true</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effects: </strong></span> Assigns <code class="computeroutput"><span class="identifier">ref</span></code>
|
||||
with expression <code class="computeroutput"><span class="identifier">rhs</span><span class="special">.</span><span class="identifier">ref</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>returns:</strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="special">==</span> <span class="keyword">bool</span><span class="special">(</span><span class="identifier">rhs</span><span class="special">)</span></code>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_assign_R"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">R</span><span class="special">></span> <span class="identifier">optional</span><span class="special">&</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">R</span><span class="special">&&</span> <span class="identifier">r</span>
|
||||
<span class="special">)</span> <span class="keyword">noexcept</span>
|
||||
<span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effects: </strong></span> Assigns <code class="computeroutput"><span class="identifier">ref</span></code>
|
||||
with expression <code class="computeroutput"><span class="identifier">r</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>returns:</strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="special">==</span> <span class="keyword">true</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Remarks:</strong></span> Unless <code class="computeroutput"><span class="identifier">R</span></code>
|
||||
is an lvalue reference, the program is ill-formed. This function does
|
||||
not participate in overload resolution if <code class="computeroutput"><span class="identifier">decay</span><span class="special"><</span><span class="identifier">R</span><span class="special">></span></code> is an instance of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Example:</strong></span>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">a</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span> <span class="identifier">b</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span>
|
||||
<span class="identifier">T</span><span class="special">&</span> <span class="identifier">ra</span> <span class="special">=</span> <span class="identifier">a</span> <span class="special">;</span>
|
||||
<span class="identifier">T</span><span class="special">&</span> <span class="identifier">rb</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">def</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">opt</span><span class="special">(</span><span class="identifier">ra</span><span class="special">)</span> <span class="special">;</span>
|
||||
|
||||
<span class="identifier">def</span> <span class="special">=</span> <span class="identifier">rb</span> <span class="special">;</span> <span class="comment">// binds 'def' to 'b' through 'rb'</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">def</span> <span class="special">==</span> <span class="identifier">b</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">*</span><span class="identifier">def</span> <span class="special">=</span> <span class="identifier">a</span> <span class="special">;</span> <span class="comment">// changes the value of 'b' to a copy of the value of 'a'</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">b</span> <span class="special">==</span> <span class="identifier">a</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span> <span class="identifier">c</span> <span class="special">=</span> <span class="number">3</span><span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">rc</span> <span class="special">=</span> <span class="identifier">c</span> <span class="special">;</span>
|
||||
<span class="identifier">opt</span> <span class="special">=</span> <span class="identifier">rc</span> <span class="special">;</span> <span class="comment">// REBINDS to 'c' through 'rc'</span>
|
||||
<span class="identifier">c</span> <span class="special">=</span> <span class="number">4</span> <span class="special">;</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">opt</span> <span class="special">==</span> <span class="number">4</span> <span class="special">)</span> <span class="special">;</span>
|
||||
</pre>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_emplace_R"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">void</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="identifier">emplace</span><span class="special">(</span> <span class="identifier">R</span><span class="special">&&</span> <span class="identifier">r</span>
|
||||
<span class="special">)</span> <span class="keyword">noexcept</span>
|
||||
<span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effects: </strong></span> Assigns <code class="computeroutput"><span class="identifier">ref</span></code>
|
||||
with expression <code class="computeroutput"><span class="identifier">r</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="special">==</span> <span class="keyword">true</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Remarks:</strong></span> Unless <code class="computeroutput"><span class="identifier">R</span></code>
|
||||
is an lvalue reference, the program is ill-formed. This function does
|
||||
not participate in overload resolution if <code class="computeroutput"><span class="identifier">decay</span><span class="special"><</span><span class="identifier">R</span><span class="special">></span></code> is an instance of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span></code>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_get"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">T</span><span class="special">&</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="identifier">get</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">T</span><span class="special">&</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="keyword">operator</span> <span class="special">*()</span>
|
||||
<span class="keyword">const</span> <span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="special">==</span> <span class="keyword">true</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effects: </strong></span> Returns <code class="computeroutput"><span class="special">*</span><span class="identifier">ref</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Throws: </strong></span> Nothing.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Example:</strong></span>
|
||||
<pre class="programlisting"><span class="identifier">T</span> <span class="identifier">v</span> <span class="special">;</span>
|
||||
<span class="identifier">T</span><span class="special">&</span> <span class="identifier">vref</span> <span class="special">=</span> <span class="identifier">v</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span> <span class="identifier">opt</span> <span class="special">(</span> <span class="identifier">vref</span> <span class="special">);</span>
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">vref2</span> <span class="special">=</span> <span class="special">*</span><span class="identifier">opt</span><span class="special">;</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">vref2</span> <span class="special">==</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">++</span> <span class="identifier">v</span> <span class="special">;</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">opt</span> <span class="special">==</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span>
|
||||
</pre>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_arrow"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">T</span><span class="special">*</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="keyword">operator</span> <span class="special">-></span>
|
||||
<span class="special">()</span> <span class="keyword">const</span>
|
||||
<span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="special">==</span> <span class="keyword">true</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effects: </strong></span> Returns <code class="computeroutput"><span class="identifier">ref</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Throws: </strong></span> Nothing.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">T</span><span class="special">&</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="identifier">value</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<span class="bold"><strong>Effects:</strong></span> Equivalent to <code class="computeroutput"><span class="keyword">return</span> <span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="special">?</span> <span class="special">*</span><span class="identifier">val</span> <span class="special">:</span> <span class="keyword">throw</span> <span class="identifier">bad_optional_access</span><span class="special">();</span></code>.
|
||||
</li></ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_value_or"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">R</span><span class="special">></span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="identifier">value_or</span><span class="special">(</span> <span class="identifier">R</span><span class="special">&&</span> <span class="identifier">r</span>
|
||||
<span class="special">)</span> <span class="keyword">const</span>
|
||||
<span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effects:</strong></span> Equivalent to <code class="computeroutput"><span class="keyword">if</span> <span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="keyword">return</span> <span class="special">**</span><span class="keyword">this</span><span class="special">;</span> <span class="keyword">else</span> <span class="keyword">return</span>
|
||||
<span class="identifier">r</span><span class="special">;</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Remarks:</strong></span> Unless <code class="computeroutput"><span class="identifier">R</span></code>
|
||||
is an lvalue reference, the program is ill-formed.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_value_or_eval"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">F</span><span class="special">></span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="identifier">value_or</span><span class="special">(</span> <span class="identifier">F</span> <span class="identifier">f</span> <span class="special">)</span> <span class="keyword">const</span> <span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effects:</strong></span> Equivalent to <code class="computeroutput"><span class="keyword">if</span> <span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="keyword">return</span> <span class="special">**</span><span class="keyword">this</span><span class="special">;</span> <span class="keyword">else</span> <span class="keyword">return</span>
|
||||
<span class="identifier">f</span><span class="special">();</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Remarks:</strong></span> Unless <code class="computeroutput"><span class="keyword">decltype</span><span class="special">(</span><span class="identifier">f</span><span class="special">())</span></code> is an lvalue reference, the program
|
||||
is ill-formed.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_map"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">F</span><span class="special">></span> <span class="keyword">auto</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="identifier">map</span><span class="special">(</span> <span class="identifier">F</span> <span class="identifier">f</span> <span class="special">)</span> <span class="keyword">const</span> <span class="special">-></span> </code><span class="emphasis"><em>see below</em></span><code class="computeroutput"><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effects:</strong></span> Equivalent to <code class="computeroutput"><span class="keyword">if</span> <span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="keyword">return</span> <span class="identifier">f</span><span class="special">(**</span><span class="keyword">this</span><span class="special">);</span> <span class="keyword">else</span> <span class="keyword">return</span> <span class="identifier">none</span><span class="special">;</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Remarks:</strong></span> The return type of this function
|
||||
is <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">decltype</span><span class="special">(</span><span class="identifier">f</span><span class="special">(**</span><span class="keyword">this</span><span class="special">))></span></code>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_flat_map"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">F</span><span class="special">></span> <span class="keyword">auto</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="identifier">flat_map</span><span class="special">(</span>
|
||||
<span class="identifier">F</span> <span class="identifier">f</span>
|
||||
<span class="special">)</span> <span class="keyword">const</span>
|
||||
<span class="special">-></span> </code><span class="emphasis"><em>see below</em></span><code class="computeroutput"><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Requires:</strong></span> The return type of expression
|
||||
<code class="computeroutput"><span class="identifier">f</span><span class="special">(**</span><span class="keyword">this</span><span class="special">)</span></code>
|
||||
is <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span></code>
|
||||
for some object or reference type <code class="computeroutput"><span class="identifier">U</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effects:</strong></span> Equivalent to <code class="computeroutput"><span class="keyword">if</span> <span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="keyword">return</span> <span class="identifier">f</span><span class="special">(**</span><span class="keyword">this</span><span class="special">);</span> <span class="keyword">else</span> <span class="keyword">return</span> <span class="identifier">none</span><span class="special">;</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Remarks:</strong></span> The return type of this function
|
||||
is <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span></code>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_get_ptr"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">T</span><span class="special">*</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="identifier">get_ptr</span> <span class="special">()</span>
|
||||
<span class="keyword">const</span> <span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="identifier">ref</span></code>.
|
||||
</li></ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_operator_bool"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="identifier">has_value</span><span class="special">()</span> <span class="keyword">const</span> <span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="keyword">operator</span> <span class="keyword">bool</span>
|
||||
<span class="special">()</span> <span class="keyword">const</span>
|
||||
<span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(</span><span class="identifier">ref</span><span class="special">)</span></code>.
|
||||
</li></ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_operator_not"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="keyword">operator</span> <span class="special">!</span> <span class="special">()</span> <span class="keyword">const</span> <span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="special">!</span><span class="keyword">bool</span><span class="special">(</span><span class="identifier">ref</span><span class="special">)</span></code>.
|
||||
</li></ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_reset"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">void</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="identifier">reset</span><span class="special">()</span> <span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<span class="bold"><strong>Effects:</strong></span> Same as <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span> <span class="special">=</span> <span class="identifier">none</span></code>.
|
||||
</li></ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_reset_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">R</span><span class="special">></span> <span class="keyword">void</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="identifier">reset</span> <span class="special">(</span> <span class="identifier">R</span><span class="special">&&</span>
|
||||
<span class="identifier">r</span><span class="special">)</span>
|
||||
<span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effects:</strong></span> Equivalent to <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">forward</span><span class="special"><</span><span class="identifier">R</span><span class="special">>(</span><span class="identifier">r</span><span class="special">)</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Remarks:</strong></span> This function is deprecated.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_is_initialized"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="identifier">is_initialized</span><span class="special">()</span> <span class="keyword">const</span> <span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effects:</strong></span> Equivalent to <code class="computeroutput"><span class="keyword">return</span> <span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Remarks:</strong></span> This function is deprecated.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_get_value_or_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">R</span><span class="special">></span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="identifier">get_value_or</span><span class="special">(</span> <span class="identifier">R</span><span class="special">&&</span> <span class="identifier">r</span>
|
||||
<span class="special">)</span> <span class="keyword">const</span>
|
||||
<span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effects:</strong></span> Equivalent to <code class="computeroutput"><span class="keyword">return</span> <span class="identifier">value_or</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">forward</span><span class="special"><</span><span class="identifier">R</span><span class="special">>(</span><span class="identifier">r</span><span class="special">);</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Remarks:</strong></span> This function is deprecated.
|
||||
</li>
|
||||
</ul></div>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
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|
||||
<boost/optional.hpp></a>
|
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|
||||
<p>
|
||||
This is an alias for header <a class="link" href="../../../optional/reference/header__boost_optional_optional_hpp_.html#boost_optional.reference.header__boost_optional_optional_hpp_.header_optional_optional" title="Synopsis"><code class="computeroutput"><span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">optional</span><span class="special">/</span><span class="identifier">optional</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code></a>.
|
||||
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2016 Andrzej Krzemieński<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
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<a name="boost_optional.reference.header__boost_optional_optional_hpp_.header_optional_in_place_init"></a><a class="link" href="header_optional_in_place_init.html" title="Initialization tags">Initialization
|
||||
tags</a>
|
||||
</h4></div></div></div>
|
||||
<a name="reference_in_place_init"></a><a name="reference_in_place_init_if"></a><pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span>
|
||||
|
||||
<span class="keyword">class</span> <span class="identifier">in_place_init_t</span> <span class="special">{</span> <span class="comment">/* see below */</span> <span class="special">}</span> <span class="special">;</span>
|
||||
<span class="keyword">const</span> <span class="identifier">in_place_init_t</span> <span class="identifier">in_place_init</span> <span class="special">(</span> <span class="comment">/* see below */</span> <span class="special">)</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">class</span> <span class="identifier">in_place_init_if_t</span> <span class="special">{</span> <span class="comment">/*see below*/</span> <span class="special">}</span> <span class="special">;</span>
|
||||
<span class="keyword">const</span> <span class="identifier">in_place_init_if_t</span> <span class="identifier">in_place_init_if</span> <span class="special">(</span> <span class="comment">/*see below*/</span> <span class="special">)</span> <span class="special">;</span>
|
||||
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
Classes <code class="computeroutput"><span class="identifier">in_place_init_t</span></code>
|
||||
and <code class="computeroutput"><span class="identifier">in_place_init_if_t</span></code>
|
||||
are empty classes. Their purpose is to control overload resolution in the
|
||||
initialization of optional objects. They are empty, trivially copyable
|
||||
classes with disabled default constructor.
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
|
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Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
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</p>
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|
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<html>
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
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<title>Optional References</title>
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<link rel="stylesheet" href="../../../../../../../doc/src/boostbook.css" type="text/css">
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<meta name="generator" content="DocBook XSL Stylesheets Vsnapshot">
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<link rel="home" href="../../../index.html" title="Boost.Optional">
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<table cellpadding="2" width="100%"><tr>
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<div class="section">
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<div class="titlepage"><div><div><h4 class="title">
|
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<a name="boost_optional.reference.header__boost_optional_optional_hpp_.header_optional_optional_refs"></a><a class="link" href="header_optional_optional_refs.html" title="Optional References">Optional
|
||||
References</a>
|
||||
</h4></div></div></div>
|
||||
<a name="reference_operator_template_spec"></a><pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span>
|
||||
<span class="keyword">class</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span> <span class="comment">// specialization for lvalue references</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">public</span> <span class="special">:</span>
|
||||
|
||||
<span class="keyword">typedef</span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">value_type</span><span class="special">;</span>
|
||||
<span class="keyword">typedef</span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">reference_type</span><span class="special">;</span>
|
||||
<span class="keyword">typedef</span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">reference_const_type</span><span class="special">;</span> <span class="comment">// no const propagation</span>
|
||||
<span class="keyword">typedef</span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">rval_reference_type</span><span class="special">;</span>
|
||||
<span class="keyword">typedef</span> <span class="identifier">T</span><span class="special">*</span> <span class="identifier">pointer_type</span><span class="special">;</span>
|
||||
<span class="keyword">typedef</span> <span class="identifier">T</span><span class="special">*</span> <span class="identifier">pointer_const_type</span><span class="special">;</span> <span class="comment">// no const propagation</span>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">()</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_default_ctor"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">none_t</span> <span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_default_ctor"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">R</span><span class="special">></span> <span class="identifier">optional</span><span class="special">(</span><span class="identifier">R</span><span class="special">&&</span> <span class="identifier">r</span><span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_value_ctor"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">R</span><span class="special">></span> <span class="identifier">optional</span><span class="special">(</span><span class="keyword">bool</span> <span class="identifier">cond</span><span class="special">,</span> <span class="identifier">R</span><span class="special">&&</span> <span class="identifier">r</span><span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_cond_value_ctor"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">optional</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_copy_ctor"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="keyword">explicit</span> <span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">&></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_ctor_from_opt_U"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">none_t</span> <span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_assign_none_t"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="keyword">noexcept</span><span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_copy_assign"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">&></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_assign_optional_U"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">R</span><span class="special">></span> <span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span><span class="identifier">R</span><span class="special">&&</span> <span class="identifier">r</span><span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_assign_R"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">R</span><span class="special">></span> <span class="keyword">void</span> <span class="identifier">emplace</span> <span class="special">(</span> <span class="identifier">R</span><span class="special">&&</span> <span class="identifier">r</span> <span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_emplace_R"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">T</span><span class="special">&</span> <span class="identifier">get</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_get"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">*()</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_get"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">T</span><span class="special">*</span> <span class="keyword">operator</span> <span class="special">->()</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_arrow"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">T</span><span class="special">&</span> <span class="identifier">value</span><span class="special">()</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_value"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">R</span><span class="special">></span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">value_or</span><span class="special">(</span> <span class="identifier">R</span> <span class="special">&&</span> <span class="identifier">r</span> <span class="special">)</span> <span class="keyword">const</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_value_or"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">F</span><span class="special">></span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">value_or_eval</span><span class="special">(</span> <span class="identifier">F</span> <span class="identifier">f</span> <span class="special">)</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_value_or_eval"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">F</span><span class="special">></span> <span class="keyword">auto</span> <span class="identifier">map</span><span class="special">(</span> <span class="identifier">F</span> <span class="identifier">f</span> <span class="special">)</span> <span class="keyword">const</span> <span class="special">-></span> <span class="emphasis"><em>see below</em></span><span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_map"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">F</span><span class="special">></span> <span class="keyword">auto</span> <span class="identifier">flat_map</span><span class="special">(</span> <span class="identifier">F</span> <span class="identifier">f</span> <span class="special">)</span> <span class="keyword">const</span> <span class="special">-></span> <span class="emphasis"><em>see below</em></span><span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_flat_map"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">T</span><span class="special">*</span> <span class="identifier">get_ptr</span><span class="special">()</span> <span class="keyword">const</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_get_ptr"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">bool</span> <span class="identifier">has_value</span><span class="special">()</span> <span class="keyword">const</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_operator_bool"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">explicit</span> <span class="keyword">operator</span> <span class="keyword">bool</span><span class="special">()</span> <span class="keyword">const</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_operator_bool"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">!()</span> <span class="keyword">const</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_operator_not"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">reset</span><span class="special">()</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_reset"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="comment">// deprecated methods</span>
|
||||
|
||||
<span class="comment">// (deprecated)</span>
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">R</span><span class="special">></span> <span class="keyword">void</span> <span class="identifier">reset</span> <span class="special">(</span> <span class="identifier">R</span> <span class="special">&&</span> <span class="identifier">r</span> <span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_reset_value"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="comment">// (deprecated)</span>
|
||||
<span class="keyword">bool</span> <span class="identifier">is_initialized</span><span class="special">()</span> <span class="keyword">const</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_is_initialized"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="comment">// (deprecated)</span>
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">R</span><span class="special">></span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">get_value_or</span><span class="special">(</span> <span class="identifier">R</span> <span class="special">&&</span> <span class="identifier">r</span> <span class="special">)</span> <span class="identifier">constnoexcept</span><span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_get_value_or_value"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">private</span><span class="special">:</span>
|
||||
<span class="identifier">T</span><span class="special">*</span> <span class="identifier">ref</span><span class="special">;</span> <span class="comment">// exposition only</span>
|
||||
<span class="special">};</span>
|
||||
</pre>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
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<div class="spirit-nav">
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<a accesskey="p" href="header_optional_optional_values.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../../optional/reference/header__boost_optional_optional_hpp_.html"><img src="../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="detailed_semantics___optional_values.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a>
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</html>
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-153
@@ -1,153 +0,0 @@
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||||
<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
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<meta name="generator" content="DocBook XSL Stylesheets Vsnapshot">
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<link rel="home" href="../../../index.html" title="Boost.Optional">
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<div class="section">
|
||||
<div class="titlepage"><div><div><h4 class="title">
|
||||
<a name="boost_optional.reference.header__boost_optional_optional_hpp_.header_optional_optional_values"></a><a class="link" href="header_optional_optional_values.html" title="Optional Values">Optional
|
||||
Values</a>
|
||||
</h4></div></div></div>
|
||||
<a name="reference_operator_template"></a><pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span>
|
||||
<span class="keyword">class</span> <span class="identifier">optional</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">public</span> <span class="special">:</span>
|
||||
|
||||
<span class="keyword">typedef</span> <span class="identifier">T</span> <span class="identifier">value_type</span> <span class="special">;</span>
|
||||
<span class="keyword">typedef</span> <span class="identifier">T</span> <span class="special">&</span> <span class="identifier">reference_type</span> <span class="special">;</span>
|
||||
<span class="keyword">typedef</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">reference_const_type</span> <span class="special">;</span>
|
||||
<span class="keyword">typedef</span> <span class="identifier">T</span> <span class="special">&&</span> <span class="identifier">rval_reference_type</span> <span class="special">;</span>
|
||||
<span class="keyword">typedef</span> <span class="identifier">T</span> <span class="special">*</span> <span class="identifier">pointer_type</span> <span class="special">;</span>
|
||||
<span class="keyword">typedef</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">*</span> <span class="identifier">pointer_const_type</span> <span class="special">;</span>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">()</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_constructor"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">none_t</span> <span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_constructor_none_t"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_constructor_value"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">T</span><span class="special">&&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_constructor_move_value"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="keyword">bool</span> <span class="identifier">condition</span><span class="special">,</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_constructor_bool_value"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">optional</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_constructor_optional"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special">&&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="keyword">noexcept</span><span class="special">(</span><span class="emphasis"><em>see below</em></span><span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_move_constructor_optional"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="keyword">explicit</span> <span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_constructor_other_optional"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="keyword">explicit</span> <span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">>&&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_move_constructor_other_optional"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span><span class="special">...</span> <span class="identifier">Args</span><span class="special">></span> <span class="keyword">explicit</span> <span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">in_place_init_t</span><span class="special">,</span> <span class="identifier">Args</span><span class="special">&&...</span> <span class="identifier">args</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_in_place_init"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span><span class="special">...</span> <span class="identifier">Args</span><span class="special">></span> <span class="keyword">explicit</span> <span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">in_place_init_if_t</span><span class="special">,</span> <span class="keyword">bool</span> <span class="identifier">condition</span><span class="special">,</span> <span class="identifier">Args</span><span class="special">&&...</span> <span class="identifier">args</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_in_place_init_if"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">InPlaceFactory</span><span class="special">></span> <span class="keyword">explicit</span> <span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">InPlaceFactory</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_constructor_factory"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">TypedInPlaceFactory</span><span class="special">></span> <span class="keyword">explicit</span> <span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">TypedInPlaceFactory</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_constructor_factory"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">none_t</span> <span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_operator_equal_none_t"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_operator_equal_value"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">T</span><span class="special">&&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_operator_move_equal_value"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_operator_equal_optional"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special">&&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="keyword">noexcept</span><span class="special">(</span><span class="emphasis"><em>see below</em></span><span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_operator_move_equal_optional"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_operator_equal_other_optional"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">>&&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_operator_move_equal_other_optional"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span><span class="special">...</span> <span class="identifier">Args</span><span class="special">></span> <span class="keyword">void</span> <span class="identifier">emplace</span> <span class="special">(</span> <span class="identifier">Args</span><span class="special">&&...</span> <span class="identifier">args</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_emplace"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">InPlaceFactory</span><span class="special">></span> <span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">InPlaceFactory</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_operator_equal_factory"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">TypedInPlaceFactory</span><span class="special">></span> <span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">TypedInPlaceFactory</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_operator_equal_factory"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">get</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_get"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">&</span> <span class="identifier">get</span><span class="special">()</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_get"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">*</span> <span class="keyword">operator</span> <span class="special">->()</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_operator_arrow"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">*</span> <span class="keyword">operator</span> <span class="special">->()</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_operator_arrow"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">*()</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_operator_asterisk"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">*()</span> <span class="special">&</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_operator_asterisk"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">&&</span> <span class="keyword">operator</span> <span class="special">*()</span> <span class="special">&&</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_operator_asterisk_move"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">value</span><span class="special">()</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_value"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">&</span> <span class="identifier">value</span><span class="special">()</span> <span class="special">&</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_value"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">&&</span> <span class="identifier">value</span><span class="special">()</span> <span class="special">&&</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_value_move"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="identifier">T</span> <span class="identifier">value_or</span><span class="special">(</span> <span class="identifier">U</span> <span class="special">&&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_value_or"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="identifier">T</span> <span class="identifier">value_or</span><span class="special">(</span> <span class="identifier">U</span> <span class="special">&&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">&&</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_value_or_move"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">F</span><span class="special">></span> <span class="identifier">T</span> <span class="identifier">value_or_eval</span><span class="special">(</span> <span class="identifier">F</span> <span class="identifier">f</span> <span class="special">)</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_value_or_call"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">F</span><span class="special">></span> <span class="identifier">T</span> <span class="identifier">value_or_eval</span><span class="special">(</span> <span class="identifier">F</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">&&</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_value_or_call_move"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">F</span><span class="special">></span> <span class="keyword">auto</span> <span class="identifier">map</span><span class="special">(</span> <span class="identifier">F</span> <span class="identifier">f</span> <span class="special">)</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">-></span> <span class="emphasis"><em>see below</em></span><span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_map"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">F</span><span class="special">></span> <span class="keyword">auto</span> <span class="identifier">map</span><span class="special">(</span> <span class="identifier">F</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">&</span> <span class="special">-></span> <span class="emphasis"><em>see below</em></span><span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_map"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">F</span><span class="special">></span> <span class="keyword">auto</span> <span class="identifier">map</span><span class="special">(</span> <span class="identifier">F</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">&&</span> <span class="special">-></span> <span class="emphasis"><em>see below</em></span><span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_map_move"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">F</span><span class="special">></span> <span class="keyword">auto</span> <span class="identifier">flat_map</span><span class="special">(</span> <span class="identifier">F</span> <span class="identifier">f</span> <span class="special">)</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">-></span> <span class="emphasis"><em>see below</em></span><span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_flat_map"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">F</span><span class="special">></span> <span class="keyword">auto</span> <span class="identifier">flat_map</span><span class="special">(</span> <span class="identifier">F</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">&</span> <span class="special">-></span> <span class="emphasis"><em>see below</em></span><span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_flat_map"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">F</span><span class="special">></span> <span class="keyword">auto</span> <span class="identifier">flat_map</span><span class="special">(</span> <span class="identifier">F</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">&&</span> <span class="special">-></span> <span class="emphasis"><em>see below</em></span><span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_flat_map_move"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">*</span> <span class="identifier">get_ptr</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_get_ptr"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">*</span> <span class="identifier">get_ptr</span><span class="special">()</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_get_ptr"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">bool</span> <span class="identifier">has_value</span><span class="special">()</span> <span class="keyword">const</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_operator_bool"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">explicit</span> <span class="keyword">operator</span> <span class="keyword">bool</span><span class="special">()</span> <span class="keyword">const</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_operator_bool"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">!()</span> <span class="keyword">const</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_operator_not"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">reset</span><span class="special">()</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_reset"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="comment">// deprecated methods</span>
|
||||
|
||||
<span class="comment">// (deprecated)</span>
|
||||
<span class="keyword">void</span> <span class="identifier">reset</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_reset_value"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="comment">// (deprecated)</span>
|
||||
<span class="keyword">bool</span> <span class="identifier">is_initialized</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_is_initialized"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="comment">// (deprecated)</span>
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">get_value_or</span><span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="keyword">default</span> <span class="special">)</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_get_value_or_value"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="special">};</span>
|
||||
</pre>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="header_optional_in_place_init.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../../optional/reference/header__boost_optional_optional_hpp_.html"><img src="../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="header_optional_optional_refs.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a>
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</div>
|
||||
</body>
|
||||
</html>
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@@ -1,72 +0,0 @@
|
||||
<html>
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||||
<head>
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||||
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
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<title>Header <boost/optional/optional_io.hpp></title>
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<link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css">
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<meta name="generator" content="DocBook XSL Stylesheets Vsnapshot">
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<link rel="home" href="../../index.html" title="Boost.Optional">
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<link rel="up" href="../../optional/reference.html" title="Reference">
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<link rel="prev" href="header__boost_optional_bad_optional_access_hpp_/detailed_semantics.html" title="Detailed semantics">
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<link rel="next" href="io_header/io_semantics.html" title="Detailed semantics">
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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<table cellpadding="2" width="100%"><tr>
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<td align="center"><a href="../../../../../../index.html">Home</a></td>
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<td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td>
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<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
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<a accesskey="p" href="header__boost_optional_bad_optional_access_hpp_/detailed_semantics.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/reference.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="io_header/io_semantics.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.reference.io_header"></a><a class="link" href="io_header.html" title="Header <boost/optional/optional_io.hpp>">Header <boost/optional/optional_io.hpp></a>
|
||||
</h3></div></div></div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h4 class="title">
|
||||
<a name="boost_optional.reference.io_header.io_synop"></a><a class="link" href="io_header.html#boost_optional.reference.io_header.io_synop" title="Synopsis">Synopsis</a>
|
||||
</h4></div></div></div>
|
||||
<p>
|
||||
</p>
|
||||
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">istream</span><span class="special">></span>
|
||||
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">ostream</span><span class="special">></span>
|
||||
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">optional</span><span class="special">/</span><span class="identifier">optional</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
|
||||
|
||||
<span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span>
|
||||
|
||||
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">CharType</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">CharTrait</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_ostream</span><span class="special"><</span><span class="identifier">CharType</span><span class="special">,</span> <span class="identifier">CharTrait</span><span class="special">>&</span>
|
||||
<span class="keyword">operator</span><span class="special"><<(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_ostream</span><span class="special"><</span><span class="identifier">CharType</span><span class="special">,</span> <span class="identifier">CharTrait</span><span class="special">>&</span> <span class="identifier">out</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span><span class="special">);</span> <a class="link" href="io_header/io_semantics.html#reference_operator_ostream"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">CharType</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">CharTrait</span><span class="special">></span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_ostream</span><span class="special"><</span><span class="identifier">CharType</span><span class="special">,</span> <span class="identifier">CharTrait</span><span class="special">>&</span>
|
||||
<span class="keyword">operator</span><span class="special"><<(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_ostream</span><span class="special"><</span><span class="identifier">CharType</span><span class="special">,</span> <span class="identifier">CharTrait</span><span class="special">>&</span> <span class="identifier">out</span><span class="special">,</span> <span class="identifier">none_t</span> <span class="keyword">const</span><span class="special">&);</span> <a class="link" href="io_header/io_semantics.html#reference_operator_ostream_none"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">CharType</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">CharTrait</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_istream</span><span class="special"><</span><span class="identifier">CharType</span><span class="special">,</span> <span class="identifier">CharTrait</span><span class="special">>&</span>
|
||||
<span class="keyword">operator</span><span class="special">>>(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_istream</span><span class="special"><</span><span class="identifier">CharType</span><span class="special">,</span> <span class="identifier">CharTrait</span><span class="special">>&</span> <span class="identifier">in</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="identifier">v</span><span class="special">);</span> <a class="link" href="io_header/io_semantics.html#reference_operator_istream"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="special">}</span> <span class="comment">// namespace boost</span>
|
||||
</pre>
|
||||
<p>
|
||||
</p>
|
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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<a name="boost_optional.reference.io_header.io_semantics"></a><a class="link" href="io_semantics.html" title="Detailed semantics">Detailed
|
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semantics</a>
|
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</h4></div></div></div>
|
||||
<p>
|
||||
<a name="reference_operator_ostream"></a><code class="computeroutput"><span class="keyword">template</span>
|
||||
<span class="special"><</span><span class="keyword">class</span>
|
||||
<span class="identifier">CharType</span><span class="special">,</span>
|
||||
<span class="keyword">class</span> <span class="identifier">CharTrait</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span></code>
|
||||
<br> <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_ostream</span><span class="special"><</span><span class="identifier">CharType</span><span class="special">,</span> <span class="identifier">CharTrait</span><span class="special">>&</span></code> <br> <code class="computeroutput"><span class="keyword">operator</span><span class="special"><<(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_ostream</span><span class="special"><</span><span class="identifier">CharType</span><span class="special">,</span> <span class="identifier">CharTrait</span><span class="special">>&</span>
|
||||
<span class="identifier">out</span><span class="special">,</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span><span class="special">);</span></code>
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effect:</strong></span> Outputs an implementation-defined
|
||||
string. The output contains the information about whether the optional
|
||||
object contains a value or not. If <code class="computeroutput"><span class="identifier">v</span></code>
|
||||
contains a value, the output contains result of calling <code class="computeroutput"><span class="identifier">out</span> <span class="special"><<</span>
|
||||
<span class="special">*</span><span class="identifier">v</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="identifier">out</span></code>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span> <a name="reference_operator_ostream_none"></a>
|
||||
</p>
|
||||
<p>
|
||||
<code class="computeroutput"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">CharType</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">CharTrait</span><span class="special">,</span>
|
||||
<span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span></code> <br> <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_ostream</span><span class="special"><</span><span class="identifier">CharType</span><span class="special">,</span> <span class="identifier">CharTrait</span><span class="special">>&</span></code>
|
||||
<br> <code class="computeroutput"><span class="keyword">operator</span><span class="special"><<(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_ostream</span><span class="special"><</span><span class="identifier">CharType</span><span class="special">,</span> <span class="identifier">CharTrait</span><span class="special">>&</span> <span class="identifier">out</span><span class="special">,</span> <span class="identifier">none_t</span><span class="special">);</span></code>
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effect:</strong></span> Outputs an implementation-defined
|
||||
string.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="identifier">out</span></code>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span> <a name="reference_operator_istream"></a>
|
||||
</p>
|
||||
<p>
|
||||
<code class="computeroutput"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">CharType</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">CharTrait</span><span class="special">,</span>
|
||||
<span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span></code> <br> <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_ostream</span><span class="special"><</span><span class="identifier">CharType</span><span class="special">,</span> <span class="identifier">CharTrait</span><span class="special">>&</span></code>
|
||||
<br> <code class="computeroutput"><span class="keyword">operator</span><span class="special">>>(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_istream</span><span class="special"><</span><span class="identifier">CharType</span><span class="special">,</span> <span class="identifier">CharTrait</span><span class="special">>&</span> <span class="identifier">in</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="identifier">v</span><span class="special">);</span></code>
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
is <a href="http://www.sgi.com/tech/stl/DefaultConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">DefaultConstructible</span></code></a> and
|
||||
<code class="computeroutput"><span class="identifier">MoveConstructible</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effect:</strong></span> Reads the value of optional
|
||||
object from <code class="computeroutput"><span class="identifier">in</span></code>. If
|
||||
the string representation indicates that the optional object should
|
||||
contain a value, <code class="computeroutput"><span class="identifier">v</span></code>
|
||||
contains a value and its contained value is obtained as if by default-constructing
|
||||
an object <code class="computeroutput"><span class="identifier">o</span></code> of type
|
||||
<code class="computeroutput"><span class="identifier">T</span></code> and then calling
|
||||
<code class="computeroutput"><span class="identifier">in</span> <span class="special">>></span>
|
||||
<span class="identifier">o</span></code>; otherwise <code class="computeroutput"><span class="identifier">v</span></code> does not contain a value, and the
|
||||
previously contained value (if any) has been destroyed.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="identifier">out</span></code>.
|
||||
</li>
|
||||
</ul></div>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
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<hr>
|
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<div class="spirit-nav">
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@@ -1,183 +0,0 @@
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<html>
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<head>
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<div class="titlepage"><div><div><h3 class="title">
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<a name="boost_optional.reference.relnotes"></a><a class="link" href="relnotes.html" title="Release Notes">Release Notes</a>
|
||||
</h3></div></div></div>
|
||||
<h5>
|
||||
<a name="boost_optional.reference.relnotes.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.reference.relnotes.boost_release_x_xx"></a></span><a class="link" href="relnotes.html#boost_optional.reference.relnotes.boost_release_x_xx">Boost Release
|
||||
X.XX</a>
|
||||
</h5>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
Now <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span></code> is specialized for reference
|
||||
parameters. This way the <code class="computeroutput"><span class="keyword">sizeof</span></code>
|
||||
of optional reference is that of a pointer, and a number of bugs is avoided.
|
||||
</li></ul></div>
|
||||
<h5>
|
||||
<a name="boost_optional.reference.relnotes.h1"></a>
|
||||
<span class="phrase"><a name="boost_optional.reference.relnotes.boost_release_1_60"></a></span><a class="link" href="relnotes.html#boost_optional.reference.relnotes.boost_release_1_60">Boost Release
|
||||
1.60</a>
|
||||
</h5>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
Changed the implementation of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code>
|
||||
again. Now it is a const object with internal linkage (as any other tag).
|
||||
This fixes <a href="https://svn.boost.org/trac/boost/ticket/11203" target="_top">Trac
|
||||
#11203</a>.
|
||||
</li></ul></div>
|
||||
<h5>
|
||||
<a name="boost_optional.reference.relnotes.h2"></a>
|
||||
<span class="phrase"><a name="boost_optional.reference.relnotes.boost_release_1_59"></a></span><a class="link" href="relnotes.html#boost_optional.reference.relnotes.boost_release_1_59">Boost Release
|
||||
1.59</a>
|
||||
</h5>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
For C++03 compilers, added 0-argument overload for member function <code class="computeroutput"><span class="identifier">emplace</span><span class="special">()</span></code>,
|
||||
and therewith removed the dependency on <code class="computeroutput"><span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">utility</span><span class="special">/</span><span class="identifier">in_place_factory</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Fixed <a href="https://svn.boost.org/trac/boost/ticket/11241" target="_top">Trac
|
||||
#11241</a>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<h5>
|
||||
<a name="boost_optional.reference.relnotes.h3"></a>
|
||||
<span class="phrase"><a name="boost_optional.reference.relnotes.boost_release_1_58"></a></span><a class="link" href="relnotes.html#boost_optional.reference.relnotes.boost_release_1_58">Boost Release
|
||||
1.58</a>
|
||||
</h5>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none_t</span></code> is no longer convertible from
|
||||
literal <code class="computeroutput"><span class="number">0</span></code>. This avoids a
|
||||
bug where <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">rational</span><span class="special"><</span><span class="keyword">int</span><span class="special">>></span>
|
||||
<span class="identifier">oi</span> <span class="special">=</span>
|
||||
<span class="number">0</span><span class="special">;</span></code>
|
||||
would initialize an optional object with no contained value.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Improved the trick that prevents streaming out <code class="computeroutput"><span class="identifier">optional</span></code>
|
||||
without header <code class="computeroutput"><span class="identifier">optional_io</span><span class="special">.</span><span class="identifier">hpp</span></code>
|
||||
by using safe-bool idiom. This addresses <a href="https://svn.boost.org/trac/boost/ticket/10825" target="_top">Trac
|
||||
#10825</a>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
IOStream operators are now mentioned in documentation.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Added a way to manually disable move semantics: just define macro <code class="computeroutput"><span class="identifier">BOOST_OPTIONAL_CONFIG_NO_RVALUE_REFERENCES</span></code>.
|
||||
This can be used to work around <a href="https://svn.boost.org/trac/boost/ticket/10399" target="_top">Trac
|
||||
#10399</a>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
It is no longer possible to assign <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span></code> to <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> when <code class="computeroutput"><span class="identifier">U</span></code>
|
||||
is not assignable or convertible to <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
(<a href="https://svn.boost.org/trac/boost/ticket/11087" target="_top">Trac #11087</a>).
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Value accessors now work correctly on rvalues of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code> (<a href="https://svn.boost.org/trac/boost/ticket/10839" target="_top">Trac
|
||||
#10839</a>).
|
||||
</li>
|
||||
</ul></div>
|
||||
<h5>
|
||||
<a name="boost_optional.reference.relnotes.h4"></a>
|
||||
<span class="phrase"><a name="boost_optional.reference.relnotes.boost_release_1_57"></a></span><a class="link" href="relnotes.html#boost_optional.reference.relnotes.boost_release_1_57">Boost Release
|
||||
1.57</a>
|
||||
</h5>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<a href="https://github.com/boostorg/optional/pull/9" target="_top">Git pull #9</a>:
|
||||
<span class="emphasis"><em>"Supply <code class="computeroutput"><span class="special"><</span><span class="identifier">string</span><span class="special">></span></code>
|
||||
to fix C++03 compile error on <code class="computeroutput"><span class="identifier">logic_error</span><span class="special">(</span><span class="string">"..."</span><span class="special">)</span></code>"</em></span>.
|
||||
</li></ul></div>
|
||||
<h5>
|
||||
<a name="boost_optional.reference.relnotes.h5"></a>
|
||||
<span class="phrase"><a name="boost_optional.reference.relnotes.boost_release_1_56"></a></span><a class="link" href="relnotes.html#boost_optional.reference.relnotes.boost_release_1_56">Boost Release
|
||||
1.56</a>
|
||||
</h5>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
Added support for rvalue references. Now <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> works with moveable but non-copyable
|
||||
<code class="computeroutput"><span class="identifier">T</span></code>'s,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Improved <code class="computeroutput"><span class="identifier">swap</span></code> (now uses
|
||||
move operations),
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Added function <code class="computeroutput"><span class="identifier">emplace</span><span class="special">()</span></code>. This is the last of the requests from
|
||||
<a href="https://svn.boost.org/trac/boost/ticket/1841" target="_top">Trac #1841</a>,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span></code> is moveable,
|
||||
including conditional <code class="computeroutput"><span class="keyword">noexcept</span></code>
|
||||
specifications, which make it <code class="computeroutput"><span class="identifier">move_if_noexcept</span></code>-friendly,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Using explicit operator bool() on platforms that support it (<a href="https://svn.boost.org/trac/boost/ticket/4227" target="_top">Trac
|
||||
#4227</a>) (breaking change),
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Forward declaration of <code class="computeroutput"><span class="keyword">operator</span><span class="special"><<(</span><span class="identifier">ostream</span><span class="special">&,</span> <span class="identifier">optional</span>
|
||||
<span class="keyword">const</span><span class="special">&)</span></code>
|
||||
to prevent inadvertent incorrect serialization of optional objects,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Removed deprecated function <code class="computeroutput"><span class="identifier">reset</span><span class="special">()</span></code> from examples (<a href="https://svn.boost.org/trac/boost/ticket/9005" target="_top">Trac
|
||||
#9005</a>),
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Equality comparison with <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code>
|
||||
does not require that <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
be EqualityComparable,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Optional rvalue references are explicitly disallowed,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Binding temporaries to optional references is explicitly disallowed (breaking
|
||||
change),
|
||||
</li>
|
||||
<li class="listitem">
|
||||
More ways to access the contained value, functions <code class="computeroutput"><span class="identifier">value</span><span class="special">()</span></code>, <code class="computeroutput"><span class="identifier">value_or</span><span class="special">()</span></code>, <code class="computeroutput"><span class="identifier">value_or_eval</span><span class="special">()</span></code>,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Updated and reorganized documentation, added tutorial and quick guide
|
||||
sections.
|
||||
</li>
|
||||
</ul></div>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2016 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
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<a accesskey="p" href="dependencies_and_portability/optional_reference_binding.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/reference.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="acknowledgements.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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</html>
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||||
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|
||||
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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<table cellpadding="2" width="100%"><tr>
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<a accesskey="p" href="dependencies_and_portability/optional_reference_binding.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="acknowledgements.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
|
||||
<a name="boost_optional.relnotes"></a><a class="link" href="relnotes.html" title="Release Notes">Release Notes</a>
|
||||
</h2></div></div></div>
|
||||
<h4>
|
||||
<a name="boost_optional.relnotes.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_79"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_79">Boost
|
||||
Release 1.79</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
Fixed <a href="https://github.com/boostorg/optional/issues/98" target="_top">issue
|
||||
#98</a>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Fixed <a href="https://github.com/boostorg/optional/issues/92" target="_top">issue
|
||||
#92</a>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Added support for <code class="computeroutput"><span class="identifier">BOOST_NO_IOSTREAM</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Now aligned storage uses <code class="computeroutput"><span class="keyword">unsigned</span>
|
||||
<span class="keyword">char</span></code> rather than <code class="computeroutput"><span class="keyword">char</span></code>
|
||||
to avoid UB.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Now using cv-unqualified <code class="computeroutput"><span class="identifier">value_type</span></code>
|
||||
with placement <code class="computeroutput"><span class="keyword">new</span></code> to avoid
|
||||
UB.
|
||||
</li>
|
||||
</ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.relnotes.h1"></a>
|
||||
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_76"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_76">Boost
|
||||
Release 1.76</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
Fixed MSVC warning C4702.
|
||||
</li></ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.relnotes.h2"></a>
|
||||
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_75"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_75">Boost
|
||||
Release 1.75</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code> is <code class="computeroutput"><span class="keyword">constexpr</span></code>-declared.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Fixed <a href="https://github.com/boostorg/optional/issues/78" target="_top">issue
|
||||
#78</a>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.relnotes.h3"></a>
|
||||
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_73"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_73">Boost
|
||||
Release 1.73</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
Fixed <a href="https://github.com/boostorg/optional/issues/78" target="_top">issue
|
||||
#78</a>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code> is now declared as an inline variable
|
||||
(on compilers that support it): there is only one instance of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code> across all translation units.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Fixed a number of compilation errors in GCC 4.4.7 in <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> for trivial <code class="computeroutput"><span class="identifier">T</span></code>s.
|
||||
Thanks to Robert Leahy for the fix. For details see <a href="https://github.com/boostorg/optional/pull/80" target="_top">pr
|
||||
#78</a>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Now suppressing warning <code class="computeroutput"><span class="special">-</span><span class="identifier">Wweak</span><span class="special">-</span><span class="identifier">vtables</span></code>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.relnotes.h4"></a>
|
||||
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_69"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_69">Boost
|
||||
Release 1.69</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
Remove deprecation mark from <code class="computeroutput"><span class="identifier">reset</span><span class="special">()</span></code> method (without arguments).
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Fixed <a href="https://github.com/boostorg/optional/issues/59" target="_top">issue
|
||||
#59</a>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Fixed bug with initialization of certain wrapper types in clang with -std=c++03.
|
||||
See <a href="https://github.com/boostorg/optional/pull/64" target="_top">pr #64</a>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.relnotes.h5"></a>
|
||||
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_68"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_68">Boost
|
||||
Release 1.68</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
Added member function <code class="computeroutput"><span class="identifier">has_value</span><span class="special">()</span></code> for compatibility with <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">optional</span></code> (<a href="https://github.com/boostorg/optional/issues/52" target="_top">issue
|
||||
#52</a>).
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Added member function <code class="computeroutput"><span class="identifier">map</span><span class="special">()</span></code> for transforming <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> into <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span></code> using a function of type <code class="computeroutput"><span class="identifier">T</span> <span class="special">-></span> <span class="identifier">U</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Added member function <code class="computeroutput"><span class="identifier">flat_map</span><span class="special">()</span></code> for transforming <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> into <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span></code> using a function of type <code class="computeroutput"><span class="identifier">T</span> <span class="special">-></span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span></code>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.relnotes.h6"></a>
|
||||
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_67"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_67">Boost
|
||||
Release 1.67</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
Fixed <a href="https://github.com/boostorg/optional/issues/46" target="_top">issue
|
||||
#46</a>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Fixed <code class="computeroutput"><span class="special">-</span><span class="identifier">Wzero</span><span class="special">-</span><span class="identifier">as</span><span class="special">-</span><span class="identifier">null</span><span class="special">-</span><span class="identifier">pointer</span><span class="special">-</span><span class="identifier">constant</span></code> warnings.
|
||||
</li>
|
||||
</ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.relnotes.h7"></a>
|
||||
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_66"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_66">Boost
|
||||
Release 1.66</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
On newer compilers <code class="computeroutput"><span class="identifier">optional</span></code>
|
||||
is now trivially-copyable for scalar <code class="computeroutput"><span class="identifier">T</span></code>s.
|
||||
This uses a different storage (just <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
rather than <code class="computeroutput"><span class="identifier">aligned_storage</span></code>).
|
||||
We require the compiler to support defaulted functions.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Changed the implementation of <code class="computeroutput"><span class="keyword">operator</span><span class="special">==</span></code> to get rid of the <code class="computeroutput"><span class="special">-</span><span class="identifier">Wmaybe</span><span class="special">-</span><span class="identifier">uninitialized</span></code> false-positive warning
|
||||
from GCC.
|
||||
</li>
|
||||
</ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.relnotes.h8"></a>
|
||||
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_63"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_63">Boost
|
||||
Release 1.63</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
Added two new in-place constructors. They work similarly to <code class="computeroutput"><span class="identifier">emplace</span><span class="special">()</span></code>
|
||||
functions: they initialize the contained value by perfect-forwarding the
|
||||
obtained arguments. One constructor always initializes the contained value,
|
||||
the other based on a boolean condition.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Syntax <code class="computeroutput"><span class="identifier">o</span> <span class="special">=</span>
|
||||
<span class="special">{}</span></code> now correctly un-initializes
|
||||
optional, just like in <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">optional</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Fixed <a href="https://svn.boost.org/trac/boost/ticket/12203" target="_top">Trac #12203</a>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Fixed <a href="https://svn.boost.org/trac/boost/ticket/12563" target="_top">Trac #12563</a>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.relnotes.h9"></a>
|
||||
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_62"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_62">Boost
|
||||
Release 1.62</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
Fixed <a href="https://svn.boost.org/trac/boost/ticket/12179" target="_top">Trac #12179</a>.
|
||||
</li></ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.relnotes.h10"></a>
|
||||
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_61"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_61">Boost
|
||||
Release 1.61</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
Now <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span></code> is specialized for reference
|
||||
parameters. This addresses a couple of issues:
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; ">
|
||||
<li class="listitem">
|
||||
the <code class="computeroutput"><span class="keyword">sizeof</span></code> of optional
|
||||
reference is that of a pointer,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
some bugs connected to copying optional references are gone,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
all run-time bugs caused by incorrect reference binding on some compilers
|
||||
are now turned into compile-time errors,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
you can swap optional references: it is like swapping pointers: shallow,
|
||||
underlying objects are not affected,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
optional references to abstract types work.
|
||||
</li>
|
||||
</ul></div>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Documented nested typedefs (<a href="https://svn.boost.org/trac/boost/ticket/5193" target="_top">Trac
|
||||
#5193</a>).
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Made the perfect-forwarding constructor SFINAE-friendly, which fixes <a href="https://svn.boost.org/trac/boost/ticket/12002" target="_top">Trac #12002</a>.
|
||||
However, this only works in the newer platforms that correctly implement
|
||||
C++11 <code class="computeroutput"><span class="special"><</span><span class="identifier">type_traits</span><span class="special">></span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Fixed <a href="https://svn.boost.org/trac/boost/ticket/10445" target="_top">Trac #10445</a>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.relnotes.h11"></a>
|
||||
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_60"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_60">Boost
|
||||
Release 1.60</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
Changed the implementation of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code>
|
||||
again. Now it is a const object with internal linkage (as any other tag).
|
||||
This fixes <a href="https://svn.boost.org/trac/boost/ticket/11203" target="_top">Trac
|
||||
#11203</a>.
|
||||
</li></ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.relnotes.h12"></a>
|
||||
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_59"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_59">Boost
|
||||
Release 1.59</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
For C++03 compilers, added 0-argument overload for member function <code class="computeroutput"><span class="identifier">emplace</span><span class="special">()</span></code>,
|
||||
and therewith removed the dependency on <code class="computeroutput"><span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">utility</span><span class="special">/</span><span class="identifier">in_place_factory</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Fixed <a href="https://svn.boost.org/trac/boost/ticket/11241" target="_top">Trac #11241</a>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.relnotes.h13"></a>
|
||||
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_58"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_58">Boost
|
||||
Release 1.58</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none_t</span></code> is no longer convertible from
|
||||
literal <code class="computeroutput"><span class="number">0</span></code>. This avoids a bug
|
||||
where <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">rational</span><span class="special"><</span><span class="keyword">int</span><span class="special">>></span> <span class="identifier">oi</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span></code> would
|
||||
initialize an optional object with no contained value.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Improved the trick that prevents streaming out <code class="computeroutput"><span class="identifier">optional</span></code>
|
||||
without header <code class="computeroutput"><span class="identifier">optional_io</span><span class="special">.</span><span class="identifier">hpp</span></code>
|
||||
by using safe-bool idiom. This addresses <a href="https://svn.boost.org/trac/boost/ticket/10825" target="_top">Trac
|
||||
#10825</a>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
IOStream operators are now mentioned in documentation.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Added a way to manually disable move semantics: just define macro <code class="computeroutput"><span class="identifier">BOOST_OPTIONAL_CONFIG_NO_RVALUE_REFERENCES</span></code>.
|
||||
This can be used to work around <a href="https://svn.boost.org/trac/boost/ticket/10399" target="_top">Trac
|
||||
#10399</a>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
It is no longer possible to assign <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span></code> to <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> when <code class="computeroutput"><span class="identifier">U</span></code>
|
||||
is not assignable or convertible to <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
(<a href="https://svn.boost.org/trac/boost/ticket/11087" target="_top">Trac #11087</a>).
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Value accessors now work correctly on rvalues of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code> (<a href="https://svn.boost.org/trac/boost/ticket/10839" target="_top">Trac
|
||||
#10839</a>).
|
||||
</li>
|
||||
</ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.relnotes.h14"></a>
|
||||
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_57"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_57">Boost
|
||||
Release 1.57</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<a href="https://github.com/boostorg/optional/pull/9" target="_top">Git pull #9</a>:
|
||||
<span class="emphasis"><em>"Supply <code class="computeroutput"><span class="special"><</span><span class="identifier">string</span><span class="special">></span></code>
|
||||
to fix C++03 compile error on <code class="computeroutput"><span class="identifier">logic_error</span><span class="special">(</span><span class="string">"..."</span><span class="special">)</span></code>"</em></span>.
|
||||
</li></ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.relnotes.h15"></a>
|
||||
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_56"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_56">Boost
|
||||
Release 1.56</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
Added support for rvalue references. Now <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> works with moveable but non-copyable
|
||||
<code class="computeroutput"><span class="identifier">T</span></code>'s,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Improved <code class="computeroutput"><span class="identifier">swap</span></code> (now uses
|
||||
move operations),
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Added function <code class="computeroutput"><span class="identifier">emplace</span><span class="special">()</span></code>. This is the last of the requests from
|
||||
<a href="https://svn.boost.org/trac/boost/ticket/1841" target="_top">Trac #1841</a>,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span></code> is moveable, including
|
||||
conditional <code class="computeroutput"><span class="keyword">noexcept</span></code> specifications,
|
||||
which make it <code class="computeroutput"><span class="identifier">move_if_noexcept</span></code>-friendly,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Using explicit operator bool() on platforms that support it (<a href="https://svn.boost.org/trac/boost/ticket/4227" target="_top">Trac
|
||||
#4227</a>) (breaking change),
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Forward declaration of <code class="computeroutput"><span class="keyword">operator</span><span class="special"><<(</span><span class="identifier">ostream</span><span class="special">&,</span> <span class="identifier">optional</span>
|
||||
<span class="keyword">const</span><span class="special">&)</span></code>
|
||||
to prevent inadvertent incorrect serialization of optional objects,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Removed deprecated function <code class="computeroutput"><span class="identifier">reset</span><span class="special">()</span></code> from examples (<a href="https://svn.boost.org/trac/boost/ticket/9005" target="_top">Trac
|
||||
#9005</a>),
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Equality comparison with <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code>
|
||||
does not require that <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
be EqualityComparable,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Optional rvalue references are explicitly disallowed,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Binding temporaries to optional references is explicitly disallowed (breaking
|
||||
change),
|
||||
</li>
|
||||
<li class="listitem">
|
||||
More ways to access the contained value, functions <code class="computeroutput"><span class="identifier">value</span><span class="special">()</span></code>, <code class="computeroutput"><span class="identifier">value_or</span><span class="special">()</span></code>, <code class="computeroutput"><span class="identifier">value_or_eval</span><span class="special">()</span></code>,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Updated and reorganized documentation, added tutorial and quick guide sections.
|
||||
</li>
|
||||
</ul></div>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
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@@ -1,174 +0,0 @@
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
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<div class="section">
|
||||
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
|
||||
<a name="boost_optional.synopsis"></a><a class="link" href="synopsis.html" title="Synopsis">Synopsis</a>
|
||||
</h2></div></div></div>
|
||||
<pre class="programlisting"><code class="computeroutput"><span class="comment">// In Header: <</span></code><a href="../../../../../boost/optional/optional.hpp" target="_top">boost/optional/optional.hpp</a><span class="comment">></span>
|
||||
|
||||
<span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span>
|
||||
<span class="keyword">class</span> <span class="identifier">optional</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">public</span> <span class="special">:</span>
|
||||
|
||||
<span class="comment">// (If T is of reference type, the parameters and results by reference are by value)</span>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">()</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_constructor"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">none_t</span> <span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_constructor_none_t"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_constructor_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">T</span><span class="special">&&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_constructor_move_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="comment">// [new in 1.34]</span>
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="keyword">bool</span> <span class="identifier">condition</span><span class="special">,</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_constructor_bool_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">optional</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_constructor_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special">&&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="keyword">noexcept</span><span class="special">(</span><span class="emphasis"><em>see below</em></span><span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_move_constructor_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="keyword">explicit</span> <span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_constructor_other_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="keyword">explicit</span> <span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">>&&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_move_constructor_other_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">InPlaceFactory</span><span class="special">></span> <span class="keyword">explicit</span> <span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">InPlaceFactory</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_constructor_factory"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">TypedInPlaceFactory</span><span class="special">></span> <span class="keyword">explicit</span> <span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">TypedInPlaceFactory</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_constructor_factory"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">none_t</span> <span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_equal_none_t"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_equal_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">T</span><span class="special">&&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_move_equal_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_equal_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special">&&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="keyword">noexcept</span><span class="special">(</span><span class="emphasis"><em>see below</em></span><span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_move_equal_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_equal_other_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">>&&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_move_equal_other_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span><span class="special">...</span> <span class="identifier">Args</span><span class="special">></span> <span class="keyword">void</span> <span class="identifier">emplace</span> <span class="special">(</span> <span class="identifier">Args</span><span class="special">...&&</span> <span class="identifier">args</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_emplace"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">InPlaceFactory</span><span class="special">></span> <span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">InPlaceFactory</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_equal_factory"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">TypedInPlaceFactory</span><span class="special">></span> <span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">TypedInPlaceFactory</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_equal_factory"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">get</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_get"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">&</span> <span class="identifier">get</span><span class="special">()</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_get"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">*</span> <span class="keyword">operator</span> <span class="special">->()</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_arrow"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">*</span> <span class="keyword">operator</span> <span class="special">->()</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_arrow"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">*()</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_asterisk"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">*()</span> <span class="special">&;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_asterisk"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">&&</span> <span class="keyword">operator</span> <span class="special">*()</span> <span class="special">&&;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_asterisk"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">value</span><span class="special">()</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">&</span> <span class="identifier">value</span><span class="special">()</span> <span class="special">&</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">&&</span> <span class="identifier">value</span><span class="special">()</span> <span class="special">&&</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="identifier">T</span> <span class="identifier">value_or</span><span class="special">(</span> <span class="identifier">U</span> <span class="special">&&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_value_or"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="identifier">T</span> <span class="identifier">value_or</span><span class="special">(</span> <span class="identifier">U</span> <span class="special">&&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">&&</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_value_or"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">*</span> <span class="identifier">get_ptr</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_get_ptr"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">*</span> <span class="identifier">get_ptr</span><span class="special">()</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_get_ptr"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">explicit</span> <span class="keyword">operator</span> <span class="keyword">bool</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_bool"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">!()</span> <span class="keyword">const</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_not"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="comment">// deprecated methods</span>
|
||||
|
||||
<span class="comment">// (deprecated)</span>
|
||||
<span class="keyword">void</span> <span class="identifier">reset</span><span class="special">()</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_reset"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="comment">// (deprecated)</span>
|
||||
<span class="keyword">void</span> <span class="identifier">reset</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_reset_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="comment">// (deprecated)</span>
|
||||
<span class="keyword">bool</span> <span class="identifier">is_initialized</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_is_initialized"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="comment">// (deprecated)</span>
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">get_value_or</span><span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="keyword">default</span> <span class="special">)</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_get_value_or_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="special">};</span>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special">==</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_operator_compare_equal_optional_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special">!=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_operator_compare_not_equal_optional_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special"><</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_operator_compare_less_optional_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special">></span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_operator_compare_greater_optional_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special"><=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_operator_compare_less_or_equal_optional_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special">>=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_operator_compare_greater_or_equal_optional_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special">==</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">none_t</span> <span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_operator_compare_equal_optional_none"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special">!=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">none_t</span> <span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_operator_compare_not_equal_optional_none"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">make_optional</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_make_optional_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">make_optional</span> <span class="special">(</span> <span class="keyword">bool</span> <span class="identifier">condition</span><span class="special">,</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_make_optional_bool_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">get_optional_value_or</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">opt</span><span class="special">,</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="keyword">default</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_get_value_or_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">get</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_get"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">get</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="special">&</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_get"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">*</span> <span class="identifier">get</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">*</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_get"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">T</span><span class="special">*</span> <span class="identifier">get</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>*</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_get"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">*</span> <span class="identifier">get_pointer</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_get_ptr"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">T</span><span class="special">*</span> <span class="identifier">get_pointer</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="special">&</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_get_ptr"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">void</span> <span class="identifier">swap</span><span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_swap_optional_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="special">}</span> <span class="comment">// namespace boost</span>
|
||||
</pre>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
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<div class="spirit-nav">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="boost_optional.tutorial"></a><a class="link" href="tutorial.html" title="Tutorial">Tutorial</a>
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||||
</h2></div></div></div>
|
||||
<div class="toc"><dl class="toc">
|
||||
<dt><span class="section"><a href="tutorial.html#boost_optional.tutorial.motivation">Motivation</a></span></dt>
|
||||
<dt><span class="section"><a href="tutorial.html#boost_optional.tutorial.design_overview">Design Overview</a></span></dt>
|
||||
</dl></div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.tutorial.motivation"></a><a class="link" href="tutorial.html#boost_optional.tutorial.motivation" title="Motivation">Motivation</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
Consider these functions which should return a value but which might not
|
||||
have a value to return:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
(A) <code class="computeroutput"><span class="keyword">double</span> <span class="identifier">sqrt</span><span class="special">(</span><span class="keyword">double</span> <span class="identifier">n</span> <span class="special">);</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
(B) <code class="computeroutput"><span class="keyword">char</span> <span class="identifier">get_async_input</span><span class="special">();</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
(C) <code class="computeroutput"><span class="identifier">point</span> <span class="identifier">polygon</span><span class="special">::</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span></code>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
There are different approaches to the issue of not having a value to return.
|
||||
</p>
|
||||
<p>
|
||||
A typical approach is to consider the existence of a valid return value as
|
||||
a postcondition, so that if the function cannot compute the value to return,
|
||||
it has either undefined behavior (and can use assert in a debug build) or
|
||||
uses a runtime check and throws an exception if the postcondition is violated.
|
||||
This is a reasonable choice for example, for function (A), because the lack
|
||||
of a proper return value is directly related to an invalid parameter (out
|
||||
of domain argument), so it is appropriate to require the callee to supply
|
||||
only parameters in a valid domain for execution to continue normally.
|
||||
</p>
|
||||
<p>
|
||||
However, function (B), because of its asynchronous nature, does not fail
|
||||
just because it can't find a value to return; so it is incorrect to consider
|
||||
such a situation an error and assert or throw an exception. This function
|
||||
must return, and somehow, must tell the callee that it is not returning a
|
||||
meaningful value.
|
||||
</p>
|
||||
<p>
|
||||
A similar situation occurs with function (C): it is conceptually an error
|
||||
to ask a <span class="emphasis"><em>null-area</em></span> polygon to return a point inside
|
||||
itself, but in many applications, it is just impractical for performance
|
||||
reasons to treat this as an error (because detecting that the polygon has
|
||||
no area might be too expensive to be required to be tested previously), and
|
||||
either an arbitrary point (typically at infinity) is returned, or some efficient
|
||||
way to tell the callee that there is no such point is used.
|
||||
</p>
|
||||
<p>
|
||||
There are various mechanisms to let functions communicate that the returned
|
||||
value is not valid. One such mechanism, which is quite common since it has
|
||||
zero or negligible overhead, is to use a special value which is reserved
|
||||
to communicate this. Classical examples of such special values are <code class="computeroutput"><span class="identifier">EOF</span></code>, <code class="computeroutput"><span class="identifier">string</span><span class="special">::</span><span class="identifier">npos</span></code>,
|
||||
points at infinity, etc...
|
||||
</p>
|
||||
<p>
|
||||
When those values exist, i.e. the return type can hold all meaningful values
|
||||
<span class="emphasis"><em>plus</em></span> the <span class="emphasis"><em>signal</em></span> value, this mechanism
|
||||
is quite appropriate and well known. Unfortunately, there are cases when
|
||||
such values do not exist. In these cases, the usual alternative is either
|
||||
to use a wider type, such as <code class="computeroutput"><span class="keyword">int</span></code>
|
||||
in place of <code class="computeroutput"><span class="keyword">char</span></code>; or a compound
|
||||
type, such as <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span></code>.
|
||||
</p>
|
||||
<p>
|
||||
Returning a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span></code>, thus attaching a boolean flag to the
|
||||
result which indicates if the result is meaningful, has the advantage that
|
||||
can be turned into a consistent idiom since the first element of the pair
|
||||
can be whatever the function would conceptually return. For example, the
|
||||
last two functions could have the following interface:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="keyword">char</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">get_async_input</span><span class="special">();</span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">polygon</span><span class="special">::</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span>
|
||||
</pre>
|
||||
<p>
|
||||
These functions use a consistent interface for dealing with possibly nonexistent
|
||||
results:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">poly</span><span class="special">.</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">second</span> <span class="special">)</span>
|
||||
<span class="identifier">flood_fill</span><span class="special">(</span><span class="identifier">p</span><span class="special">.</span><span class="identifier">first</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
However, not only is this quite a burden syntactically, it is also error
|
||||
prone since the user can easily use the function result (first element of
|
||||
the pair) without ever checking if it has a valid value.
|
||||
</p>
|
||||
<p>
|
||||
Clearly, we need a better idiom.
|
||||
</p>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.tutorial.design_overview"></a><a class="link" href="tutorial.html#boost_optional.tutorial.design_overview" title="Design Overview">Design Overview</a>
|
||||
</h3></div></div></div>
|
||||
<div class="toc"><dl class="toc">
|
||||
<dt><span class="section"><a href="tutorial.html#boost_optional.tutorial.design_overview.the_models">The
|
||||
models</a></span></dt>
|
||||
<dt><span class="section"><a href="tutorial.html#boost_optional.tutorial.design_overview.the_semantics">The
|
||||
semantics</a></span></dt>
|
||||
<dt><span class="section"><a href="tutorial.html#boost_optional.tutorial.design_overview.the_interface">The
|
||||
Interface</a></span></dt>
|
||||
</dl></div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h4 class="title">
|
||||
<a name="boost_optional.tutorial.design_overview.the_models"></a><a class="link" href="tutorial.html#boost_optional.tutorial.design_overview.the_models" title="The models">The
|
||||
models</a>
|
||||
</h4></div></div></div>
|
||||
<p>
|
||||
In C++, we can <span class="emphasis"><em>declare</em></span> an object (a variable) of type
|
||||
<code class="computeroutput"><span class="identifier">T</span></code>, and we can give this
|
||||
variable an <span class="emphasis"><em>initial value</em></span> (through an <span class="emphasis"><em>initializer</em></span>.
|
||||
(cf. 8.5)). When a declaration includes a non-empty initializer (an initial
|
||||
value is given), it is said that the object has been initialized. If the
|
||||
declaration uses an empty initializer (no initial value is given), and
|
||||
neither default nor value initialization applies, it is said that the object
|
||||
is <span class="bold"><strong>uninitialized</strong></span>. Its actual value exist
|
||||
but has an <span class="emphasis"><em>indeterminate initial value</em></span> (cf. 8.5/11).
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
intends to formalize the notion of initialization (or lack of it) allowing
|
||||
a program to test whether an object has been initialized and stating that
|
||||
access to the value of an uninitialized object is undefined behavior. That
|
||||
is, when a variable is declared as <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> and no initial value is given, the
|
||||
variable is <span class="emphasis"><em>formally</em></span> uninitialized. A formally uninitialized
|
||||
optional object has conceptually no value at all and this situation can
|
||||
be tested at runtime. It is formally <span class="emphasis"><em>undefined behavior</em></span>
|
||||
to try to access the value of an uninitialized optional. An uninitialized
|
||||
optional can be assigned a value, in which case its initialization state
|
||||
changes to initialized. Furthermore, given the formal treatment of initialization
|
||||
states in optional objects, it is even possible to reset an optional to
|
||||
<span class="emphasis"><em>uninitialized</em></span>.
|
||||
</p>
|
||||
<p>
|
||||
In C++ there is no formal notion of uninitialized objects, which means
|
||||
that objects always have an initial value even if indeterminate. As discussed
|
||||
on the previous section, this has a drawback because you need additional
|
||||
information to tell if an object has been effectively initialized. One
|
||||
of the typical ways in which this has been historically dealt with is via
|
||||
a special value: <code class="computeroutput"><span class="identifier">EOF</span></code>,
|
||||
<code class="computeroutput"><span class="identifier">npos</span></code>, -1, etc... This is
|
||||
equivalent to adding the special value to the set of possible values of
|
||||
a given type. This super set of <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
plus some <span class="emphasis"><em>nil_t</em></span>—where <code class="computeroutput"><span class="identifier">nil_t</span></code>
|
||||
is some stateless POD—can be modeled in modern languages as a <span class="bold"><strong>discriminated union</strong></span> of T and nil_t. Discriminated
|
||||
unions are often called <span class="emphasis"><em>variants</em></span>. A variant has a
|
||||
<span class="emphasis"><em>current type</em></span>, which in our case is either <code class="computeroutput"><span class="identifier">T</span></code> or <code class="computeroutput"><span class="identifier">nil_t</span></code>.
|
||||
Using the <a href="../../../../variant/index.html" target="_top">Boost.Variant</a>
|
||||
library, this model can be implemented in terms of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">variant</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="identifier">nil_t</span><span class="special">></span></code>. There is precedent for a discriminated
|
||||
union as a model for an optional value: the <a href="http://www.haskell.org/" target="_top">Haskell</a>
|
||||
<span class="bold"><strong>Maybe</strong></span> built-in type constructor. Thus,
|
||||
a discriminated union <code class="computeroutput"><span class="identifier">T</span><span class="special">+</span><span class="identifier">nil_t</span></code>
|
||||
serves as a conceptual foundation.
|
||||
</p>
|
||||
<p>
|
||||
A <code class="computeroutput"><span class="identifier">variant</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="identifier">nil_t</span><span class="special">></span></code> follows naturally from the traditional
|
||||
idiom of extending the range of possible values adding an additional sentinel
|
||||
value with the special meaning of <span class="emphasis"><em>Nothing</em></span>. However,
|
||||
this additional <span class="emphasis"><em>Nothing</em></span> value is largely irrelevant
|
||||
for our purpose since our goal is to formalize the notion of uninitialized
|
||||
objects and, while a special extended value can be used to convey that
|
||||
meaning, it is not strictly necessary in order to do so.
|
||||
</p>
|
||||
<p>
|
||||
The observation made in the last paragraph about the irrelevant nature
|
||||
of the additional <code class="computeroutput"><span class="identifier">nil_t</span></code>
|
||||
with respect to <span class="underline">purpose</span> of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
suggests an alternative model: a <span class="emphasis"><em>container</em></span> that either
|
||||
has a value of <code class="computeroutput"><span class="identifier">T</span></code> or nothing.
|
||||
</p>
|
||||
<p>
|
||||
As of this writing I don't know of any precedent for a variable-size fixed-capacity
|
||||
(of 1) stack-based container model for optional values, yet I believe this
|
||||
is the consequence of the lack of practical implementations of such a container
|
||||
rather than an inherent shortcoming of the container model.
|
||||
</p>
|
||||
<p>
|
||||
In any event, both the discriminated-union or the single-element container
|
||||
models serve as a conceptual ground for a class representing optional—i.e.
|
||||
possibly uninitialized—objects. For instance, these models show the
|
||||
<span class="emphasis"><em>exact</em></span> semantics required for a wrapper of optional
|
||||
values:
|
||||
</p>
|
||||
<p>
|
||||
Discriminated-union:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>deep-copy</strong></span> semantics: copies of the
|
||||
variant implies copies of the value.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>deep-relational</strong></span> semantics: comparisons
|
||||
between variants matches both current types and values
|
||||
</li>
|
||||
<li class="listitem">
|
||||
If the variant's current type is <code class="computeroutput"><span class="identifier">T</span></code>,
|
||||
it is modeling an <span class="emphasis"><em>initialized</em></span> optional.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
If the variant's current type is not <code class="computeroutput"><span class="identifier">T</span></code>,
|
||||
it is modeling an <span class="emphasis"><em>uninitialized</em></span> optional.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Testing if the variant's current type is <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
models testing if the optional is initialized
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Trying to extract a <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
from a variant when its current type is not <code class="computeroutput"><span class="identifier">T</span></code>,
|
||||
models the undefined behavior of trying to access the value of an uninitialized
|
||||
optional
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
Single-element container:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>deep-copy</strong></span> semantics: copies of the
|
||||
container implies copies of the value.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>deep-relational</strong></span> semantics: comparisons
|
||||
between containers compare container size and if match, contained value
|
||||
</li>
|
||||
<li class="listitem">
|
||||
If the container is not empty (contains an object of type <code class="computeroutput"><span class="identifier">T</span></code>), it is modeling an <span class="emphasis"><em>initialized</em></span>
|
||||
optional.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
If the container is empty, it is modeling an <span class="emphasis"><em>uninitialized</em></span>
|
||||
optional.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Testing if the container is empty models testing if the optional is
|
||||
initialized
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Trying to extract a <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
from an empty container models the undefined behavior of trying to
|
||||
access the value of an uninitialized optional
|
||||
</li>
|
||||
</ul></div>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h4 class="title">
|
||||
<a name="boost_optional.tutorial.design_overview.the_semantics"></a><a class="link" href="tutorial.html#boost_optional.tutorial.design_overview.the_semantics" title="The semantics">The
|
||||
semantics</a>
|
||||
</h4></div></div></div>
|
||||
<p>
|
||||
Objects of type <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> are intended to be used in places where
|
||||
objects of type <code class="computeroutput"><span class="identifier">T</span></code> would
|
||||
but which might be uninitialized. Hence, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>'s purpose is to formalize the additional
|
||||
possibly uninitialized state. From the perspective of this role, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
can have the same operational semantics of <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
plus the additional semantics corresponding to this special state. As such,
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
could be thought of as a <span class="emphasis"><em>supertype</em></span> of <code class="computeroutput"><span class="identifier">T</span></code>. Of course, we can't do that in C++,
|
||||
so we need to compose the desired semantics using a different mechanism.
|
||||
Doing it the other way around, that is, making <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> a <span class="emphasis"><em>subtype</em></span> of
|
||||
<code class="computeroutput"><span class="identifier">T</span></code> is not only conceptually
|
||||
wrong but also impractical: it is not allowed to derive from a non-class
|
||||
type, such as a built-in type.
|
||||
</p>
|
||||
<p>
|
||||
We can draw from the purpose of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> the required basic semantics:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Default Construction:</strong></span> To introduce
|
||||
a formally uninitialized wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Direct Value Construction via copy:</strong></span>
|
||||
To introduce a formally initialized wrapped object whose value is obtained
|
||||
as a copy of some object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Deep Copy Construction:</strong></span> To obtain
|
||||
a new yet equivalent wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Direct Value Assignment (upon initialized):</strong></span>
|
||||
To assign a value to the wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Direct Value Assignment (upon uninitialized):</strong></span>
|
||||
To initialize the wrapped object with a value obtained as a copy of
|
||||
some object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Assignment (upon initialized):</strong></span> To
|
||||
assign to the wrapped object the value of another wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Assignment (upon uninitialized):</strong></span> To
|
||||
initialize the wrapped object with value of another wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Deep Relational Operations (when supported by
|
||||
the type T):</strong></span> To compare wrapped object values taking into
|
||||
account the presence of uninitialized states.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Value access:</strong></span> To unwrap the wrapped
|
||||
object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Initialization state query:</strong></span> To determine
|
||||
if the object is formally initialized or not.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Swap:</strong></span> To exchange wrapped objects.
|
||||
(with whatever exception safety guarantees are provided by <code class="computeroutput"><span class="identifier">T</span></code>'s swap).
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>De-initialization:</strong></span> To release the
|
||||
wrapped object (if any) and leave the wrapper in the uninitialized
|
||||
state.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
Additional operations are useful, such as converting constructors and converting
|
||||
assignments, in-place construction and assignment, and safe value access
|
||||
via a pointer to the wrapped object or null.
|
||||
</p>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h4 class="title">
|
||||
<a name="boost_optional.tutorial.design_overview.the_interface"></a><a class="link" href="tutorial.html#boost_optional.tutorial.design_overview.the_interface" title="The Interface">The
|
||||
Interface</a>
|
||||
</h4></div></div></div>
|
||||
<p>
|
||||
Since the purpose of optional is to allow us to use objects with a formal
|
||||
uninitialized additional state, the interface could try to follow the interface
|
||||
of the underlying <code class="computeroutput"><span class="identifier">T</span></code> type
|
||||
as much as possible. In order to choose the proper degree of adoption of
|
||||
the native <code class="computeroutput"><span class="identifier">T</span></code> interface,
|
||||
the following must be noted: Even if all the operations supported by an
|
||||
instance of type <code class="computeroutput"><span class="identifier">T</span></code> are
|
||||
defined for the entire range of values for such a type, an <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
extends such a set of values with a new value for which most (otherwise
|
||||
valid) operations are not defined in terms of <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</p>
|
||||
<p>
|
||||
Furthermore, since <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> itself is merely a <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
wrapper (modeling a <code class="computeroutput"><span class="identifier">T</span></code> supertype),
|
||||
any attempt to define such operations upon uninitialized optionals will
|
||||
be totally artificial w.r.t. <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</p>
|
||||
<p>
|
||||
This library chooses an interface which follows from <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
interface only for those operations which are well defined (w.r.t the type
|
||||
<code class="computeroutput"><span class="identifier">T</span></code>) even if any of the operands
|
||||
are uninitialized. These operations include: construction, copy-construction,
|
||||
assignment, swap and relational operations.
|
||||
</p>
|
||||
<p>
|
||||
For the value access operations, which are undefined (w.r.t the type <code class="computeroutput"><span class="identifier">T</span></code>) when the operand is uninitialized,
|
||||
a different interface is chosen (which will be explained next).
|
||||
</p>
|
||||
<p>
|
||||
Also, the presence of the possibly uninitialized state requires additional
|
||||
operations not provided by <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
itself which are supported by a special interface.
|
||||
</p>
|
||||
<h6>
|
||||
<a name="boost_optional.tutorial.design_overview.the_interface.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.tutorial.design_overview.the_interface.lexically_hinted_value_access_in_the_presence_of_possibly_uninitialized_optional_objects__the_operators___and___gt_"></a></span><a class="link" href="tutorial.html#boost_optional.tutorial.design_overview.the_interface.lexically_hinted_value_access_in_the_presence_of_possibly_uninitialized_optional_objects__the_operators___and___gt_">Lexically-hinted
|
||||
Value Access in the presence of possibly uninitialized optional objects:
|
||||
The operators * and -></a>
|
||||
</h6>
|
||||
<p>
|
||||
A relevant feature of a pointer is that it can have a <span class="bold"><strong>null
|
||||
pointer value</strong></span>. This is a <span class="emphasis"><em>special</em></span> value
|
||||
which is used to indicate that the pointer is not referring to any object
|
||||
at all. In other words, null pointer values convey the notion of nonexistent
|
||||
objects.
|
||||
</p>
|
||||
<p>
|
||||
This meaning of the null pointer value allowed pointers to became a <span class="emphasis"><em>de
|
||||
facto</em></span> standard for handling optional objects because all you
|
||||
have to do to refer to a value which you don't really have is to use a
|
||||
null pointer value of the appropriate type. Pointers have been used for
|
||||
decades—from the days of C APIs to modern C++ libraries—to <span class="emphasis"><em>refer</em></span>
|
||||
to optional (that is, possibly nonexistent) objects; particularly as optional
|
||||
arguments to a function, but also quite often as optional data members.
|
||||
</p>
|
||||
<p>
|
||||
The possible presence of a null pointer value makes the operations that
|
||||
access the pointee's value possibly undefined, therefore, expressions which
|
||||
use dereference and access operators, such as: <code class="computeroutput"><span class="special">(</span>
|
||||
<span class="special">*</span><span class="identifier">p</span>
|
||||
<span class="special">=</span> <span class="number">2</span> <span class="special">)</span></code> and <code class="computeroutput"><span class="special">(</span>
|
||||
<span class="identifier">p</span><span class="special">-></span><span class="identifier">foo</span><span class="special">()</span> <span class="special">)</span></code>, implicitly convey the notion of optionality,
|
||||
and this information is tied to the <span class="emphasis"><em>syntax</em></span> of the
|
||||
expressions. That is, the presence of operators <code class="computeroutput"><span class="special">*</span></code>
|
||||
and <code class="computeroutput"><span class="special">-></span></code> tell by themselves
|
||||
—without any additional context— that the expression will be undefined
|
||||
unless the implied pointee actually exist.
|
||||
</p>
|
||||
<p>
|
||||
Such a <span class="emphasis"><em>de facto</em></span> idiom for referring to optional objects
|
||||
can be formalized in the form of a concept: the <a href="../../../../utility/OptionalPointee.html" target="_top">OptionalPointee</a>
|
||||
concept. This concept captures the syntactic usage of operators <code class="computeroutput"><span class="special">*</span></code>, <code class="computeroutput"><span class="special">-></span></code>
|
||||
and contextual conversion to <code class="computeroutput"><span class="keyword">bool</span></code>
|
||||
to convey the notion of optionality.
|
||||
</p>
|
||||
<p>
|
||||
However, pointers are good to <span class="underline">refer</span>
|
||||
to optional objects, but not particularly good to handle the optional objects
|
||||
in all other respects, such as initializing or moving/copying them. The
|
||||
problem resides in the shallow-copy of pointer semantics: if you need to
|
||||
effectively move or copy the object, pointers alone are not enough. The
|
||||
problem is that copies of pointers do not imply copies of pointees. For
|
||||
example, as was discussed in the motivation, pointers alone cannot be used
|
||||
to return optional objects from a function because the object must move
|
||||
outside from the function and into the caller's context.
|
||||
</p>
|
||||
<p>
|
||||
A solution to the shallow-copy problem that is often used is to resort
|
||||
to dynamic allocation and use a smart pointer to automatically handle the
|
||||
details of this. For example, if a function is to optionally return an
|
||||
object <code class="computeroutput"><span class="identifier">X</span></code>, it can use <code class="computeroutput"><span class="identifier">shared_ptr</span><span class="special"><</span><span class="identifier">X</span><span class="special">></span></code>
|
||||
as the return value. However, this requires dynamic allocation of <code class="computeroutput"><span class="identifier">X</span></code>. If <code class="computeroutput"><span class="identifier">X</span></code>
|
||||
is a built-in or small POD, this technique is very poor in terms of required
|
||||
resources. Optional objects are essentially values so it is very convenient
|
||||
to be able to use automatic storage and deep-copy semantics to manipulate
|
||||
optional values just as we do with ordinary values. Pointers do not have
|
||||
this semantics, so are inappropriate for the initialization and transport
|
||||
of optional values, yet are quite convenient for handling the access to
|
||||
the possible undefined value because of the idiomatic aid present in the
|
||||
<a href="../../../../utility/OptionalPointee.html" target="_top">OptionalPointee</a>
|
||||
concept incarnated by pointers.
|
||||
</p>
|
||||
<h6>
|
||||
<a name="boost_optional.tutorial.design_overview.the_interface.h1"></a>
|
||||
<span class="phrase"><a name="boost_optional.tutorial.design_overview.the_interface.optional_lt_t_gt__as_a_model_of_optionalpointee"></a></span><a class="link" href="tutorial.html#boost_optional.tutorial.design_overview.the_interface.optional_lt_t_gt__as_a_model_of_optionalpointee">Optional<T>
|
||||
as a model of OptionalPointee</a>
|
||||
</h6>
|
||||
<p>
|
||||
For value access operations <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code> uses operators <code class="computeroutput"><span class="special">*</span></code>
|
||||
and <code class="computeroutput"><span class="special">-></span></code> to lexically warn
|
||||
about the possibly uninitialized state appealing to the familiar pointer
|
||||
semantics w.r.t. to null pointers.
|
||||
</p>
|
||||
<div class="warning"><table border="0" summary="Warning">
|
||||
<tr>
|
||||
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../doc/src/images/warning.png"></td>
|
||||
<th align="left">Warning</th>
|
||||
</tr>
|
||||
<tr><td align="left" valign="top"><p>
|
||||
However, it is particularly important to note that <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code> objects are not pointers. <span class="underline"><code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code> is not, and does not model, a
|
||||
pointer</span>.
|
||||
</p></td></tr>
|
||||
</table></div>
|
||||
<p>
|
||||
For instance, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code> does not have shallow-copy so does
|
||||
not alias: two different optionals never refer to the <span class="emphasis"><em>same</em></span>
|
||||
value unless <code class="computeroutput"><span class="identifier">T</span></code> itself is
|
||||
a reference (but may have <span class="emphasis"><em>equivalent</em></span> values). The
|
||||
difference between an <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> and a pointer must be kept in mind,
|
||||
particularly because the semantics of relational operators are different:
|
||||
since <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
is a value-wrapper, relational operators are deep: they compare optional
|
||||
values; but relational operators for pointers are shallow: they do not
|
||||
compare pointee values. As a result, you might be able to replace <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
by <code class="computeroutput"><span class="identifier">T</span><span class="special">*</span></code>
|
||||
on some situations but not always. Specifically, on generic code written
|
||||
for both, you cannot use relational operators directly, and must use the
|
||||
template functions <a href="../../../../utility/OptionalPointee.html#equal" target="_top"><code class="computeroutput"><span class="identifier">equal_pointees</span><span class="special">()</span></code></a>
|
||||
and <a href="../../../../utility/OptionalPointee.html#less" target="_top"><code class="computeroutput"><span class="identifier">less_pointees</span><span class="special">()</span></code></a>
|
||||
instead.
|
||||
</p>
|
||||
</div>
|
||||
</div>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
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<a accesskey="p" href="quick_start.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="synopsis.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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||||
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|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
|
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<div class="section">
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||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.tutorial.a_note_about_optional_bool_"></a><a class="link" href="a_note_about_optional_bool_.html" title="A note about optional<bool>">A
|
||||
note about optional<bool></a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span></code> should
|
||||
be used with special caution and consideration.
|
||||
</p>
|
||||
<p>
|
||||
First, it is functionally similar to a tristate boolean (false, maybe, true)
|
||||
—such as <a href="../../../../../../doc/html/tribool.html" target="_top">boost::tribool</a>—
|
||||
except that in a tristate boolean, the maybe state <span class="underline">represents
|
||||
a valid value</span>, unlike the corresponding state of an uninitialized
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span></code>.
|
||||
It should be carefully considered if an <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span></code>
|
||||
instead of a <code class="computeroutput"><span class="identifier">tribool</span></code> is really
|
||||
needed.
|
||||
</p>
|
||||
<p>
|
||||
Second, although <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code> provides a contextual conversion
|
||||
to <code class="computeroutput"><span class="keyword">bool</span></code> in C++11, this falls
|
||||
back to an implicit conversion on older compilers. This conversion refers
|
||||
to the initialization state and not to the contained value. Using <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span></code> can
|
||||
lead to subtle errors due to the implicit <code class="computeroutput"><span class="keyword">bool</span></code>
|
||||
conversion:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">foo</span> <span class="special">(</span> <span class="keyword">bool</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="keyword">void</span> <span class="identifier">bar</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">v</span> <span class="special">=</span> <span class="keyword">try</span><span class="special">();</span>
|
||||
|
||||
<span class="comment">// The following intended to pass the value of 'v' to foo():</span>
|
||||
<span class="identifier">foo</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span>
|
||||
<span class="comment">// But instead, the initialization state is passed</span>
|
||||
<span class="comment">// due to a typo: it should have been foo(*v).</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
The only implicit conversion is to <code class="computeroutput"><span class="keyword">bool</span></code>,
|
||||
and it is safe in the sense that typical integral promotions don't apply
|
||||
(i.e. if <code class="computeroutput"><span class="identifier">foo</span><span class="special">()</span></code>
|
||||
takes an <code class="computeroutput"><span class="keyword">int</span></code> instead, it won't
|
||||
compile).
|
||||
</p>
|
||||
<p>
|
||||
Third, mixed comparisons with <code class="computeroutput"><span class="keyword">bool</span></code>
|
||||
work differently than similar mixed comparisons between pointers and <code class="computeroutput"><span class="keyword">bool</span></code>, so the results might surprise you:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">oEmpty</span><span class="special">(</span><span class="identifier">none</span><span class="special">),</span> <span class="identifier">oTrue</span><span class="special">(</span><span class="keyword">true</span><span class="special">),</span> <span class="identifier">oFalse</span><span class="special">(</span><span class="keyword">false</span><span class="special">);</span>
|
||||
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oEmpty</span> <span class="special">==</span> <span class="identifier">none</span><span class="special">);</span> <span class="comment">// renders true</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oEmpty</span> <span class="special">==</span> <span class="keyword">false</span><span class="special">);</span> <span class="comment">// renders false!</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oEmpty</span> <span class="special">==</span> <span class="keyword">true</span><span class="special">);</span> <span class="comment">// renders false!</span>
|
||||
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oFalse</span> <span class="special">==</span> <span class="identifier">none</span><span class="special">);</span> <span class="comment">// renders false</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oFalse</span> <span class="special">==</span> <span class="keyword">false</span><span class="special">);</span> <span class="comment">// renders true!</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oFalse</span> <span class="special">==</span> <span class="keyword">true</span><span class="special">);</span> <span class="comment">// renders false</span>
|
||||
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oTrue</span> <span class="special">==</span> <span class="identifier">none</span><span class="special">);</span> <span class="comment">// renders false</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oTrue</span> <span class="special">==</span> <span class="keyword">false</span><span class="special">);</span> <span class="comment">// renders false</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oTrue</span> <span class="special">==</span> <span class="keyword">true</span><span class="special">);</span> <span class="comment">// renders true</span>
|
||||
</pre>
|
||||
<p>
|
||||
In other words, for <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code>, the following assertion does not
|
||||
hold:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">assert</span><span class="special">((</span><span class="identifier">opt</span> <span class="special">==</span> <span class="keyword">false</span><span class="special">)</span> <span class="special">==</span> <span class="special">(!</span><span class="identifier">opt</span><span class="special">));</span>
|
||||
</pre>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2016 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
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<hr>
|
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<div class="spirit-nav">
|
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<a accesskey="p" href="in_place_factories.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="exception_safety_guarantees.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
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<title>Design Overview</title>
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<link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css">
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<meta name="generator" content="DocBook XSL Stylesheets Vsnapshot">
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<link rel="home" href="../../index.html" title="Boost.Optional">
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<link rel="up" href="../../optional/tutorial.html" title="Tutorial">
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<link rel="prev" href="../../optional/tutorial.html" title="Tutorial">
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<link rel="next" href="design_overview/the_semantics.html" title="The semantics">
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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<table cellpadding="2" width="100%"><tr>
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<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
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<a accesskey="p" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="design_overview/the_semantics.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.tutorial.design_overview"></a><a class="link" href="design_overview.html" title="Design Overview">Design Overview</a>
|
||||
</h3></div></div></div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h4 class="title">
|
||||
<a name="boost_optional.tutorial.design_overview.the_models"></a><a class="link" href="design_overview.html#boost_optional.tutorial.design_overview.the_models" title="The models">The
|
||||
models</a>
|
||||
</h4></div></div></div>
|
||||
<p>
|
||||
In C++, we can <span class="emphasis"><em>declare</em></span> an object (a variable) of type
|
||||
<code class="computeroutput"><span class="identifier">T</span></code>, and we can give this
|
||||
variable an <span class="emphasis"><em>initial value</em></span> (through an <span class="emphasis"><em>initializer</em></span>.
|
||||
(cf. 8.5)). When a declaration includes a non-empty initializer (an initial
|
||||
value is given), it is said that the object has been initialized. If the
|
||||
declaration uses an empty initializer (no initial value is given), and
|
||||
neither default nor value initialization applies, it is said that the object
|
||||
is <span class="bold"><strong>uninitialized</strong></span>. Its actual value exist
|
||||
but has an <span class="emphasis"><em>indeterminate initial value</em></span> (cf. 8.5/11).
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
intends to formalize the notion of initialization (or lack of it) allowing
|
||||
a program to test whether an object has been initialized and stating that
|
||||
access to the value of an uninitialized object is undefined behavior. That
|
||||
is, when a variable is declared as <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> and no initial value is given, the
|
||||
variable is <span class="emphasis"><em>formally</em></span> uninitialized. A formally uninitialized
|
||||
optional object has conceptually no value at all and this situation can
|
||||
be tested at runtime. It is formally <span class="emphasis"><em>undefined behavior</em></span>
|
||||
to try to access the value of an uninitialized optional. An uninitialized
|
||||
optional can be assigned a value, in which case its initialization state
|
||||
changes to initialized. Furthermore, given the formal treatment of initialization
|
||||
states in optional objects, it is even possible to reset an optional to
|
||||
<span class="emphasis"><em>uninitialized</em></span>.
|
||||
</p>
|
||||
<p>
|
||||
In C++ there is no formal notion of uninitialized objects, which means
|
||||
that objects always have an initial value even if indeterminate. As discussed
|
||||
on the previous section, this has a drawback because you need additional
|
||||
information to tell if an object has been effectively initialized. One
|
||||
of the typical ways in which this has been historically dealt with is via
|
||||
a special value: <code class="computeroutput"><span class="identifier">EOF</span></code>,
|
||||
<code class="computeroutput"><span class="identifier">npos</span></code>, -1, etc... This is
|
||||
equivalent to adding the special value to the set of possible values of
|
||||
a given type. This super set of <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
plus some <span class="emphasis"><em>nil_t</em></span>—where <code class="computeroutput"><span class="identifier">nil_t</span></code>
|
||||
is some stateless POD—can be modeled in modern languages as a <span class="bold"><strong>discriminated union</strong></span> of T and nil_t. Discriminated
|
||||
unions are often called <span class="emphasis"><em>variants</em></span>. A variant has a
|
||||
<span class="emphasis"><em>current type</em></span>, which in our case is either <code class="computeroutput"><span class="identifier">T</span></code> or <code class="computeroutput"><span class="identifier">nil_t</span></code>.
|
||||
Using the <a href="../../../../../variant/index.html" target="_top">Boost.Variant</a>
|
||||
library, this model can be implemented in terms of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">variant</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="identifier">nil_t</span><span class="special">></span></code>. There is precedent for a discriminated
|
||||
union as a model for an optional value: the <a href="http://www.haskell.org/" target="_top">Haskell</a>
|
||||
<span class="bold"><strong>Maybe</strong></span> built-in type constructor. Thus,
|
||||
a discriminated union <code class="computeroutput"><span class="identifier">T</span><span class="special">+</span><span class="identifier">nil_t</span></code>
|
||||
serves as a conceptual foundation.
|
||||
</p>
|
||||
<p>
|
||||
A <code class="computeroutput"><span class="identifier">variant</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="identifier">nil_t</span><span class="special">></span></code> follows naturally from the traditional
|
||||
idiom of extending the range of possible values adding an additional sentinel
|
||||
value with the special meaning of <span class="emphasis"><em>Nothing</em></span>. However,
|
||||
this additional <span class="emphasis"><em>Nothing</em></span> value is largely irrelevant
|
||||
for our purpose since our goal is to formalize the notion of uninitialized
|
||||
objects and, while a special extended value can be used to convey that
|
||||
meaning, it is not strictly necessary in order to do so.
|
||||
</p>
|
||||
<p>
|
||||
The observation made in the last paragraph about the irrelevant nature
|
||||
of the additional <code class="computeroutput"><span class="identifier">nil_t</span></code>
|
||||
with respect to <span class="underline">purpose</span> of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
suggests an alternative model: a <span class="emphasis"><em>container</em></span> that either
|
||||
has a value of <code class="computeroutput"><span class="identifier">T</span></code> or nothing.
|
||||
</p>
|
||||
<p>
|
||||
As of this writing I don't know of any precedent for a variable-size fixed-capacity
|
||||
(of 1) stack-based container model for optional values, yet I believe this
|
||||
is the consequence of the lack of practical implementations of such a container
|
||||
rather than an inherent shortcoming of the container model.
|
||||
</p>
|
||||
<p>
|
||||
In any event, both the discriminated-union or the single-element container
|
||||
models serve as a conceptual ground for a class representing optional—i.e.
|
||||
possibly uninitialized—objects. For instance, these models show the
|
||||
<span class="emphasis"><em>exact</em></span> semantics required for a wrapper of optional
|
||||
values:
|
||||
</p>
|
||||
<p>
|
||||
Discriminated-union:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>deep-copy</strong></span> semantics: copies of the
|
||||
variant implies copies of the value.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>deep-relational</strong></span> semantics: comparisons
|
||||
between variants matches both current types and values
|
||||
</li>
|
||||
<li class="listitem">
|
||||
If the variant's current type is <code class="computeroutput"><span class="identifier">T</span></code>,
|
||||
it is modeling an <span class="emphasis"><em>initialized</em></span> optional.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
If the variant's current type is not <code class="computeroutput"><span class="identifier">T</span></code>,
|
||||
it is modeling an <span class="emphasis"><em>uninitialized</em></span> optional.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Testing if the variant's current type is <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
models testing if the optional is initialized
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Trying to extract a <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
from a variant when its current type is not <code class="computeroutput"><span class="identifier">T</span></code>,
|
||||
models the undefined behavior of trying to access the value of an uninitialized
|
||||
optional
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
Single-element container:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>deep-copy</strong></span> semantics: copies of the
|
||||
container implies copies of the value.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>deep-relational</strong></span> semantics: comparisons
|
||||
between containers compare container size and if match, contained value
|
||||
</li>
|
||||
<li class="listitem">
|
||||
If the container is not empty (contains an object of type <code class="computeroutput"><span class="identifier">T</span></code>), it is modeling an <span class="emphasis"><em>initialized</em></span>
|
||||
optional.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
If the container is empty, it is modeling an <span class="emphasis"><em>uninitialized</em></span>
|
||||
optional.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Testing if the container is empty models testing if the optional is
|
||||
initialized
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Trying to extract a <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
from an empty container models the undefined behavior of trying to
|
||||
access the value of an uninitialized optional
|
||||
</li>
|
||||
</ul></div>
|
||||
</div>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
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<td align="center"><a href="../../../../../../../libs/libraries.htm">Libraries</a></td>
|
||||
<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
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||||
<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
|
||||
<td align="center"><a href="../../../../../../../more/index.htm">More</a></td>
|
||||
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||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="the_semantics.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../design_overview.html"><img src="../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="../when_to_use_optional.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h4 class="title">
|
||||
<a name="boost_optional.tutorial.design_overview.the_interface"></a><a class="link" href="the_interface.html" title="The Interface">The
|
||||
Interface</a>
|
||||
</h4></div></div></div>
|
||||
<p>
|
||||
Since the purpose of optional is to allow us to use objects with a formal
|
||||
uninitialized additional state, the interface could try to follow the interface
|
||||
of the underlying <code class="computeroutput"><span class="identifier">T</span></code> type
|
||||
as much as possible. In order to choose the proper degree of adoption of
|
||||
the native <code class="computeroutput"><span class="identifier">T</span></code> interface,
|
||||
the following must be noted: Even if all the operations supported by an
|
||||
instance of type <code class="computeroutput"><span class="identifier">T</span></code> are
|
||||
defined for the entire range of values for such a type, an <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
extends such a set of values with a new value for which most (otherwise
|
||||
valid) operations are not defined in terms of <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</p>
|
||||
<p>
|
||||
Furthermore, since <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> itself is merely a <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
wrapper (modeling a <code class="computeroutput"><span class="identifier">T</span></code> supertype),
|
||||
any attempt to define such operations upon uninitialized optionals will
|
||||
be totally artificial w.r.t. <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</p>
|
||||
<p>
|
||||
This library chooses an interface which follows from <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
interface only for those operations which are well defined (w.r.t the type
|
||||
<code class="computeroutput"><span class="identifier">T</span></code>) even if any of the operands
|
||||
are uninitialized. These operations include: construction, copy-construction,
|
||||
assignment, swap and relational operations.
|
||||
</p>
|
||||
<p>
|
||||
For the value access operations, which are undefined (w.r.t the type <code class="computeroutput"><span class="identifier">T</span></code>) when the operand is uninitialized,
|
||||
a different interface is chosen (which will be explained next).
|
||||
</p>
|
||||
<p>
|
||||
Also, the presence of the possibly uninitialized state requires additional
|
||||
operations not provided by <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
itself which are supported by a special interface.
|
||||
</p>
|
||||
<h6>
|
||||
<a name="boost_optional.tutorial.design_overview.the_interface.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.tutorial.design_overview.the_interface.lexically_hinted_value_access_in_the_presence_of_possibly_uninitialized_optional_objects__the_operators___and___gt_"></a></span><a class="link" href="the_interface.html#boost_optional.tutorial.design_overview.the_interface.lexically_hinted_value_access_in_the_presence_of_possibly_uninitialized_optional_objects__the_operators___and___gt_">Lexically-hinted
|
||||
Value Access in the presence of possibly uninitialized optional objects:
|
||||
The operators * and -></a>
|
||||
</h6>
|
||||
<p>
|
||||
A relevant feature of a pointer is that it can have a <span class="bold"><strong>null
|
||||
pointer value</strong></span>. This is a <span class="emphasis"><em>special</em></span> value
|
||||
which is used to indicate that the pointer is not referring to any object
|
||||
at all. In other words, null pointer values convey the notion of nonexistent
|
||||
objects.
|
||||
</p>
|
||||
<p>
|
||||
This meaning of the null pointer value allowed pointers to became a <span class="emphasis"><em>de
|
||||
facto</em></span> standard for handling optional objects because all you
|
||||
have to do to refer to a value which you don't really have is to use a
|
||||
null pointer value of the appropriate type. Pointers have been used for
|
||||
decades—from the days of C APIs to modern C++ libraries—to <span class="emphasis"><em>refer</em></span>
|
||||
to optional (that is, possibly nonexistent) objects; particularly as optional
|
||||
arguments to a function, but also quite often as optional data members.
|
||||
</p>
|
||||
<p>
|
||||
The possible presence of a null pointer value makes the operations that
|
||||
access the pointee's value possibly undefined, therefore, expressions which
|
||||
use dereference and access operators, such as: <code class="computeroutput"><span class="special">(</span>
|
||||
<span class="special">*</span><span class="identifier">p</span>
|
||||
<span class="special">=</span> <span class="number">2</span> <span class="special">)</span></code> and <code class="computeroutput"><span class="special">(</span>
|
||||
<span class="identifier">p</span><span class="special">-></span><span class="identifier">foo</span><span class="special">()</span> <span class="special">)</span></code>, implicitly convey the notion of optionality,
|
||||
and this information is tied to the <span class="emphasis"><em>syntax</em></span> of the
|
||||
expressions. That is, the presence of operators <code class="computeroutput"><span class="special">*</span></code>
|
||||
and <code class="computeroutput"><span class="special">-></span></code> tell by themselves
|
||||
—without any additional context— that the expression will be undefined
|
||||
unless the implied pointee actually exist.
|
||||
</p>
|
||||
<p>
|
||||
Such a <span class="emphasis"><em>de facto</em></span> idiom for referring to optional objects
|
||||
can be formalized in the form of a concept: the <a href="../../../../../../utility/OptionalPointee.html" target="_top"><code class="computeroutput"><span class="identifier">OptionalPointee</span></code></a> concept. This
|
||||
concept captures the syntactic usage of operators <code class="computeroutput"><span class="special">*</span></code>,
|
||||
<code class="computeroutput"><span class="special">-></span></code> and contextual conversion
|
||||
to <code class="computeroutput"><span class="keyword">bool</span></code> to convey the notion
|
||||
of optionality.
|
||||
</p>
|
||||
<p>
|
||||
However, pointers are good to <span class="underline">refer</span>
|
||||
to optional objects, but not particularly good to handle the optional objects
|
||||
in all other respects, such as initializing or moving/copying them. The
|
||||
problem resides in the shallow-copy of pointer semantics: if you need to
|
||||
effectively move or copy the object, pointers alone are not enough. The
|
||||
problem is that copies of pointers do not imply copies of pointees. For
|
||||
example, as was discussed in the motivation, pointers alone cannot be used
|
||||
to return optional objects from a function because the object must move
|
||||
outside from the function and into the caller's context.
|
||||
</p>
|
||||
<p>
|
||||
A solution to the shallow-copy problem that is often used is to resort
|
||||
to dynamic allocation and use a smart pointer to automatically handle the
|
||||
details of this. For example, if a function is to optionally return an
|
||||
object <code class="computeroutput"><span class="identifier">X</span></code>, it can use <code class="computeroutput"><span class="identifier">shared_ptr</span><span class="special"><</span><span class="identifier">X</span><span class="special">></span></code>
|
||||
as the return value. However, this requires dynamic allocation of <code class="computeroutput"><span class="identifier">X</span></code>. If <code class="computeroutput"><span class="identifier">X</span></code>
|
||||
is a built-in or small POD, this technique is very poor in terms of required
|
||||
resources. Optional objects are essentially values so it is very convenient
|
||||
to be able to use automatic storage and deep-copy semantics to manipulate
|
||||
optional values just as we do with ordinary values. Pointers do not have
|
||||
this semantics, so are inappropriate for the initialization and transport
|
||||
of optional values, yet are quite convenient for handling the access to
|
||||
the possible undefined value because of the idiomatic aid present in the
|
||||
<a href="../../../../../../utility/OptionalPointee.html" target="_top"><code class="computeroutput"><span class="identifier">OptionalPointee</span></code></a>
|
||||
concept incarnated by pointers.
|
||||
</p>
|
||||
<h6>
|
||||
<a name="boost_optional.tutorial.design_overview.the_interface.h1"></a>
|
||||
<span class="phrase"><a name="boost_optional.tutorial.design_overview.the_interface.optional_lt_t_gt__as_a_model_of_optionalpointee"></a></span><a class="link" href="the_interface.html#boost_optional.tutorial.design_overview.the_interface.optional_lt_t_gt__as_a_model_of_optionalpointee">Optional<T>
|
||||
as a model of OptionalPointee</a>
|
||||
</h6>
|
||||
<p>
|
||||
For value access operations <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code> uses operators <code class="computeroutput"><span class="special">*</span></code>
|
||||
and <code class="computeroutput"><span class="special">-></span></code> to lexically warn
|
||||
about the possibly uninitialized state appealing to the familiar pointer
|
||||
semantics w.r.t. to null pointers.
|
||||
</p>
|
||||
<div class="caution"><table border="0" summary="Caution">
|
||||
<tr>
|
||||
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Caution]" src="../../../../../../../doc/src/images/caution.png"></td>
|
||||
<th align="left">Caution</th>
|
||||
</tr>
|
||||
<tr><td align="left" valign="top"><p>
|
||||
However, it is particularly important to note that <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code> objects are not pointers. <span class="underline"><code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code> is not, and does not model, a
|
||||
pointer</span>.
|
||||
</p></td></tr>
|
||||
</table></div>
|
||||
<p>
|
||||
For instance, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code> does not have shallow-copy so does
|
||||
not alias: two different optionals never refer to the <span class="emphasis"><em>same</em></span>
|
||||
value unless <code class="computeroutput"><span class="identifier">T</span></code> itself is
|
||||
a reference (but may have <span class="emphasis"><em>equivalent</em></span> values). The
|
||||
difference between an <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> and a pointer must be kept in mind,
|
||||
particularly because the semantics of relational operators are different:
|
||||
since <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
is a value-wrapper, relational operators are deep: they compare optional
|
||||
values; but relational operators for pointers are shallow: they do not
|
||||
compare pointee values. As a result, you might be able to replace <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
by <code class="computeroutput"><span class="identifier">T</span><span class="special">*</span></code>
|
||||
on some situations but not always. Specifically, on generic code written
|
||||
for both, you cannot use relational operators directly, and must use the
|
||||
template functions <a href="../../../../../../utility/OptionalPointee.html#equal" target="_top"><code class="computeroutput"><span class="identifier">equal_pointees</span><span class="special">()</span></code></a>
|
||||
and <a href="../../../../../../utility/OptionalPointee.html#less" target="_top"><code class="computeroutput"><span class="identifier">less_pointees</span><span class="special">()</span></code></a>
|
||||
instead.
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="the_semantics.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../design_overview.html"><img src="../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="../when_to_use_optional.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a>
|
||||
</div>
|
||||
</body>
|
||||
</html>
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@@ -1,121 +0,0 @@
|
||||
<html>
|
||||
<head>
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||||
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
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<title>The semantics</title>
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<link rel="stylesheet" href="../../../../../../../doc/src/boostbook.css" type="text/css">
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<meta name="generator" content="DocBook XSL Stylesheets Vsnapshot">
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<link rel="home" href="../../../index.html" title="Boost.Optional">
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<link rel="up" href="../design_overview.html" title="Design Overview">
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<link rel="prev" href="../design_overview.html" title="Design Overview">
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<link rel="next" href="the_interface.html" title="The Interface">
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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<table cellpadding="2" width="100%"><tr>
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<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../../boost.png"></td>
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<td align="center"><a href="../../../../../../../index.html">Home</a></td>
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<td align="center"><a href="../../../../../../../libs/libraries.htm">Libraries</a></td>
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<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
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<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
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<td align="center"><a href="../../../../../../../more/index.htm">More</a></td>
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</tr></table>
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<hr>
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<div class="spirit-nav">
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||||
<a accesskey="p" href="../design_overview.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../design_overview.html"><img src="../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="the_interface.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a>
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||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h4 class="title">
|
||||
<a name="boost_optional.tutorial.design_overview.the_semantics"></a><a class="link" href="the_semantics.html" title="The semantics">The
|
||||
semantics</a>
|
||||
</h4></div></div></div>
|
||||
<p>
|
||||
Objects of type <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> are intended to be used in places where
|
||||
objects of type <code class="computeroutput"><span class="identifier">T</span></code> would
|
||||
but which might be uninitialized. Hence, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>'s purpose is to formalize the additional
|
||||
possibly uninitialized state. From the perspective of this role, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
can have the same operational semantics of <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
plus the additional semantics corresponding to this special state. As such,
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
could be thought of as a <span class="emphasis"><em>supertype</em></span> of <code class="computeroutput"><span class="identifier">T</span></code>. Of course, we can't do that in C++,
|
||||
so we need to compose the desired semantics using a different mechanism.
|
||||
Doing it the other way around, that is, making <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> a <span class="emphasis"><em>subtype</em></span> of
|
||||
<code class="computeroutput"><span class="identifier">T</span></code> is not only conceptually
|
||||
wrong but also impractical: it is not allowed to derive from a non-class
|
||||
type, such as a built-in type.
|
||||
</p>
|
||||
<p>
|
||||
We can draw from the purpose of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> the required basic semantics:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Default Construction:</strong></span> To introduce
|
||||
a formally uninitialized wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Direct Value Construction via copy:</strong></span>
|
||||
To introduce a formally initialized wrapped object whose value is obtained
|
||||
as a copy of some object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Deep Copy Construction:</strong></span> To obtain
|
||||
a new yet equivalent wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Direct Value Assignment (upon initialized):</strong></span>
|
||||
To assign a value to the wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Direct Value Assignment (upon uninitialized):</strong></span>
|
||||
To initialize the wrapped object with a value obtained as a copy of
|
||||
some object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Assignment (upon initialized):</strong></span> To
|
||||
assign to the wrapped object the value of another wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Assignment (upon uninitialized):</strong></span> To
|
||||
initialize the wrapped object with value of another wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Deep Relational Operations (when supported by
|
||||
the type T):</strong></span> To compare wrapped object values taking into
|
||||
account the presence of uninitialized states.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Value access:</strong></span> To unwrap the wrapped
|
||||
object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Initialization state query:</strong></span> To determine
|
||||
if the object is formally initialized or not.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Swap:</strong></span> To exchange wrapped objects.
|
||||
(with whatever exception safety guarantees are provided by <code class="computeroutput"><span class="identifier">T</span></code>'s swap).
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>De-initialization:</strong></span> To release the
|
||||
wrapped object (if any) and leave the wrapper in the uninitialized
|
||||
state.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
Additional operations are useful, such as converting constructors and converting
|
||||
assignments, in-place construction and assignment, and safe value access
|
||||
via a pointer to the wrapped object or null.
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="../design_overview.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../design_overview.html"><img src="../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="the_interface.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a>
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</div>
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||||
</body>
|
||||
</html>
|
||||
@@ -1,175 +0,0 @@
|
||||
<html>
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<div class="titlepage"><div><div><h3 class="title">
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<a name="boost_optional.tutorial.exception_safety_guarantees"></a><a class="link" href="exception_safety_guarantees.html" title="Exception Safety Guarantees">Exception
|
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Safety Guarantees</a>
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</h3></div></div></div>
|
||||
<p>
|
||||
This library assumes that <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
destructor does not throw exceptions. If it does, the behaviour of many operations
|
||||
on <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is
|
||||
undefined.
|
||||
</p>
|
||||
<p>
|
||||
The following mutating operations never throw exceptions:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">none_t</span>
|
||||
<span class="special">)</span> <span class="keyword">noexcept</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">reset</span><span class="special">()</span> <span class="keyword">noexcept</span></code>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
In addition, the following constructors and the destructor never throw exceptions:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">optional</span><span class="special">()</span>
|
||||
<span class="keyword">noexcept</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">optional</span><span class="special">(</span>
|
||||
<span class="identifier">none_t</span> <span class="special">)</span>
|
||||
<span class="keyword">noexcept</span></code>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
Regarding the following assignment functions:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">InPlaceFactory</span><span class="special">></span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">InPlaceFactory</span>
|
||||
<span class="keyword">const</span><span class="special">&</span>
|
||||
<span class="special">)</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">TypedInPlaceFactory</span><span class="special">></span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">TypedInPlaceFactory</span>
|
||||
<span class="keyword">const</span><span class="special">&</span>
|
||||
<span class="special">)</span> </code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">reset</span><span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span></code>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
They forward calls to the corresponding <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
constructors or assignments (depending on whether the optional object is
|
||||
initialized or not); so if both <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
constructor and the assignment provide strong exception safety guarantee,
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>'s
|
||||
assignment also provides strong exception safety guarantee; otherwise we
|
||||
only get the basic guarantee. Additionally, if both involved <code class="computeroutput"><span class="identifier">T</span></code>'s constructor and the assignment never
|
||||
throw, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>'s
|
||||
assignment also never throws.
|
||||
</p>
|
||||
<p>
|
||||
Unless <code class="computeroutput"><span class="identifier">T</span></code>'s constructor or
|
||||
assignment throws, assignments to <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
do not throw anything else on its own. A throw during assignment never changes
|
||||
the initialization state of any optional object involved:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">opt1</span><span class="special">(</span><span class="identifier">val1</span><span class="special">);</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">opt2</span><span class="special">(</span><span class="identifier">val2</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">opt1</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">opt2</span><span class="special">);</span>
|
||||
|
||||
<span class="keyword">try</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">opt1</span> <span class="special">=</span> <span class="identifier">opt2</span><span class="special">;</span> <span class="comment">// throws</span>
|
||||
<span class="special">}</span>
|
||||
<span class="keyword">catch</span><span class="special">(...)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">opt1</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">opt2</span><span class="special">);</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
This also applies to move assignments/constructors. However, move operations
|
||||
are made no-throw more often.
|
||||
</p>
|
||||
<p>
|
||||
Operation <code class="computeroutput"><span class="identifier">emplace</span></code> provides
|
||||
basic exception safety guarantee. If it throws, the optional object becomes
|
||||
uninitialized regardless of its initial state, and its previous contained
|
||||
value (if any) is destroyed. It doesn't call any assignment or move/copy
|
||||
constructor on <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</p>
|
||||
<h5>
|
||||
<a name="boost_optional.tutorial.exception_safety_guarantees.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.tutorial.exception_safety_guarantees.swap"></a></span><a class="link" href="exception_safety_guarantees.html#boost_optional.tutorial.exception_safety_guarantees.swap">Swap</a>
|
||||
</h5>
|
||||
<p>
|
||||
Unless <code class="computeroutput"><span class="identifier">swap</span></code> on optional is
|
||||
customized, its primary implementation forwards calls to <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
<code class="computeroutput"><span class="identifier">swap</span></code> or move constructor
|
||||
(depending on the initialization state of the optional objects). Thus, if
|
||||
both <code class="computeroutput"><span class="identifier">T</span></code>'s <code class="computeroutput"><span class="identifier">swap</span></code>
|
||||
and move constructor never throw, <code class="computeroutput"><span class="identifier">swap</span></code>
|
||||
on <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> never
|
||||
throws. similarly, if both <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
<code class="computeroutput"><span class="identifier">swap</span></code> and move constructor
|
||||
offer strong guarantee, <code class="computeroutput"><span class="identifier">swap</span></code>
|
||||
on <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> also
|
||||
offers a strong guarantee.
|
||||
</p>
|
||||
<p>
|
||||
In case <code class="computeroutput"><span class="identifier">swap</span></code> on optional
|
||||
is customized, the call to <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
move constructor are replaced with the calls to <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
default constructor followed by <code class="computeroutput"><span class="identifier">swap</span></code>.
|
||||
(This is more useful on older compilers that do not support move semantics,
|
||||
when one wants to achieve stronger exception safety guarantees.) In this
|
||||
case the exception safety guarantees for <code class="computeroutput"><span class="identifier">swap</span></code>
|
||||
are reliant on the guarantees of <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
<code class="computeroutput"><span class="identifier">swap</span></code> and default constructor
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
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<div class="spirit-nav">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="boost_optional.tutorial.gotchas"></a><a class="link" href="gotchas.html" title="Gotchas">Gotchas</a>
|
||||
</h3></div></div></div>
|
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<div class="section">
|
||||
<div class="titlepage"><div><div><h4 class="title">
|
||||
<a name="boost_optional.tutorial.gotchas.a_note_about_optional_bool_"></a><a class="link" href="gotchas.html#boost_optional.tutorial.gotchas.a_note_about_optional_bool_" title="A note about optional<bool>">A
|
||||
note about optional<bool></a>
|
||||
</h4></div></div></div>
|
||||
<p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span></code>
|
||||
should be used with special caution and consideration.
|
||||
</p>
|
||||
<p>
|
||||
First, it is functionally similar to a tristate boolean (false, maybe,
|
||||
true) —such as <a href="../../../../../../doc/html/tribool.html" target="_top">boost::tribool</a>—
|
||||
except that in a tristate boolean, the maybe state <span class="underline">represents
|
||||
a valid value</span>, unlike the corresponding state of an uninitialized
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span></code>.
|
||||
It should be carefully considered if an <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span></code> instead of a <code class="computeroutput"><span class="identifier">tribool</span></code>
|
||||
is really needed.
|
||||
</p>
|
||||
<p>
|
||||
Second, although <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code> provides a contextual conversion
|
||||
to <code class="computeroutput"><span class="keyword">bool</span></code> in C++11, this falls
|
||||
back to an implicit conversion on older compilers. This conversion refers
|
||||
to the initialization state and not to the contained value. Using <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span></code>
|
||||
can lead to subtle errors due to the implicit <code class="computeroutput"><span class="keyword">bool</span></code>
|
||||
conversion:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">foo</span> <span class="special">(</span> <span class="keyword">bool</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="keyword">void</span> <span class="identifier">bar</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">v</span> <span class="special">=</span> <span class="keyword">try</span><span class="special">();</span>
|
||||
|
||||
<span class="comment">// The following intended to pass the value of 'v' to foo():</span>
|
||||
<span class="identifier">foo</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span>
|
||||
<span class="comment">// But instead, the initialization state is passed</span>
|
||||
<span class="comment">// due to a typo: it should have been foo(*v).</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
The only implicit conversion is to <code class="computeroutput"><span class="keyword">bool</span></code>,
|
||||
and it is safe in the sense that typical integral promotions don't apply
|
||||
(i.e. if <code class="computeroutput"><span class="identifier">foo</span><span class="special">()</span></code>
|
||||
takes an <code class="computeroutput"><span class="keyword">int</span></code> instead, it won't
|
||||
compile).
|
||||
</p>
|
||||
<p>
|
||||
Third, mixed comparisons with <code class="computeroutput"><span class="keyword">bool</span></code>
|
||||
work differently than similar mixed comparisons between pointers and <code class="computeroutput"><span class="keyword">bool</span></code>, so the results might surprise you:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">oEmpty</span><span class="special">(</span><span class="identifier">none</span><span class="special">),</span> <span class="identifier">oTrue</span><span class="special">(</span><span class="keyword">true</span><span class="special">),</span> <span class="identifier">oFalse</span><span class="special">(</span><span class="keyword">false</span><span class="special">);</span>
|
||||
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oEmpty</span> <span class="special">==</span> <span class="identifier">none</span><span class="special">);</span> <span class="comment">// renders true</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oEmpty</span> <span class="special">==</span> <span class="keyword">false</span><span class="special">);</span> <span class="comment">// renders false!</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oEmpty</span> <span class="special">==</span> <span class="keyword">true</span><span class="special">);</span> <span class="comment">// renders false!</span>
|
||||
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oFalse</span> <span class="special">==</span> <span class="identifier">none</span><span class="special">);</span> <span class="comment">// renders false</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oFalse</span> <span class="special">==</span> <span class="keyword">false</span><span class="special">);</span> <span class="comment">// renders true!</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oFalse</span> <span class="special">==</span> <span class="keyword">true</span><span class="special">);</span> <span class="comment">// renders false</span>
|
||||
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oTrue</span> <span class="special">==</span> <span class="identifier">none</span><span class="special">);</span> <span class="comment">// renders false</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oTrue</span> <span class="special">==</span> <span class="keyword">false</span><span class="special">);</span> <span class="comment">// renders false</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oTrue</span> <span class="special">==</span> <span class="keyword">true</span><span class="special">);</span> <span class="comment">// renders true</span>
|
||||
</pre>
|
||||
<p>
|
||||
In other words, for <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code>, the following assertion does not
|
||||
hold:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">assert</span><span class="special">((</span><span class="identifier">opt</span> <span class="special">==</span> <span class="keyword">false</span><span class="special">)</span> <span class="special">==</span> <span class="special">(!</span><span class="identifier">opt</span><span class="special">));</span>
|
||||
</pre>
|
||||
</div>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
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<a accesskey="p" href="in_place_factories.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="gotchas/moved_from__optional_.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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|
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|
||||
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|
||||
-77
@@ -1,77 +0,0 @@
|
||||
<html>
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
|
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|
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|
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|
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<div class="section">
|
||||
<div class="titlepage"><div><div><h4 class="title">
|
||||
<a name="boost_optional.tutorial.gotchas.false_positive_with__wmaybe_uninitialized"></a><a class="link" href="false_positive_with__wmaybe_uninitialized.html" title="False positive with -Wmaybe-uninitialized">False
|
||||
positive with -Wmaybe-uninitialized</a>
|
||||
</h4></div></div></div>
|
||||
<p>
|
||||
Sometimes on GCC compilers below version 5.1 you may get an -Wmaybe-uninitialized
|
||||
warning when compiling with option -02 on a perfectly valid <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span></code> usage. For instance in this
|
||||
program:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">optional</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
|
||||
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">getitem</span><span class="special">();</span>
|
||||
|
||||
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">argc</span><span class="special">,</span> <span class="keyword">const</span> <span class="keyword">char</span> <span class="special">*[])</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">a</span> <span class="special">=</span> <span class="identifier">getitem</span><span class="special">();</span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">b</span><span class="special">;</span>
|
||||
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">argc</span> <span class="special">></span> <span class="number">0</span><span class="special">)</span>
|
||||
<span class="identifier">b</span> <span class="special">=</span> <span class="identifier">argc</span><span class="special">;</span>
|
||||
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">a</span> <span class="special">!=</span> <span class="identifier">b</span><span class="special">)</span>
|
||||
<span class="keyword">return</span> <span class="number">1</span><span class="special">;</span>
|
||||
|
||||
<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
This is a bug in the compiler. As a workaround (provided in <a href="http://stackoverflow.com/questions/21755206/how-to-get-around-gcc-void-b-4-may-be-used-uninitialized-in-this-funct" target="_top">this
|
||||
Stack Overflow question</a>) use the following way of initializing
|
||||
an optional containing no value:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">b</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">make_optional</span><span class="special">(</span><span class="keyword">false</span><span class="special">,</span> <span class="keyword">int</span><span class="special">());</span>
|
||||
</pre>
|
||||
<p>
|
||||
This is obviously redundant, but makes the warning disappear.
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="mixed_relational_comparisons.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../gotchas.html"><img src="../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="../exception_safety_guarantees.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a>
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|
||||
@@ -1,59 +0,0 @@
|
||||
<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
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</div>
|
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<div class="section">
|
||||
<div class="titlepage"><div><div><h4 class="title">
|
||||
<a name="boost_optional.tutorial.gotchas.mixed_relational_comparisons"></a><a class="link" href="mixed_relational_comparisons.html" title="Mixed relational comparisons">Mixed
|
||||
relational comparisons</a>
|
||||
</h4></div></div></div>
|
||||
<p>
|
||||
Because <code class="computeroutput"><span class="identifier">T</span></code> is convertible
|
||||
to <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
and because <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
is <a href="http://www.sgi.com/tech/stl/LessThanComparable.html" target="_top"><code class="computeroutput"><span class="identifier">LessThanComparable</span></code></a> when <code class="computeroutput"><span class="identifier">T</span></code> is <a href="http://www.sgi.com/tech/stl/LessThanComparable.html" target="_top"><code class="computeroutput"><span class="identifier">LessThanComparable</span></code></a>, you can sometimes
|
||||
get an unexpected runtime result where you would rather expect a compiler
|
||||
error:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span> <span class="identifier">Flight_plan</span><span class="special">::</span><span class="identifier">weight</span><span class="special">();</span> <span class="comment">// sometimes no weight can be returned</span>
|
||||
|
||||
<span class="keyword">bool</span> <span class="identifier">is_aircraft_too_heavy</span><span class="special">(</span><span class="identifier">Flight_plan</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">p</span><span class="special">)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">return</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">weight</span><span class="special">()</span> <span class="special">></span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">aircraft</span><span class="special">().</span><span class="identifier">max_weight</span><span class="special">();</span> <span class="comment">// compiles!</span>
|
||||
<span class="special">}</span> <span class="comment">// returns false when the optional contains no value </span>
|
||||
</pre>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="moved_from__optional_.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../gotchas.html"><img src="../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="false_positive_with__wmaybe_uninitialized.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a>
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|
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|
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</html>
|
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@@ -1,63 +0,0 @@
|
||||
<html>
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
|
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<title>Moved-from optional</title>
|
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<meta name="generator" content="DocBook XSL Stylesheets Vsnapshot">
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<a accesskey="p" href="../gotchas.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../gotchas.html"><img src="../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="mixed_relational_comparisons.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a>
|
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</div>
|
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<div class="section">
|
||||
<div class="titlepage"><div><div><h4 class="title">
|
||||
<a name="boost_optional.tutorial.gotchas.moved_from__optional_"></a><a class="link" href="moved_from__optional_.html" title="Moved-from optional">Moved-from
|
||||
<code class="computeroutput"><span class="identifier">optional</span></code></a>
|
||||
</h4></div></div></div>
|
||||
<p>
|
||||
When an optional object that contains a value is moved from (is a source
|
||||
of move constructor or assignment) it still contains a value and its contained
|
||||
value is left in a moved-from state. This can be illustrated with the following
|
||||
example.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">unique_ptr</span><span class="special"><</span><span class="keyword">int</span><span class="special">>></span> <span class="identifier">opi</span> <span class="special">{</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">make_unique</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="number">1</span><span class="special">)};</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">unique_ptr</span><span class="special"><</span><span class="keyword">int</span><span class="special">>></span> <span class="identifier">opj</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(</span><span class="identifier">opi</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span><span class="identifier">opi</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(*</span><span class="identifier">opi</span> <span class="special">==</span> <span class="keyword">nullptr</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
Quite a lot of people expect that when an object that contains a value
|
||||
is moved from, its contained value should be destroyed. This is not so,
|
||||
for performance reasons. Current semantics allow the implementation of
|
||||
<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
to be trivially copyable when <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
is trivial.
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
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<a name="boost_optional.tutorial.in_place_factories"></a><a class="link" href="in_place_factories.html" title="In-Place Factories">In-Place
|
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Factories</a>
|
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</h3></div></div></div>
|
||||
<p>
|
||||
One of the typical problems with wrappers and containers is that their interfaces
|
||||
usually provide an operation to initialize or assign the contained object
|
||||
as a copy of some other object. This not only requires the underlying type
|
||||
to be <a href="../../../../../utility/CopyConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">CopyConstructible</span></code></a>, but also requires
|
||||
the existence of a fully constructed object, often temporary, just to follow
|
||||
the copy from:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">X</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">X</span> <span class="special">(</span> <span class="keyword">int</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">}</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">class</span> <span class="identifier">W</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">public</span><span class="special">:</span>
|
||||
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">X</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span> <span class="special">)</span> <span class="special">:</span> <span class="identifier">wrapped_</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">{}</span>
|
||||
<span class="special">}</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="comment">// Temporary object created.</span>
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">X</span><span class="special">(</span><span class="number">123</span><span class="special">,</span><span class="string">"hello"</span><span class="special">)</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
A solution to this problem is to support direct construction of the contained
|
||||
object right in the container's storage. In this scheme, the user only needs
|
||||
to supply the arguments to the constructor to use in the wrapped object construction.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">W</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">public</span><span class="special">:</span>
|
||||
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">X</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span> <span class="special">)</span> <span class="special">:</span> <span class="identifier">wrapped_</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">{}</span>
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="keyword">int</span> <span class="identifier">a0</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">a1</span><span class="special">)</span> <span class="special">:</span> <span class="identifier">wrapped_</span><span class="special">(</span><span class="identifier">a0</span><span class="special">,</span><span class="identifier">a1</span><span class="special">)</span> <span class="special">{}</span>
|
||||
<span class="special">}</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="comment">// Wrapped object constructed in-place</span>
|
||||
<span class="comment">// No temporary created.</span>
|
||||
<span class="identifier">W</span> <span class="special">(</span><span class="number">123</span><span class="special">,</span><span class="string">"hello"</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
A limitation of this method is that it doesn't scale well to wrapped objects
|
||||
with multiple constructors nor to generic code were the constructor overloads
|
||||
are unknown.
|
||||
</p>
|
||||
<p>
|
||||
The solution presented in this library is the family of <span class="bold"><strong>InPlaceFactories</strong></span>
|
||||
and <span class="bold"><strong>TypedInPlaceFactories</strong></span>. These factories
|
||||
are a family of classes which encapsulate an increasing number of arbitrary
|
||||
constructor parameters and supply a method to construct an object of a given
|
||||
type using those parameters at an address specified by the user via placement
|
||||
new.
|
||||
</p>
|
||||
<p>
|
||||
For example, one member of this family looks like:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span><span class="keyword">class</span> <span class="identifier">A0</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">A1</span><span class="special">></span>
|
||||
<span class="keyword">class</span> <span class="identifier">TypedInPlaceFactory2</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">A0</span> <span class="identifier">m_a0</span> <span class="special">;</span> <span class="identifier">A1</span> <span class="identifier">m_a1</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">public</span><span class="special">:</span>
|
||||
|
||||
<span class="identifier">TypedInPlaceFactory2</span><span class="special">(</span> <span class="identifier">A0</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">a0</span><span class="special">,</span> <span class="identifier">A1</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">a1</span> <span class="special">)</span> <span class="special">:</span> <span class="identifier">m_a0</span><span class="special">(</span><span class="identifier">a0</span><span class="special">),</span> <span class="identifier">m_a1</span><span class="special">(</span><span class="identifier">a1</span><span class="special">)</span> <span class="special">{}</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">construct</span> <span class="special">(</span> <span class="keyword">void</span><span class="special">*</span> <span class="identifier">p</span> <span class="special">)</span> <span class="special">{</span> <span class="keyword">new</span> <span class="special">(</span><span class="identifier">p</span><span class="special">)</span> <span class="identifier">T</span><span class="special">(</span><span class="identifier">m_a0</span><span class="special">,</span><span class="identifier">m_a1</span><span class="special">)</span> <span class="special">;</span> <span class="special">}</span>
|
||||
<span class="special">}</span> <span class="special">;</span>
|
||||
</pre>
|
||||
<p>
|
||||
A wrapper class aware of this can use it as:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">W</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">public</span><span class="special">:</span>
|
||||
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">X</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span> <span class="special">)</span> <span class="special">:</span> <span class="identifier">wrapped_</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">{}</span>
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">TypedInPlaceFactory2</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">fac</span> <span class="special">)</span> <span class="special">{</span> <span class="identifier">fac</span><span class="special">.</span><span class="identifier">construct</span><span class="special">(&</span><span class="identifier">wrapped_</span><span class="special">)</span> <span class="special">;</span> <span class="special">}</span>
|
||||
<span class="special">}</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="comment">// Wrapped object constructed in-place via a TypedInPlaceFactory.</span>
|
||||
<span class="comment">// No temporary created.</span>
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">TypedInPlaceFactory2</span><span class="special"><</span><span class="identifier">X</span><span class="special">,</span><span class="keyword">int</span><span class="special">,</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">>(</span><span class="number">123</span><span class="special">,</span><span class="string">"hello"</span><span class="special">))</span> <span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
The factories are divided in two groups:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="underline">TypedInPlaceFactories</span>: those which
|
||||
take the target type as a primary template parameter.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="underline">InPlaceFactories</span>: those with a
|
||||
template <code class="computeroutput"><span class="identifier">construct</span><span class="special">(</span><span class="keyword">void</span><span class="special">*)</span></code>
|
||||
member function taking the target type.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
Within each group, all the family members differ only in the number of parameters
|
||||
allowed.
|
||||
</p>
|
||||
<p>
|
||||
This library provides an overloaded set of helper template functions to construct
|
||||
these factories without requiring unnecessary template parameters:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">A0</span><span class="special">,...,</span><span class="keyword">class</span> <span class="identifier">AN</span><span class="special">></span>
|
||||
<span class="identifier">InPlaceFactoryN</span> <span class="special"><</span><span class="identifier">A0</span><span class="special">,...,</span><span class="identifier">AN</span><span class="special">></span> <span class="identifier">in_place</span> <span class="special">(</span> <span class="identifier">A0</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">a0</span><span class="special">,</span> <span class="special">...,</span> <span class="identifier">AN</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">aN</span><span class="special">)</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span><span class="keyword">class</span> <span class="identifier">A0</span><span class="special">,...,</span><span class="keyword">class</span> <span class="identifier">AN</span><span class="special">></span>
|
||||
<span class="identifier">TypedInPlaceFactoryN</span> <span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="identifier">A0</span><span class="special">,...,</span><span class="identifier">AN</span><span class="special">></span> <span class="identifier">in_place</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">a0</span><span class="special">,</span> <span class="identifier">A0</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">a0</span><span class="special">,</span> <span class="special">...,</span> <span class="identifier">AN</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">aN</span><span class="special">)</span> <span class="special">;</span>
|
||||
</pre>
|
||||
<p>
|
||||
In-place factories can be used generically by the wrapper and user as follows:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">W</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">public</span><span class="special">:</span>
|
||||
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">X</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span> <span class="special">)</span> <span class="special">:</span> <span class="identifier">wrapped_</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">{}</span>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span> <span class="keyword">class</span> <span class="identifier">InPlaceFactory</span> <span class="special">></span>
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">InPlaceFactory</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">fac</span> <span class="special">)</span> <span class="special">{</span> <span class="identifier">fac</span><span class="special">.</span><span class="keyword">template</span> <span class="special"><</span><span class="identifier">X</span><span class="special">></span><span class="identifier">construct</span><span class="special">(&</span><span class="identifier">wrapped_</span><span class="special">)</span> <span class="special">;</span> <span class="special">}</span>
|
||||
|
||||
<span class="special">}</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="comment">// Wrapped object constructed in-place via a InPlaceFactory.</span>
|
||||
<span class="comment">// No temporary created.</span>
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">in_place</span><span class="special">(</span><span class="number">123</span><span class="special">,</span><span class="string">"hello"</span><span class="special">)</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
The factories are implemented in the headers: <a href="../../../../../../boost/utility/in_place_factory.hpp" target="_top">in_place_factory.hpp</a>
|
||||
and <a href="../../../../../../boost/utility/typed_in_place_factory.hpp" target="_top">typed_in_place_factory.hpp</a>
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
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|
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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||||
<a name="boost_optional.tutorial.io_operators"></a><a class="link" href="io_operators.html" title="IO operators">IO operators</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
It is possible to use <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
with IO streams, provided that <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
can be used with streams. IOStream operators are defined in a separate header.
|
||||
</p>
|
||||
<p>
|
||||
</p>
|
||||
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span>
|
||||
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">optional</span><span class="special">/</span><span class="identifier">optional_io</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
|
||||
|
||||
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">o1</span> <span class="special">=</span> <span class="number">1</span><span class="special">,</span> <span class="identifier">oN</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span><span class="special">;</span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">o1</span><span class="special">;</span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cin</span> <span class="special">>></span> <span class="identifier">oN</span><span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
</p>
|
||||
<p>
|
||||
The current implementation does not guarantee any particular output. What
|
||||
it guarantees is that if streaming out and then back in <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
gives the same value, then streaming out and then back in <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
will also give back the same result:
|
||||
</p>
|
||||
<p>
|
||||
</p>
|
||||
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">cassert</span><span class="special">></span>
|
||||
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">sstream</span><span class="special">></span>
|
||||
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">optional</span><span class="special">/</span><span class="identifier">optional_io</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
|
||||
|
||||
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">o1</span> <span class="special">=</span> <span class="number">1</span><span class="special">,</span> <span class="identifier">oN</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span><span class="special">;</span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">x1</span><span class="special">,</span> <span class="identifier">x2</span><span class="special">;</span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">stringstream</span> <span class="identifier">s</span><span class="special">;</span>
|
||||
<span class="identifier">s</span> <span class="special"><<</span> <span class="identifier">o1</span> <span class="special"><<</span> <span class="identifier">oN</span><span class="special">;</span>
|
||||
<span class="identifier">s</span> <span class="special">>></span> <span class="identifier">x1</span> <span class="special">>></span> <span class="identifier">x2</span><span class="special">;</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span><span class="identifier">o1</span> <span class="special">==</span> <span class="identifier">x1</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span><span class="identifier">oN</span> <span class="special">==</span> <span class="identifier">x2</span><span class="special">);</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="relational_operators.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="optional_references.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
|
||||
</div>
|
||||
</body>
|
||||
</html>
|
||||
@@ -1,129 +0,0 @@
|
||||
<html>
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
|
||||
<title>Motivation</title>
|
||||
<link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css">
|
||||
<meta name="generator" content="DocBook XSL Stylesheets V1.78.1">
|
||||
<link rel="home" href="../../index.html" title="Chapter 1. Boost.Optional">
|
||||
<link rel="up" href="../../optional/tutorial.html" title="Tutorial">
|
||||
<link rel="prev" href="../../optional/tutorial.html" title="Tutorial">
|
||||
<link rel="next" href="design_overview.html" title="Design Overview">
|
||||
</head>
|
||||
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
|
||||
<table cellpadding="2" width="100%"><tr>
|
||||
<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td>
|
||||
<td align="center"><a href="../../../../../../index.html">Home</a></td>
|
||||
<td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td>
|
||||
<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
|
||||
<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
|
||||
<td align="center"><a href="../../../../../../more/index.htm">More</a></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="design_overview.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.tutorial.motivation"></a><a class="link" href="motivation.html" title="Motivation">Motivation</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
Consider these functions which should return a value but which might not
|
||||
have a value to return:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
(A) <code class="computeroutput"><span class="keyword">double</span> <span class="identifier">sqrt</span><span class="special">(</span><span class="keyword">double</span> <span class="identifier">n</span> <span class="special">);</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
(B) <code class="computeroutput"><span class="keyword">char</span> <span class="identifier">get_async_input</span><span class="special">();</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
(C) <code class="computeroutput"><span class="identifier">point</span> <span class="identifier">polygon</span><span class="special">::</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span></code>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
There are different approaches to the issue of not having a value to return.
|
||||
</p>
|
||||
<p>
|
||||
A typical approach is to consider the existence of a valid return value as
|
||||
a postcondition, so that if the function cannot compute the value to return,
|
||||
it has either undefined behavior (and can use assert in a debug build) or
|
||||
uses a runtime check and throws an exception if the postcondition is violated.
|
||||
This is a reasonable choice for example, for function (A), because the lack
|
||||
of a proper return value is directly related to an invalid parameter (out
|
||||
of domain argument), so it is appropriate to require the callee to supply
|
||||
only parameters in a valid domain for execution to continue normally.
|
||||
</p>
|
||||
<p>
|
||||
However, function (B), because of its asynchronous nature, does not fail
|
||||
just because it can't find a value to return; so it is incorrect to consider
|
||||
such a situation an error and assert or throw an exception. This function
|
||||
must return, and somehow, must tell the callee that it is not returning a
|
||||
meaningful value.
|
||||
</p>
|
||||
<p>
|
||||
A similar situation occurs with function (C): it is conceptually an error
|
||||
to ask a <span class="emphasis"><em>null-area</em></span> polygon to return a point inside
|
||||
itself, but in many applications, it is just impractical for performance
|
||||
reasons to treat this as an error (because detecting that the polygon has
|
||||
no area might be too expensive to be required to be tested previously), and
|
||||
either an arbitrary point (typically at infinity) is returned, or some efficient
|
||||
way to tell the callee that there is no such point is used.
|
||||
</p>
|
||||
<p>
|
||||
There are various mechanisms to let functions communicate that the returned
|
||||
value is not valid. One such mechanism, which is quite common since it has
|
||||
zero or negligible overhead, is to use a special value which is reserved
|
||||
to communicate this. Classical examples of such special values are <code class="computeroutput"><span class="identifier">EOF</span></code>, <code class="computeroutput"><span class="identifier">string</span><span class="special">::</span><span class="identifier">npos</span></code>,
|
||||
points at infinity, etc...
|
||||
</p>
|
||||
<p>
|
||||
When those values exist, i.e. the return type can hold all meaningful values
|
||||
<span class="emphasis"><em>plus</em></span> the <span class="emphasis"><em>signal</em></span> value, this mechanism
|
||||
is quite appropriate and well known. Unfortunately, there are cases when
|
||||
such values do not exist. In these cases, the usual alternative is either
|
||||
to use a wider type, such as <code class="computeroutput"><span class="keyword">int</span></code>
|
||||
in place of <code class="computeroutput"><span class="keyword">char</span></code>; or a compound
|
||||
type, such as <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span></code>.
|
||||
</p>
|
||||
<p>
|
||||
Returning a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span></code>, thus attaching a boolean flag to the
|
||||
result which indicates if the result is meaningful, has the advantage that
|
||||
can be turned into a consistent idiom since the first element of the pair
|
||||
can be whatever the function would conceptually return. For example, the
|
||||
last two functions could have the following interface:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="keyword">char</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">get_async_input</span><span class="special">();</span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">polygon</span><span class="special">::</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span>
|
||||
</pre>
|
||||
<p>
|
||||
These functions use a consistent interface for dealing with possibly nonexistent
|
||||
results:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">poly</span><span class="special">.</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">second</span> <span class="special">)</span>
|
||||
<span class="identifier">flood_fill</span><span class="special">(</span><span class="identifier">p</span><span class="special">.</span><span class="identifier">first</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
However, not only is this quite a burden syntactically, it is also error
|
||||
prone since the user can easily use the function result (first element of
|
||||
the pair) without ever checking if it has a valid value.
|
||||
</p>
|
||||
<p>
|
||||
Clearly, we need a better idiom.
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="design_overview.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
|
||||
</div>
|
||||
</body>
|
||||
</html>
|
||||
@@ -1,117 +0,0 @@
|
||||
<html>
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
|
||||
<title>Optional references</title>
|
||||
<link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css">
|
||||
<meta name="generator" content="DocBook XSL Stylesheets Vsnapshot">
|
||||
<link rel="home" href="../../index.html" title="Boost.Optional">
|
||||
<link rel="up" href="../../optional/tutorial.html" title="Tutorial">
|
||||
<link rel="prev" href="io_operators.html" title="IO operators">
|
||||
<link rel="next" href="optional_references/rebinding_semantics_for_assignment_of_optional_references.html" title="Rebinding semantics for assignment of optional references">
|
||||
</head>
|
||||
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
|
||||
<table cellpadding="2" width="100%"><tr>
|
||||
<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td>
|
||||
<td align="center"><a href="../../../../../../index.html">Home</a></td>
|
||||
<td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td>
|
||||
<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
|
||||
<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
|
||||
<td align="center"><a href="../../../../../../more/index.htm">More</a></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="io_operators.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="optional_references/rebinding_semantics_for_assignment_of_optional_references.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.tutorial.optional_references"></a><a class="link" href="optional_references.html" title="Optional references">Optional
|
||||
references</a>
|
||||
</h3></div></div></div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h4 class="title">
|
||||
<a name="boost_optional.tutorial.optional_references.overview"></a><a class="link" href="optional_references.html#boost_optional.tutorial.optional_references.overview" title="Overview">Overview</a>
|
||||
</h4></div></div></div>
|
||||
<p>
|
||||
This library allows the template parameter <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
to be of reference type: <code class="computeroutput"><span class="identifier">T</span><span class="special">&</span></code>, and to some extent, <code class="computeroutput"><span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span></code>.
|
||||
</p>
|
||||
<p>
|
||||
However, since references are not real objects some restrictions apply
|
||||
and some operations are not available in this case:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
Converting constructors
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Converting assignment
|
||||
</li>
|
||||
<li class="listitem">
|
||||
InPlace construction
|
||||
</li>
|
||||
<li class="listitem">
|
||||
InPlace assignment
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Value-access via pointer
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
Also, even though <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code> treats it wrapped pseudo-object
|
||||
much as a real value, a true real reference is stored so aliasing will
|
||||
occur:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
Copies of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code> will copy the references but
|
||||
all these references will nonetheless refer to the same object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Value-access will actually provide access to the referenced object
|
||||
rather than the reference itself.
|
||||
</li>
|
||||
</ul></div>
|
||||
<div class="caution"><table border="0" summary="Caution">
|
||||
<tr>
|
||||
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Caution]" src="../../../../../../doc/src/images/caution.png"></td>
|
||||
<th align="left">Caution</th>
|
||||
</tr>
|
||||
<tr><td align="left" valign="top"><p>
|
||||
On compilers that do not conform to Standard C++ rules of reference binding,
|
||||
some operations on optional references are disabled in order to prevent
|
||||
subtle bugs. For more details see <a class="link" href="../dependencies_and_portability/optional_reference_binding.html" title="Optional Reference Binding">Dependencies
|
||||
and Portability section</a>.
|
||||
</p></td></tr>
|
||||
</table></div>
|
||||
<h6>
|
||||
<a name="boost_optional.tutorial.optional_references.overview.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.tutorial.optional_references.overview.rvalue_references"></a></span><a class="link" href="optional_references.html#boost_optional.tutorial.optional_references.overview.rvalue_references">Rvalue
|
||||
references</a>
|
||||
</h6>
|
||||
<p>
|
||||
Rvalue references and lvalue references to const have the ability in C++
|
||||
to extend the life time of a temporary they bind to. Optional references
|
||||
do not have this capability, therefore to avoid surprising effects it is
|
||||
not possible to initialize an optional references from a temporary. Optional
|
||||
rvalue references are disabled altogether. Also, the initialization and
|
||||
assignment of an optional reference to const from rvalue reference is disabled.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">1</span><span class="special">;</span> <span class="comment">// legal</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&></span> <span class="identifier">oi</span> <span class="special">=</span> <span class="number">1</span><span class="special">;</span> <span class="comment">// illegal</span>
|
||||
</pre>
|
||||
</div>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
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<div class="spirit-nav">
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
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<div class="section">
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<div class="titlepage"><div><div><h4 class="title">
|
||||
<a name="boost_optional.tutorial.optional_references.rebinding_semantics_for_assignment_of_optional_references"></a><a class="link" href="rebinding_semantics_for_assignment_of_optional_references.html" title="Rebinding semantics for assignment of optional references">Rebinding
|
||||
semantics for assignment of optional references</a>
|
||||
</h4></div></div></div>
|
||||
<p>
|
||||
If you assign to an <span class="emphasis"><em>uninitialized </em></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code> the effect is to bind (for the
|
||||
first time) to the object. Clearly, there is no other choice.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">rx</span> <span class="special">=</span> <span class="identifier">x</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">ora</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">orb</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">ora</span> <span class="special">=</span> <span class="identifier">orb</span> <span class="special">;</span> <span class="comment">// now 'ora' is bound to 'x' through 'rx'</span>
|
||||
<span class="special">*</span><span class="identifier">ora</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span> <span class="comment">// Changes value of 'x' through 'ora'</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">x</span><span class="special">==</span><span class="number">2</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
If you assign to a bare C++ reference, the assignment is forwarded to the
|
||||
referenced object; its value changes but the reference is never rebound.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">a</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">ra</span> <span class="special">=</span> <span class="identifier">a</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span> <span class="identifier">b</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">rb</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">;</span>
|
||||
<span class="identifier">ra</span> <span class="special">=</span> <span class="identifier">rb</span> <span class="special">;</span> <span class="comment">// Changes the value of 'a' to 'b'</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">a</span><span class="special">==</span><span class="identifier">b</span><span class="special">);</span>
|
||||
<span class="identifier">b</span> <span class="special">=</span> <span class="number">3</span> <span class="special">;</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">ra</span><span class="special">!=</span><span class="identifier">b</span><span class="special">);</span> <span class="comment">// 'ra' is not rebound to 'b'</span>
|
||||
</pre>
|
||||
<p>
|
||||
Now, if you assign to an <span class="emphasis"><em>initialized </em></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>,
|
||||
the effect is to <span class="bold"><strong>rebind</strong></span> to the new object
|
||||
instead of assigning the referee. This is unlike bare C++ references.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">a</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span> <span class="identifier">b</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">ra</span> <span class="special">=</span> <span class="identifier">a</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">rb</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">ora</span><span class="special">(</span><span class="identifier">ra</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">orb</span><span class="special">(</span><span class="identifier">rb</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">ora</span> <span class="special">=</span> <span class="identifier">orb</span> <span class="special">;</span> <span class="comment">// 'ora' is rebound to 'b'</span>
|
||||
<span class="special">*</span><span class="identifier">ora</span> <span class="special">=</span> <span class="number">3</span> <span class="special">;</span> <span class="comment">// Changes value of 'b' (not 'a')</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">a</span><span class="special">==</span><span class="number">1</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">b</span><span class="special">==</span><span class="number">3</span><span class="special">);</span>
|
||||
</pre>
|
||||
<h6>
|
||||
<a name="boost_optional.tutorial.optional_references.rebinding_semantics_for_assignment_of_optional_references.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.tutorial.optional_references.rebinding_semantics_for_assignment_of_optional_references.rationale"></a></span><a class="link" href="rebinding_semantics_for_assignment_of_optional_references.html#boost_optional.tutorial.optional_references.rebinding_semantics_for_assignment_of_optional_references.rationale">Rationale</a>
|
||||
</h6>
|
||||
<p>
|
||||
Rebinding semantics for the assignment of <span class="emphasis"><em>initialized </em></span>
|
||||
<code class="computeroutput"><span class="identifier">optional</span></code> references has
|
||||
been chosen to provide <span class="bold"><strong>consistency among initialization
|
||||
states</strong></span> even at the expense of lack of consistency with the semantics
|
||||
of bare C++ references. It is true that <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span></code> strives to behave as much as possible
|
||||
as <code class="computeroutput"><span class="identifier">U</span></code> does whenever it is
|
||||
initialized; but in the case when <code class="computeroutput"><span class="identifier">U</span></code>
|
||||
is <code class="computeroutput"><span class="identifier">T</span><span class="special">&</span></code>,
|
||||
doing so would result in inconsistent behavior w.r.t to the lvalue initialization
|
||||
state.
|
||||
</p>
|
||||
<p>
|
||||
Imagine <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>
|
||||
forwarding assignment to the referenced object (thus changing the referenced
|
||||
object value but not rebinding), and consider the following code:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">a</span> <span class="special">=</span> <span class="identifier">get</span><span class="special">();</span>
|
||||
<span class="keyword">int</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">rx</span> <span class="special">=</span> <span class="identifier">x</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">b</span><span class="special">(</span><span class="identifier">rx</span><span class="special">);</span>
|
||||
<span class="identifier">a</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">;</span>
|
||||
</pre>
|
||||
<p>
|
||||
What does the assignment do?
|
||||
</p>
|
||||
<p>
|
||||
If <code class="computeroutput"><span class="identifier">a</span></code> is <span class="emphasis"><em>uninitialized</em></span>,
|
||||
the answer is clear: it binds to <code class="computeroutput"><span class="identifier">x</span></code>
|
||||
(we now have another reference to <code class="computeroutput"><span class="identifier">x</span></code>).
|
||||
But what if <code class="computeroutput"><span class="identifier">a</span></code> is already
|
||||
<span class="emphasis"><em>initialized</em></span>? it would change the value of the referenced
|
||||
object (whatever that is); which is inconsistent with the other possible
|
||||
case.
|
||||
</p>
|
||||
<p>
|
||||
If <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>
|
||||
would assign just like <code class="computeroutput"><span class="identifier">T</span><span class="special">&</span></code> does, you would never be able to use
|
||||
Optional's assignment without explicitly handling the previous initialization
|
||||
state unless your code is capable of functioning whether after the assignment,
|
||||
<code class="computeroutput"><span class="identifier">a</span></code> aliases the same object
|
||||
as <code class="computeroutput"><span class="identifier">b</span></code> or not.
|
||||
</p>
|
||||
<p>
|
||||
That is, you would have to discriminate in order to be consistent.
|
||||
</p>
|
||||
<p>
|
||||
If in your code rebinding to another object is not an option, then it is
|
||||
very likely that binding for the first time isn't either. In such case,
|
||||
assignment to an <span class="emphasis"><em>uninitialized </em></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code> shall be prohibited. It is quite
|
||||
possible that in such a scenario it is a precondition that the lvalue must
|
||||
be already initialized. If it isn't, then binding for the first time is
|
||||
OK while rebinding is not which is IMO very unlikely. In such a scenario,
|
||||
you can assign the value itself directly, as in:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">assert</span><span class="special">(!!</span><span class="identifier">opt</span><span class="special">);</span>
|
||||
<span class="special">*</span><span class="identifier">opt</span><span class="special">=</span><span class="identifier">value</span><span class="special">;</span>
|
||||
</pre>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
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<div class="spirit-nav">
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<html>
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<a accesskey="p" href="type_requirements.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="../../optional/reference.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.tutorial.performance_considerations"></a><a class="link" href="performance_considerations.html" title="Performance considerations">Performance
|
||||
considerations</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
Technical details aside, the memory layout of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
for a generic <code class="computeroutput"><span class="identifier">T</span></code> is more-less
|
||||
this:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span>
|
||||
<span class="keyword">class</span> <span class="identifier">optional</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">bool</span> <span class="identifier">_initialized</span><span class="special">;</span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">aligned_storage_t</span><span class="special"><</span><span class="keyword">sizeof</span><span class="special">(</span><span class="identifier">t</span><span class="special">),</span> <span class="keyword">alignof</span><span class="special">(</span><span class="identifier">T</span><span class="special">)></span> <span class="identifier">_storage</span><span class="special">;</span>
|
||||
<span class="special">};</span>
|
||||
</pre>
|
||||
<p>
|
||||
Lifetime of the <code class="computeroutput"><span class="identifier">T</span></code> inside
|
||||
<code class="computeroutput"><span class="identifier">_storage</span></code> is manually controlled
|
||||
with placement-<code class="computeroutput"><span class="keyword">new</span></code>s and pseudo-destructor
|
||||
calls. However, for scalar <code class="computeroutput"><span class="identifier">T</span></code>s
|
||||
we use a different way of storage, by simply holding a <code class="computeroutput"><span class="identifier">T</span></code>:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span>
|
||||
<span class="keyword">class</span> <span class="identifier">optional</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">bool</span> <span class="identifier">_initialized</span><span class="special">;</span>
|
||||
<span class="identifier">T</span> <span class="identifier">_storage</span><span class="special">;</span>
|
||||
<span class="special">};</span>
|
||||
</pre>
|
||||
<p>
|
||||
We call it a <span class="emphasis"><em>direct</em></span> storage. This makes <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> a
|
||||
trivially-copyable type for scalar <code class="computeroutput"><span class="identifier">T</span></code>s.
|
||||
This only works for compilers that support defaulted functions (including
|
||||
defaulted move assignment and constructor). On compilers without defaulted
|
||||
functions we still use the direct storage, but <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
is no longer recognized as trivially-copyable. Apart from scalar types, we
|
||||
leave the programmer a way of customizing her type, so that it is recognized
|
||||
by <code class="computeroutput"><span class="identifier">optional</span></code> as candidate
|
||||
for optimized storage, by specializing type trait <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional_config</span><span class="special">::</span><span class="identifier">optional_uses_direct_storage_for</span></code>:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">X</span> <span class="comment">// not trivial</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">X</span><span class="special">()</span> <span class="special">{}</span>
|
||||
<span class="special">};</span>
|
||||
|
||||
<span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">optional_config</span> <span class="special">{</span>
|
||||
|
||||
<span class="keyword">template</span> <span class="special"><></span> <span class="keyword">struct</span> <span class="identifier">optional_uses_direct_storage_for</span><span class="special"><</span><span class="identifier">X</span><span class="special">></span> <span class="special">:</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">true_type</span> <span class="special">{};</span>
|
||||
|
||||
<span class="special">}}</span>
|
||||
</pre>
|
||||
<h5>
|
||||
<a name="boost_optional.tutorial.performance_considerations.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.tutorial.performance_considerations.controlling_the_size"></a></span><a class="link" href="performance_considerations.html#boost_optional.tutorial.performance_considerations.controlling_the_size">Controlling
|
||||
the size</a>
|
||||
</h5>
|
||||
<p>
|
||||
For the purpose of the following analysis, considering memory layouts, we
|
||||
can think of it as:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span>
|
||||
<span class="keyword">class</span> <span class="identifier">optional</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">bool</span> <span class="identifier">_initialized</span><span class="special">;</span>
|
||||
<span class="identifier">T</span> <span class="identifier">_storage</span><span class="special">;</span>
|
||||
<span class="special">};</span>
|
||||
</pre>
|
||||
<p>
|
||||
Given type <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code>, and
|
||||
assuming that <code class="computeroutput"><span class="keyword">sizeof</span><span class="special">(</span><span class="keyword">int</span><span class="special">)</span> <span class="special">==</span>
|
||||
<span class="number">4</span></code>, we will get <code class="computeroutput"><span class="keyword">sizeof</span><span class="special">(</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">>)</span>
|
||||
<span class="special">==</span> <span class="number">8</span></code>.
|
||||
This is so because of the alignment rules, for our two members we get the
|
||||
following alignment:
|
||||
</p>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../images/opt_align1.png" alt="opt_align1"></span>
|
||||
</p>
|
||||
<p>
|
||||
This means you can fit twice as many <code class="computeroutput"><span class="keyword">int</span></code>s
|
||||
as <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code>s into
|
||||
the same space of memory. Therefore, if the size of the objects is critical
|
||||
for your application (e.g., because you want to utilize your CPU cache in
|
||||
order to gain performance) and you have determined you are willing to trade
|
||||
the code clarity, it is recommended that you simply go with type <code class="computeroutput"><span class="keyword">int</span></code> and use some 'magic value' to represent
|
||||
<span class="emphasis"><em>not-an-int</em></span>, or use something like <a href="https://github.com/akrzemi1/markable" target="_top"><code class="computeroutput"><span class="identifier">markable</span></code></a> library.
|
||||
</p>
|
||||
<p>
|
||||
Even if you cannot spare any value of <code class="computeroutput"><span class="keyword">int</span></code>
|
||||
to represent <span class="emphasis"><em>not-an-int</em></span> (e.g., because every value is
|
||||
useful, or you do want to signal <span class="emphasis"><em>not-an-int</em></span> explicitly),
|
||||
at least for <code class="computeroutput"><span class="identifier">Trivial</span></code> types
|
||||
you should consider storing the value and the <code class="computeroutput"><span class="keyword">bool</span></code>
|
||||
flag representing the <span class="emphasis"><em>null-state</em></span> separately. Consider
|
||||
the following class:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">Record</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">_min</span><span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">_max</span><span class="special">;</span>
|
||||
<span class="special">};</span>
|
||||
</pre>
|
||||
<p>
|
||||
Its memory layout can be depicted as follows:
|
||||
</p>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../images/opt_align2.png" alt="opt_align2"></span>
|
||||
</p>
|
||||
<p>
|
||||
This is exactly the same as if we had the following members:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">Record</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">bool</span> <span class="identifier">_has_min</span><span class="special">;</span>
|
||||
<span class="keyword">int</span> <span class="identifier">_min</span><span class="special">;</span>
|
||||
<span class="keyword">bool</span> <span class="identifier">_has_max</span><span class="special">;</span>
|
||||
<span class="keyword">int</span> <span class="identifier">_max</span><span class="special">;</span>
|
||||
<span class="special">};</span>
|
||||
</pre>
|
||||
<p>
|
||||
But when they are stored separately, we at least have an option to reorder
|
||||
them like this:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">Record</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">bool</span> <span class="identifier">_has_min</span><span class="special">;</span>
|
||||
<span class="keyword">bool</span> <span class="identifier">_has_max</span><span class="special">;</span>
|
||||
<span class="keyword">int</span> <span class="identifier">_min</span><span class="special">;</span>
|
||||
<span class="keyword">int</span> <span class="identifier">_max</span><span class="special">;</span>
|
||||
<span class="special">};</span>
|
||||
</pre>
|
||||
<p>
|
||||
Which gives us the following layout (and smaller total size):
|
||||
</p>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../images/opt_align3.png" alt="opt_align3"></span>
|
||||
</p>
|
||||
<p>
|
||||
Sometimes it requires detailed consideration what data we make optional.
|
||||
In our case above, if we determine that both minimum and maximum value can
|
||||
be provided or not provided together, but one is never provided without the
|
||||
other, we can make only one optional member:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">Limits</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">int</span> <span class="identifier">_min</span><span class="special">;</span>
|
||||
<span class="keyword">int</span> <span class="identifier">_max</span><span class="special">;</span>
|
||||
<span class="special">};</span>
|
||||
|
||||
<span class="keyword">struct</span> <span class="identifier">Record</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">Limits</span><span class="special">></span> <span class="identifier">_limits</span><span class="special">;</span>
|
||||
<span class="special">};</span>
|
||||
</pre>
|
||||
<p>
|
||||
This would give us the following layout:
|
||||
</p>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../images/opt_align4.png" alt="opt_align4"></span>
|
||||
</p>
|
||||
<h5>
|
||||
<a name="boost_optional.tutorial.performance_considerations.h1"></a>
|
||||
<span class="phrase"><a name="boost_optional.tutorial.performance_considerations.optional_function_parameters"></a></span><a class="link" href="performance_considerations.html#boost_optional.tutorial.performance_considerations.optional_function_parameters">Optional
|
||||
function parameters</a>
|
||||
</h5>
|
||||
<p>
|
||||
Having function parameters of type <code class="computeroutput"><span class="keyword">const</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span></code>
|
||||
may incur certain unexpected run-time cost connected to copy construction
|
||||
of <code class="computeroutput"><span class="identifier">T</span></code>. Consider the following
|
||||
code.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">fun</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">Big</span><span class="special">>&</span> <span class="identifier">v</span><span class="special">)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">v</span><span class="special">)</span> <span class="identifier">doSomethingWith</span><span class="special">(*</span><span class="identifier">v</span><span class="special">);</span>
|
||||
<span class="keyword">else</span> <span class="identifier">doSomethingElse</span><span class="special">();</span>
|
||||
<span class="special">}</span>
|
||||
|
||||
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">Big</span><span class="special">></span> <span class="identifier">ov</span><span class="special">;</span>
|
||||
<span class="identifier">Big</span> <span class="identifier">v</span><span class="special">;</span>
|
||||
<span class="identifier">fun</span><span class="special">(</span><span class="identifier">none</span><span class="special">);</span>
|
||||
<span class="identifier">fun</span><span class="special">(</span><span class="identifier">ov</span><span class="special">);</span> <span class="comment">// no copy</span>
|
||||
<span class="identifier">fun</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span> <span class="comment">// copy constructor of Big</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
No copy elision or move semantics can save us from copying type <code class="computeroutput"><span class="identifier">Big</span></code> here. Not that we need any copy, but
|
||||
this is how <code class="computeroutput"><span class="identifier">optional</span></code> works.
|
||||
In order to avoid copying in this case, one could provide second overload
|
||||
of <code class="computeroutput"><span class="identifier">fun</span></code>:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">fun</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">Big</span><span class="special">&</span> <span class="identifier">v</span><span class="special">)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">doSomethingWith</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span>
|
||||
<span class="special">}</span>
|
||||
|
||||
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">Big</span><span class="special">></span> <span class="identifier">ov</span><span class="special">;</span>
|
||||
<span class="identifier">Big</span> <span class="identifier">v</span><span class="special">;</span>
|
||||
<span class="identifier">fun</span><span class="special">(</span><span class="identifier">ov</span><span class="special">);</span> <span class="comment">// no copy</span>
|
||||
<span class="identifier">fun</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span> <span class="comment">// no copy: second overload selected</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
Alternatively, you could consider using an optional reference instead:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">fun</span><span class="special">(</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">const</span> <span class="identifier">Big</span><span class="special">&></span> <span class="identifier">v</span><span class="special">)</span> <span class="comment">// note where the reference is</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">v</span><span class="special">)</span> <span class="identifier">doSomethingWith</span><span class="special">(*</span><span class="identifier">v</span><span class="special">);</span>
|
||||
<span class="keyword">else</span> <span class="identifier">doSomethingElse</span><span class="special">();</span>
|
||||
<span class="special">}</span>
|
||||
|
||||
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">Big</span><span class="special">></span> <span class="identifier">ov</span><span class="special">;</span>
|
||||
<span class="identifier">Big</span> <span class="identifier">v</span><span class="special">;</span>
|
||||
<span class="identifier">fun</span><span class="special">(</span><span class="identifier">none</span><span class="special">);</span>
|
||||
<span class="identifier">fun</span><span class="special">(</span><span class="identifier">ov</span><span class="special">);</span> <span class="comment">// doesn't compile</span>
|
||||
<span class="identifier">fun</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span> <span class="comment">// no copy</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="type_requirements.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="../../optional/reference.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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||||
</div>
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||||
</body>
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||||
</html>
|
||||
-149
@@ -1,149 +0,0 @@
|
||||
<html>
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
|
||||
<title>Rebinding semantics for assignment of optional references</title>
|
||||
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<meta name="generator" content="DocBook XSL Stylesheets V1.78.1">
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<link rel="home" href="../../index.html" title="Boost.Optional">
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<link rel="up" href="../../optional/tutorial.html" title="Tutorial">
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||||
<link rel="prev" href="optional_references.html" title="Optional references">
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||||
<link rel="next" href="in_place_factories.html" title="In-Place Factories">
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||||
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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<table cellpadding="2" width="100%"><tr>
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|
||||
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<hr>
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<div class="spirit-nav">
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||||
<a accesskey="p" href="optional_references.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="in_place_factories.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.tutorial.rebinding_semantics_for_assignment_of_optional_references"></a><a class="link" href="rebinding_semantics_for_assignment_of_optional_references.html" title="Rebinding semantics for assignment of optional references">Rebinding
|
||||
semantics for assignment of optional references</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
If you assign to an <span class="emphasis"><em>uninitialized </em></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>
|
||||
the effect is to bind (for the first time) to the object. Clearly, there
|
||||
is no other choice.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">rx</span> <span class="special">=</span> <span class="identifier">x</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">ora</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">orb</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">ora</span> <span class="special">=</span> <span class="identifier">orb</span> <span class="special">;</span> <span class="comment">// now 'ora' is bound to 'x' through 'rx'</span>
|
||||
<span class="special">*</span><span class="identifier">ora</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span> <span class="comment">// Changes value of 'x' through 'ora'</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">x</span><span class="special">==</span><span class="number">2</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
If you assign to a bare C++ reference, the assignment is forwarded to the
|
||||
referenced object; its value changes but the reference is never rebound.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">a</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">ra</span> <span class="special">=</span> <span class="identifier">a</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span> <span class="identifier">b</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">rb</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">;</span>
|
||||
<span class="identifier">ra</span> <span class="special">=</span> <span class="identifier">rb</span> <span class="special">;</span> <span class="comment">// Changes the value of 'a' to 'b'</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">a</span><span class="special">==</span><span class="identifier">b</span><span class="special">);</span>
|
||||
<span class="identifier">b</span> <span class="special">=</span> <span class="number">3</span> <span class="special">;</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">ra</span><span class="special">!=</span><span class="identifier">b</span><span class="special">);</span> <span class="comment">// 'ra' is not rebound to 'b'</span>
|
||||
</pre>
|
||||
<p>
|
||||
Now, if you assign to an <span class="emphasis"><em>initialized </em></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>,
|
||||
the effect is to <span class="bold"><strong>rebind</strong></span> to the new object
|
||||
instead of assigning the referee. This is unlike bare C++ references.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">a</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span> <span class="identifier">b</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">ra</span> <span class="special">=</span> <span class="identifier">a</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">rb</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">ora</span><span class="special">(</span><span class="identifier">ra</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">orb</span><span class="special">(</span><span class="identifier">rb</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">ora</span> <span class="special">=</span> <span class="identifier">orb</span> <span class="special">;</span> <span class="comment">// 'ora' is rebound to 'b'</span>
|
||||
<span class="special">*</span><span class="identifier">ora</span> <span class="special">=</span> <span class="number">3</span> <span class="special">;</span> <span class="comment">// Changes value of 'b' (not 'a')</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">a</span><span class="special">==</span><span class="number">1</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">b</span><span class="special">==</span><span class="number">3</span><span class="special">);</span>
|
||||
</pre>
|
||||
<h5>
|
||||
<a name="boost_optional.tutorial.rebinding_semantics_for_assignment_of_optional_references.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.tutorial.rebinding_semantics_for_assignment_of_optional_references.rationale"></a></span><a class="link" href="rebinding_semantics_for_assignment_of_optional_references.html#boost_optional.tutorial.rebinding_semantics_for_assignment_of_optional_references.rationale">Rationale</a>
|
||||
</h5>
|
||||
<p>
|
||||
Rebinding semantics for the assignment of <span class="emphasis"><em>initialized </em></span>
|
||||
<code class="computeroutput"><span class="identifier">optional</span></code> references has been
|
||||
chosen to provide <span class="bold"><strong>consistency among initialization
|
||||
states</strong></span> even at the expense of lack of consistency with the semantics
|
||||
of bare C++ references. It is true that <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span></code>
|
||||
strives to behave as much as possible as <code class="computeroutput"><span class="identifier">U</span></code>
|
||||
does whenever it is initialized; but in the case when <code class="computeroutput"><span class="identifier">U</span></code>
|
||||
is <code class="computeroutput"><span class="identifier">T</span><span class="special">&</span></code>,
|
||||
doing so would result in inconsistent behavior w.r.t to the lvalue initialization
|
||||
state.
|
||||
</p>
|
||||
<p>
|
||||
Imagine <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>
|
||||
forwarding assignment to the referenced object (thus changing the referenced
|
||||
object value but not rebinding), and consider the following code:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">a</span> <span class="special">=</span> <span class="identifier">get</span><span class="special">();</span>
|
||||
<span class="keyword">int</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">rx</span> <span class="special">=</span> <span class="identifier">x</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">b</span><span class="special">(</span><span class="identifier">rx</span><span class="special">);</span>
|
||||
<span class="identifier">a</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">;</span>
|
||||
</pre>
|
||||
<p>
|
||||
What does the assignment do?
|
||||
</p>
|
||||
<p>
|
||||
If <code class="computeroutput"><span class="identifier">a</span></code> is <span class="emphasis"><em>uninitialized</em></span>,
|
||||
the answer is clear: it binds to <code class="computeroutput"><span class="identifier">x</span></code>
|
||||
(we now have another reference to <code class="computeroutput"><span class="identifier">x</span></code>).
|
||||
But what if <code class="computeroutput"><span class="identifier">a</span></code> is already
|
||||
<span class="emphasis"><em>initialized</em></span>? it would change the value of the referenced
|
||||
object (whatever that is); which is inconsistent with the other possible
|
||||
case.
|
||||
</p>
|
||||
<p>
|
||||
If <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>
|
||||
would assign just like <code class="computeroutput"><span class="identifier">T</span><span class="special">&</span></code> does, you would never be able to use
|
||||
Optional's assignment without explicitly handling the previous initialization
|
||||
state unless your code is capable of functioning whether after the assignment,
|
||||
<code class="computeroutput"><span class="identifier">a</span></code> aliases the same object
|
||||
as <code class="computeroutput"><span class="identifier">b</span></code> or not.
|
||||
</p>
|
||||
<p>
|
||||
That is, you would have to discriminate in order to be consistent.
|
||||
</p>
|
||||
<p>
|
||||
If in your code rebinding to another object is not an option, then it is
|
||||
very likely that binding for the first time isn't either. In such case, assignment
|
||||
to an <span class="emphasis"><em>uninitialized </em></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>
|
||||
shall be prohibited. It is quite possible that in such a scenario it is a
|
||||
precondition that the lvalue must be already initialized. If it isn't, then
|
||||
binding for the first time is OK while rebinding is not which is IMO very
|
||||
unlikely. In such a scenario, you can assign the value itself directly, as
|
||||
in:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">assert</span><span class="special">(!!</span><span class="identifier">opt</span><span class="special">);</span>
|
||||
<span class="special">*</span><span class="identifier">opt</span><span class="special">=</span><span class="identifier">value</span><span class="special">;</span>
|
||||
</pre>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2016 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
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<div class="spirit-nav">
|
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<a accesskey="p" href="optional_references.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="in_place_factories.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
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</div>
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<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.tutorial.relational_operators"></a><a class="link" href="relational_operators.html" title="Relational operators">Relational
|
||||
operators</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
Type <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is
|
||||
<a href="http://www.sgi.com/tech/stl/EqualityComparable.html" target="_top"><code class="computeroutput"><span class="identifier">EqualityComparable</span></code></a> whenever <code class="computeroutput"><span class="identifier">T</span></code> is <a href="http://www.sgi.com/tech/stl/EqualityComparable.html" target="_top"><code class="computeroutput"><span class="identifier">EqualityComparable</span></code></a>. Two optional
|
||||
objects containing a value compare in the same way as their contained values.
|
||||
The uninitialized state of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
is treated as a distinct value, equal to itself, and unequal to any value
|
||||
of type <code class="computeroutput"><span class="identifier">T</span></code>:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">oN</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span><span class="special">;</span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">o0</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">o1</span> <span class="special">=</span> <span class="number">1</span><span class="special">;</span>
|
||||
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">oN</span> <span class="special">!=</span> <span class="identifier">o0</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">o1</span> <span class="special">!=</span> <span class="identifier">oN</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">o0</span> <span class="special">!=</span> <span class="identifier">o1</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">oN</span> <span class="special">==</span> <span class="identifier">oN</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">o0</span> <span class="special">==</span> <span class="identifier">o0</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
The converting constructor from <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
as well as from <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code> implies the existence and semantics
|
||||
of the mixed comparison between <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
and <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> as
|
||||
well as between <code class="computeroutput"><span class="identifier">none_t</span></code> and
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">assert</span><span class="special">(</span><span class="identifier">oN</span> <span class="special">!=</span> <span class="number">0</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">o1</span> <span class="special">!=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">o0</span> <span class="special">!=</span> <span class="number">1</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">oN</span> <span class="special">==</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">o0</span> <span class="special">==</span> <span class="number">0</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
This mixed comparison has a practical interpretation, which is occasionally
|
||||
useful:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">choice</span> <span class="special">=</span> <span class="identifier">ask_user</span><span class="special">();</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">choice</span> <span class="special">==</span> <span class="number">2</span><span class="special">)</span>
|
||||
<span class="identifier">start_procedure_2</span><span class="special">();</span>
|
||||
</pre>
|
||||
<p>
|
||||
In the above example, the meaning of the comparison is 'user chose number
|
||||
2'. If user chose nothing, he didn't choose number 2.
|
||||
</p>
|
||||
<p>
|
||||
In case where <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is
|
||||
compared to <code class="computeroutput"><span class="identifier">none</span></code>, it is not
|
||||
required that <code class="computeroutput"><span class="identifier">T</span></code> be <a href="http://www.sgi.com/tech/stl/EqualityComparable.html" target="_top"><code class="computeroutput"><span class="identifier">EqualityComparable</span></code></a>.
|
||||
</p>
|
||||
<p>
|
||||
In a similar manner, type <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
is <a href="http://www.sgi.com/tech/stl/LessThanComparable.html" target="_top"><code class="computeroutput"><span class="identifier">LessThanComparable</span></code></a> whenever <code class="computeroutput"><span class="identifier">T</span></code> is <a href="http://www.sgi.com/tech/stl/LessThanComparable.html" target="_top"><code class="computeroutput"><span class="identifier">LessThanComparable</span></code></a>. The optional
|
||||
object containing no value is compared less than any value of <code class="computeroutput"><span class="identifier">T</span></code>. To illustrate this, if the default ordering
|
||||
of <code class="computeroutput"><span class="identifier">size_t</span></code> is {<code class="computeroutput"><span class="number">0</span></code>, <code class="computeroutput"><span class="number">1</span></code>,
|
||||
<code class="computeroutput"><span class="number">2</span></code>, ...}, the default ordering
|
||||
of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">size_t</span><span class="special">></span></code>
|
||||
is {<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code>, <code class="computeroutput"><span class="number">0</span></code>,
|
||||
<code class="computeroutput"><span class="number">1</span></code>, <code class="computeroutput"><span class="number">2</span></code>,
|
||||
...}. This order does not have a practical interpretation. The goal is to
|
||||
have any semantically correct default ordering in order for <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> to
|
||||
be usable in ordered associative containers (wherever <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
is usable).
|
||||
</p>
|
||||
<p>
|
||||
Mixed relational operators are the only case where the contained value of
|
||||
an optional object can be inspected without the usage of value accessing
|
||||
function (<code class="computeroutput"><span class="keyword">operator</span><span class="special">*</span></code>,
|
||||
<code class="computeroutput"><span class="identifier">value</span></code>, <code class="computeroutput"><span class="identifier">value_or</span></code>).
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="when_to_use_optional.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="io_operators.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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@@ -1,109 +0,0 @@
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
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<title>Type requirements</title>
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<meta name="generator" content="DocBook XSL Stylesheets Vsnapshot">
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<link rel="next" href="performance_considerations.html" title="Performance considerations">
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||||
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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<table cellpadding="2" width="100%"><tr>
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<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td>
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<td align="center"><a href="../../../../../../index.html">Home</a></td>
|
||||
<td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td>
|
||||
<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
|
||||
<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
|
||||
<td align="center"><a href="../../../../../../more/index.htm">More</a></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="exception_safety_guarantees.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="performance_considerations.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.tutorial.type_requirements"></a><a class="link" href="type_requirements.html" title="Type requirements">Type requirements</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
The very minimum requirement of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
is that <code class="computeroutput"><span class="identifier">T</span></code> is a complete type
|
||||
and that it has a publicly accessible destructor. <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
doesn't even need to be constructible. You can use a very minimum interface:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">o</span><span class="special">;</span> <span class="comment">// uninitialized</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">o</span> <span class="special">==</span> <span class="identifier">none</span><span class="special">);</span> <span class="comment">// check if initialized</span>
|
||||
<span class="identifier">assert</span><span class="special">(!</span><span class="identifier">o</span><span class="special">);</span> <span class="comment">//</span>
|
||||
<span class="identifier">o</span><span class="special">.</span><span class="identifier">value</span><span class="special">();</span> <span class="comment">// always throws</span>
|
||||
</pre>
|
||||
<p>
|
||||
But this is practically useless. In order for <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
to be able to do anything useful and offer all the spectrum of ways of accessing
|
||||
the contained value, <code class="computeroutput"><span class="identifier">T</span></code> needs
|
||||
to have at least one accessible constructor. In that case you need to initialize
|
||||
the optional object with function <code class="computeroutput"><span class="identifier">emplace</span><span class="special">()</span></code>, or if your compiler does not support it,
|
||||
resort to <a class="link" href="in_place_factories.html" title="In-Place Factories">In-Place
|
||||
Factories</a>:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">o</span><span class="special">;</span>
|
||||
<span class="identifier">o</span><span class="special">.</span><span class="identifier">emplace</span><span class="special">(</span><span class="string">"T"</span><span class="special">,</span> <span class="string">"ctor"</span><span class="special">,</span> <span class="string">"params"</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
If <code class="computeroutput"><span class="identifier">T</span></code> is <code class="computeroutput"><span class="identifier">MoveConstructible</span></code>,
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is
|
||||
also <code class="computeroutput"><span class="identifier">MoveConstructible</span></code> and
|
||||
can be easily initialized from an rvalue of type <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
and be passed by value:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">o</span> <span class="special">=</span> <span class="identifier">make_T</span><span class="special">();</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>();</span>
|
||||
</pre>
|
||||
<p>
|
||||
If <code class="computeroutput"><span class="identifier">T</span></code> is <a href="../../../../../utility/CopyConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">CopyConstructible</span></code></a>, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is
|
||||
also <a href="../../../../../utility/CopyConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">CopyConstructible</span></code></a>
|
||||
and can be easily initialized from an lvalue of type <code class="computeroutput"><span class="identifier">T</span></code>:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">T</span> <span class="identifier">v</span> <span class="special">=</span> <span class="identifier">make_T</span><span class="special">();</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">o</span> <span class="special">=</span> <span class="identifier">v</span><span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">o</span><span class="special">;</span>
|
||||
</pre>
|
||||
<p>
|
||||
If <code class="computeroutput"><span class="identifier">T</span></code> is not <code class="computeroutput"><span class="identifier">MoveAssignable</span></code>, it is still possible to
|
||||
reset the value of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
using function <code class="computeroutput"><span class="identifier">emplace</span><span class="special">()</span></code>:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">const</span> <span class="identifier">T</span><span class="special">></span> <span class="identifier">o</span> <span class="special">=</span> <span class="identifier">make_T</span><span class="special">();</span>
|
||||
<span class="identifier">o</span><span class="special">.</span><span class="identifier">emplace</span><span class="special">(</span><span class="identifier">make_another_T</span><span class="special">());</span>
|
||||
</pre>
|
||||
<p>
|
||||
If <code class="computeroutput"><span class="identifier">T</span></code> is <code class="computeroutput"><span class="identifier">Moveable</span></code>
|
||||
(both <code class="computeroutput"><span class="identifier">MoveConstructible</span></code> and
|
||||
<code class="computeroutput"><span class="identifier">MoveAssignable</span></code>) then <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is
|
||||
also <code class="computeroutput"><span class="identifier">Moveable</span></code> and additionally
|
||||
can be constructed and assigned from an rvalue of type <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</p>
|
||||
<p>
|
||||
Similarly, if <code class="computeroutput"><span class="identifier">T</span></code> is <code class="computeroutput"><span class="identifier">Copyable</span></code> (both <a href="../../../../../utility/CopyConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">CopyConstructible</span></code></a> and <code class="computeroutput"><span class="identifier">CopyAssignable</span></code>) then <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
is also <code class="computeroutput"><span class="identifier">Copyable</span></code> and additionally
|
||||
can be constructed and assigned from an lvalue of type <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</p>
|
||||
<p>
|
||||
<code class="computeroutput"><span class="identifier">T</span></code> <span class="emphasis"><em>is not</em></span>
|
||||
required to be <a href="http://www.sgi.com/tech/stl/DefaultConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">DefaultConstructible</span></code></a>.
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="exception_safety_guarantees.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="performance_considerations.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
|
||||
</div>
|
||||
</body>
|
||||
</html>
|
||||
@@ -1,142 +0,0 @@
|
||||
<html>
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
|
||||
<title>When to use Optional</title>
|
||||
<link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css">
|
||||
<meta name="generator" content="DocBook XSL Stylesheets Vsnapshot">
|
||||
<link rel="home" href="../../index.html" title="Boost.Optional">
|
||||
<link rel="up" href="../../optional/tutorial.html" title="Tutorial">
|
||||
<link rel="prev" href="design_overview/the_interface.html" title="The Interface">
|
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<link rel="next" href="relational_operators.html" title="Relational operators">
|
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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<table cellpadding="2" width="100%"><tr>
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<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td>
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<td align="center"><a href="../../../../../../index.html">Home</a></td>
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<td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td>
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<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
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<td align="center"><a href="../../../../../../more/index.htm">More</a></td>
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||||
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|
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<hr>
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||||
<div class="spirit-nav">
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<a accesskey="p" href="design_overview/the_interface.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="relational_operators.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.tutorial.when_to_use_optional"></a><a class="link" href="when_to_use_optional.html" title="When to use Optional">When to
|
||||
use Optional</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
It is recommended to use <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
in situations where there is exactly one, clear (to all parties) reason for
|
||||
having no value of type <code class="computeroutput"><span class="identifier">T</span></code>,
|
||||
and where the lack of value is as natural as having any regular value of
|
||||
<code class="computeroutput"><span class="identifier">T</span></code>. One example of such situation
|
||||
is asking the user in some GUI form to optionally specify some limit on an
|
||||
<code class="computeroutput"><span class="keyword">int</span></code> value, but the user is allowed
|
||||
to say 'I want the number not to be constrained by the maximum'. For another
|
||||
example, consider a config parameter specifying how many threads the application
|
||||
should launch. Leaving this parameter unspecified means that the application
|
||||
should decide itself. For yet another example, consider a function returning
|
||||
the index of the smallest element in a <code class="computeroutput"><span class="identifier">vector</span></code>.
|
||||
We need to be prepared for the situation, where the <code class="computeroutput"><span class="identifier">vector</span></code>
|
||||
is empty. Therefore a natural signature for such function would be:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">size_t</span><span class="special">></span> <span class="identifier">find_smallest_elem</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="identifier">vec</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
Here, having received an empty <code class="computeroutput"><span class="identifier">vec</span></code>
|
||||
and having no <code class="computeroutput"><span class="identifier">size_t</span></code> to return
|
||||
is not a <span class="emphasis"><em>failure</em></span> but a <span class="emphasis"><em>normal</em></span>,
|
||||
albeit irregular, situation.
|
||||
</p>
|
||||
<p>
|
||||
Another typical situation is to indicate that we do not have a value yet,
|
||||
but we expect to have it later. This notion can be used in implementing solutions
|
||||
like lazy initialization or a two-phase initialization.
|
||||
</p>
|
||||
<p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span></code> can be used to take
|
||||
a non-<a href="http://www.sgi.com/tech/stl/DefaultConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">DefaultConstructible</span></code></a> type <code class="computeroutput"><span class="identifier">T</span></code> and create a sibling type with a default
|
||||
constructor. This is a way to add a <span class="emphasis"><em>null-state</em></span> to any
|
||||
type that doesn't have it already.
|
||||
</p>
|
||||
<p>
|
||||
Sometimes type <code class="computeroutput"><span class="identifier">T</span></code> already
|
||||
provides a built-in null-state, but it may still be useful to wrap it into
|
||||
<code class="computeroutput"><span class="identifier">optional</span></code>. Consider <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>.
|
||||
When you read a piece of text from a GUI form or a DB table, it is hardly
|
||||
ever that the empty string indicates anything else but a missing text. And
|
||||
some data bases do not even distinguish between a null string entry and a
|
||||
non-null string of length 0. Still, it may be practical to use <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">string</span><span class="special">></span></code>
|
||||
to indicate in the returned type that we want to treat the empty string in
|
||||
a special dedicated program path:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">if</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">></span> <span class="identifier">name</span> <span class="special">=</span> <span class="identifier">ask_user_name</span><span class="special">())</span> <span class="special">{</span>
|
||||
<span class="identifier">assert</span><span class="special">(*</span><span class="identifier">name</span> <span class="special">!=</span> <span class="string">""</span><span class="special">);</span>
|
||||
<span class="identifier">logon_as</span><span class="special">(*</span><span class="identifier">name</span><span class="special">);</span>
|
||||
<span class="special">}</span>
|
||||
<span class="keyword">else</span> <span class="special">{</span>
|
||||
<span class="identifier">skip_logon</span><span class="special">();</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
In the example above, the assertion indicates that if we choose to use this
|
||||
technique, we must translate the empty string state to an optional object
|
||||
with no contained value (inside function <code class="computeroutput"><span class="identifier">ask_user_name</span></code>).
|
||||
</p>
|
||||
<h5>
|
||||
<a name="boost_optional.tutorial.when_to_use_optional.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.tutorial.when_to_use_optional.not_recommended_usages"></a></span><a class="link" href="when_to_use_optional.html#boost_optional.tutorial.when_to_use_optional.not_recommended_usages">Not
|
||||
recommended usages</a>
|
||||
</h5>
|
||||
<p>
|
||||
It is not recommended to use <code class="computeroutput"><span class="identifier">optional</span></code>
|
||||
to indicate that we were not able to compute a value because of a <span class="emphasis"><em>failure</em></span>.
|
||||
It is difficult to define what a failure is, but it usually has one common
|
||||
characteristic: an associated information on the cause of the failure. This
|
||||
can be the type and member data of an exception object, or an error code.
|
||||
It is a bad design to signal a failure and not inform about the cause. If
|
||||
you do not want to use exceptions, and do not like the fact that by returning
|
||||
error codes you cannot return the computed value, you can use <a href="https://github.com/ptal/Boost.Expected" target="_top">Expected</a>
|
||||
library. It is sort of <a href="../../../../../variant/index.html" target="_top">Boost.Variant</a>
|
||||
that contains either a computed value or a reason why the computation failed.
|
||||
</p>
|
||||
<p>
|
||||
Sometimes the distinction into what is a failure and what is a valid but
|
||||
irregular result is blurry and depends on a particular usage and personal
|
||||
preference. Consider a function that converts a <code class="computeroutput"><span class="identifier">string</span></code>
|
||||
to an <code class="computeroutput"><span class="keyword">int</span></code>. Is it a failure that
|
||||
you cannot convert? It might in some cases, but in other you may call it
|
||||
exactly for the purpose of figuring out if a given <code class="computeroutput"><span class="identifier">string</span></code>
|
||||
is convertible, and you are not even interested in the resulting value. Sometimes
|
||||
when a conversion fails you may not consider it a failure, but you need to
|
||||
know why it cannot be converted; for instance at which character it is determined
|
||||
that the conversion is impossible. In this case returning <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
will not suffice. Finally, there is a use case where an input string that
|
||||
does not represent an <code class="computeroutput"><span class="keyword">int</span></code> is
|
||||
not a failure condition, but during the conversion we use resources whose
|
||||
acquisition may fail. In that case the natural representation is to both
|
||||
return <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code> and
|
||||
signal failure:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">convert1</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">string</span><span class="special">&</span> <span class="identifier">str</span><span class="special">);</span> <span class="comment">// throws</span>
|
||||
<span class="identifier">expected</span><span class="special"><</span><span class="identifier">ErrorT</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">>></span> <span class="identifier">convert2</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">string</span><span class="special">&</span> <span class="identifier">str</span><span class="special">);</span> <span class="comment">// return either optional or error</span>
|
||||
</pre>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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At the very minimum for <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
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to work with a minimum interface it is required that <code class="computeroutput"><span class="identifier">T</span></code>
|
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has a publicly accessible no-throw destructor. In that case you need to initialize
|
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the optional object with function <code class="computeroutput"><span class="identifier">emplace</span><span class="special">()</span></code> or use <span class="bold"><strong>InPlaceFactories</strong></span>.
|
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Additionally, if <code class="computeroutput"><span class="identifier">T</span></code> is <code class="computeroutput"><span class="identifier">Moveable</span></code>, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
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is also <code class="computeroutput"><span class="identifier">Moveable</span></code> and can be
|
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easily initialized from an rvalue of type <code class="computeroutput"><span class="identifier">T</span></code>
|
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and be passed by value. Additionally, if <code class="computeroutput"><span class="identifier">T</span></code>
|
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is <code class="computeroutput"><span class="identifier">Copyable</span></code>, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is
|
||||
also <code class="computeroutput"><span class="identifier">Copyable</span></code> and can be easily
|
||||
initialized from an lvalue of type <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</p>
|
||||
<p>
|
||||
<code class="computeroutput"><span class="identifier">T</span></code> <span class="underline">is
|
||||
not</span> required to be <a href="http://www.sgi.com/tech/stl/DefaultConstructible.html" target="_top">Default
|
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Constructible</a>.
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<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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<a name="optional"></a>Boost.Optional</h2></div>
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<div><div class="author"><h3 class="author">
|
||||
<span class="firstname">Fernando Luis</span> <span class="surname">Cacciola Carballal</span>
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</h3></div></div>
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<div><p class="copyright">Copyright © 2003-2007 Fernando Luis Cacciola Carballal</p></div>
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<div><p class="copyright">Copyright © 2014-2021 Andrzej Krzemieński</p></div>
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<div><div class="legalnotice">
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
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</p>
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</div></div>
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</div></div>
|
||||
<div class="toc">
|
||||
<p><b>Table of Contents</b></p>
|
||||
<dl class="toc">
|
||||
<dt><span class="section"><a href="index.html#optional.introduction">Introduction</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/quick_start.html">Quick Start</a></span></dt>
|
||||
<dd><dl>
|
||||
<dt><span class="section"><a href="boost_optional/quick_start.html#boost_optional.quick_start.optional_return_values">Optional
|
||||
return values</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/quick_start/optional_automatic_variables.html">Optional
|
||||
automatic variables</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/quick_start/optional_data_members.html">Optional
|
||||
data members</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/quick_start/bypassing_unnecessary_default_construction.html">Bypassing
|
||||
unnecessary default construction</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/quick_start/storage_in_containers.html">Storage
|
||||
in containers</a></span></dt>
|
||||
</dl></dd>
|
||||
<dt><span class="section"><a href="optional/tutorial.html">Tutorial</a></span></dt>
|
||||
<dd><dl>
|
||||
<dt><span class="section"><a href="optional/tutorial.html#boost_optional.tutorial.motivation">Motivation</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/tutorial/design_overview.html">Design Overview</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/tutorial/when_to_use_optional.html">When to
|
||||
use Optional</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/tutorial/relational_operators.html">Relational
|
||||
operators</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/tutorial/io_operators.html">IO operators</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/tutorial/optional_references.html">Optional
|
||||
references</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/tutorial/in_place_factories.html">In-Place
|
||||
Factories</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/tutorial/gotchas.html">Gotchas</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/tutorial/exception_safety_guarantees.html">Exception
|
||||
Safety Guarantees</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/tutorial/type_requirements.html">Type requirements</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/tutorial/performance_considerations.html">Performance
|
||||
considerations</a></span></dt>
|
||||
</dl></dd>
|
||||
<dt><span class="section"><a href="optional/reference.html">Reference</a></span></dt>
|
||||
<dd><dl>
|
||||
<dt><span class="section"><a href="optional/reference.html#boost_optional.reference.header__boost_none_hpp_">Header
|
||||
<boost/none.hpp></a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/reference/header__boost_optional_bad_optional_access_hpp_.html">Header
|
||||
<boost/optional/bad_optional_access.hpp></a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/reference/io_header.html">Header <boost/optional/optional_io.hpp></a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/reference/header__boost_optional_optional_fwd_hpp_.html">Header
|
||||
<boost/optional/optional_fwd.hpp></a></span></dt>
|
||||
<dt><span class="section"><a href="optional/reference/header__boost_optional_optional_hpp_.html">Header
|
||||
<boost/optional/optional.hpp></a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/reference/header__boost_optional_hpp_.html">Header
|
||||
<boost/optional.hpp></a></span></dt>
|
||||
</dl></dd>
|
||||
<dt><span class="section"><a href="boost_optional/dependencies_and_portability.html">Dependencies
|
||||
and Portability</a></span></dt>
|
||||
<dd><dl>
|
||||
<dt><span class="section"><a href="boost_optional/dependencies_and_portability.html#boost_optional.dependencies_and_portability.dependencies">Dependencies</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/dependencies_and_portability/emplace_operations_in_older_compilers.html">Emplace
|
||||
operations in older compilers</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/dependencies_and_portability/optional_reference_binding.html">Optional
|
||||
Reference Binding</a></span></dt>
|
||||
</dl></dd>
|
||||
<dt><span class="section"><a href="boost_optional/relnotes.html">Release Notes</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/acknowledgements.html">Acknowledgements</a></span></dt>
|
||||
</dl>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
|
||||
<a name="optional.introduction"></a><a class="link" href="index.html#optional.introduction" title="Introduction">Introduction</a>
|
||||
</h2></div></div></div>
|
||||
<p>
|
||||
Class template <code class="computeroutput"><span class="identifier">optional</span></code> is
|
||||
a wrapper for representing 'optional' (or 'nullable') objects who may not (yet)
|
||||
contain a valid value. Optional objects offer full value semantics; they are
|
||||
good for passing by value and usage inside STL containers. This is a header-only
|
||||
library.
|
||||
</p>
|
||||
<h4>
|
||||
<a name="optional.introduction.h0"></a>
|
||||
<span class="phrase"><a name="optional.introduction.problem"></a></span><a class="link" href="index.html#optional.introduction.problem">Problem</a>
|
||||
</h4>
|
||||
<p>
|
||||
Suppose we want to read a parameter from a config file which represents some
|
||||
integral value, let's call it <code class="computeroutput"><span class="string">"MaxValue"</span></code>.
|
||||
It is possible that this parameter is not specified; such situation is no error.
|
||||
It is valid to not specify the parameter and in that case the program is supposed
|
||||
to behave slightly differently. Also, suppose that any possible value of type
|
||||
<code class="computeroutput"><span class="keyword">int</span></code> is a valid value for <code class="computeroutput"><span class="string">"MaxValue"</span></code>, so we cannot just use
|
||||
<code class="computeroutput"><span class="special">-</span><span class="number">1</span></code>
|
||||
to represent the absence of the parameter in the config file.
|
||||
</p>
|
||||
<h4>
|
||||
<a name="optional.introduction.h1"></a>
|
||||
<span class="phrase"><a name="optional.introduction.solution"></a></span><a class="link" href="index.html#optional.introduction.solution">Solution</a>
|
||||
</h4>
|
||||
<p>
|
||||
This is how you solve it with <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span></code>:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">optional</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
|
||||
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">getConfigParam</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">name</span><span class="special">);</span> <span class="comment">// return either an int or a `not-an-int`</span>
|
||||
|
||||
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">oi</span> <span class="special">=</span> <span class="identifier">getConfigParam</span><span class="special">(</span><span class="string">"MaxValue"</span><span class="special">))</span> <span class="comment">// did I get a real int?</span>
|
||||
<span class="identifier">runWithMax</span><span class="special">(*</span><span class="identifier">oi</span><span class="special">);</span> <span class="comment">// use my int</span>
|
||||
<span class="keyword">else</span>
|
||||
<span class="identifier">runWithNoMax</span><span class="special">();</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
</div>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
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<td align="left"><p><small>Last revised: March 26, 2022 at 22:47:53 GMT</small></p></td>
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<boost/optional/optional_fwd.hpp></a></span></dt>
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<dt><span class="section"><a href="reference/header__boost_optional_optional_hpp_.html">Header
|
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<boost/optional/optional.hpp></a></span></dt>
|
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<dt><span class="section"><a href="../boost_optional/reference/header__boost_optional_hpp_.html">Header
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<boost/optional.hpp></a></span></dt>
|
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</dl></div>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="boost_optional.reference.header__boost_none_hpp_"></a><a class="link" href="reference.html#boost_optional.reference.header__boost_none_hpp_" title="Header <boost/none.hpp>">Header
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<boost/none.hpp></a>
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</h3></div></div></div>
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<div class="section">
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<div class="titlepage"><div><div><h4 class="title">
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<a name="boost_optional.reference.header__boost_none_hpp_.synopsis"></a><a class="link" href="reference.html#boost_optional.reference.header__boost_none_hpp_.synopsis" title="Synopsis">Synopsis</a>
|
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</h4></div></div></div>
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<p>
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</p>
|
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<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span>
|
||||
|
||||
<span class="keyword">class</span> <span class="identifier">none_t</span> <span class="special">{/*</span> <span class="identifier">see</span> <span class="identifier">below</span> <span class="special">*/};</span>
|
||||
|
||||
<span class="keyword">inline</span> <span class="keyword">constexpr</span> <span class="identifier">none_t</span> <span class="identifier">none</span> <span class="special">(/*</span> <span class="identifier">see</span> <span class="identifier">below</span> <span class="special">*/);</span>
|
||||
|
||||
<span class="special">}</span> <span class="comment">// namespace boost</span>
|
||||
</pre>
|
||||
<p>
|
||||
</p>
|
||||
<p>
|
||||
Class <code class="computeroutput"><span class="identifier">none_t</span></code> is meant to
|
||||
serve as a tag for selecting appropriate overloads of from <code class="computeroutput"><span class="identifier">optional</span></code>'s interface. It is an empty,
|
||||
trivially copyable class with disabled default constructor.
|
||||
</p>
|
||||
<p>
|
||||
Constant <code class="computeroutput"><span class="identifier">none</span></code> is used to
|
||||
indicate an optional object that does not contain a value in initialization,
|
||||
assignment and relational operations of <code class="computeroutput"><span class="identifier">optional</span></code>.
|
||||
</p>
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</div>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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</p>
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<boost/optional/optional.hpp></a>
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<div class="section">
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<a name="boost_optional.reference.header__boost_optional_optional_hpp_.header_optional_optional"></a><a name="ref_header_optional_optional_hpp"></a><a class="link" href="header__boost_optional_optional_hpp_.html#boost_optional.reference.header__boost_optional_optional_hpp_.header_optional_optional" title="Synopsis">Synopsis</a>
|
||||
</h4></div></div></div>
|
||||
<pre class="programlisting"><code class="computeroutput"><span class="comment">// In Header: <</span></code><a href="../../../../../../boost/optional/optional.hpp" target="_top">boost/optional/optional.hpp</a><span class="comment">></span>
|
||||
|
||||
<span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span>
|
||||
|
||||
<span class="keyword">class</span> <span class="identifier">in_place_init_t</span> <span class="special">{</span> <span class="comment">/* see below */</span> <span class="special">}</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/header_optional_in_place_init.html#reference_in_place_init"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="keyword">const</span> <span class="identifier">in_place_init_t</span> <span class="identifier">in_place_init</span> <span class="special">(</span> <span class="comment">/* see below */</span> <span class="special">)</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">class</span> <span class="identifier">in_place_init_if_t</span> <span class="special">{</span> <span class="comment">/*see below*/</span> <span class="special">}</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/header_optional_in_place_init.html#reference_in_place_init_if"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="keyword">const</span> <span class="identifier">in_place_init_if_t</span> <span class="identifier">in_place_init_if</span> <span class="special">(</span> <span class="comment">/*see below*/</span> <span class="special">)</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span>
|
||||
<span class="keyword">class</span> <span class="identifier">optional</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/header_optional_optional_values.html#reference_operator_template"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span>
|
||||
<span class="keyword">class</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/header_optional_optional_refs.html#reference_operator_template_spec"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special">==</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___free_functions.html#reference_operator_compare_equal_optional_optional"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special">!=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___free_functions.html#reference_operator_compare_not_equal_optional_optional"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special"><</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___free_functions.html#reference_operator_compare_less_optional_optional"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special">></span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___free_functions.html#reference_operator_compare_greater_optional_optional"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special"><=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___free_functions.html#reference_operator_compare_less_or_equal_optional_optional"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special">>=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___free_functions.html#reference_operator_compare_greater_or_equal_optional_optional"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special">==</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">none_t</span> <span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___free_functions.html#reference_operator_compare_equal_optional_none"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special">!=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">none_t</span> <span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___free_functions.html#reference_operator_compare_not_equal_optional_none"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">make_optional</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___free_functions.html#reference_make_optional_value"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">decay_t</span><span class="special"><</span><span class="identifier">T</span><span class="special">>></span> <span class="identifier">make_optional</span> <span class="special">(</span> <span class="identifier">T</span> <span class="special">&&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___free_functions.html#reference_make_optional_rvalue"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">make_optional</span> <span class="special">(</span> <span class="keyword">bool</span> <span class="identifier">condition</span><span class="special">,</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___free_functions.html#reference_make_optional_bool_value"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">decay_t</span><span class="special"><</span><span class="identifier">T</span><span class="special">>></span> <span class="identifier">make_optional</span> <span class="special">(</span> <span class="keyword">bool</span> <span class="identifier">condition</span><span class="special">,</span> <span class="identifier">T</span> <span class="special">&&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___free_functions.html#reference_make_optional_bool_rvalue"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">auto</span> <span class="identifier">get_optional_value_or</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">opt</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">reference_const_type</span> <span class="identifier">def</span> <span class="special">)</span> <span class="special">-></span> <span class="keyword">typename</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">reference_const_type</span><span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___free_functions.html#reference_free_get_value_or"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">auto</span> <span class="identifier">get_optional_value_or</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">opt</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">reference_type</span> <span class="identifier">def</span> <span class="special">)</span> <span class="special">-></span> <span class="keyword">typename</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">reference_type</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___free_functions.html#reference_free_get_value_or"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">get</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___optional_values.html#reference_optional_get"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">get</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="special">&</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___optional_values.html#reference_optional_get"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">*</span> <span class="identifier">get</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">*</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___optional_values.html#reference_optional_get"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">T</span><span class="special">*</span> <span class="identifier">get</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>*</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___optional_values.html#reference_optional_get"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">auto</span> <span class="identifier">get_pointer</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">-></span> <span class="emphasis"><em>see below</em></span><span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___free_functions.html#reference_free_get_pointer"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">auto</span> <span class="identifier">get_pointer</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="special">&</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">-></span> <span class="emphasis"><em>see below</em></span><span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___free_functions.html#reference_free_get_pointer"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">void</span> <span class="identifier">swap</span><span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___free_functions.html#reference_swap_optional_optional"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">void</span> <span class="identifier">swap</span><span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___free_functions.html#reference_swap_optional_reference"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="special">}</span> <span class="comment">// namespace boost</span>
|
||||
</pre>
|
||||
</div>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
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<a accesskey="p" href="../../boost_optional/reference/header__boost_optional_optional_fwd_hpp_.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../reference.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/header_optional_in_place_init.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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@@ -1,153 +0,0 @@
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
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</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
|
||||
<a name="optional.tutorial"></a><a class="link" href="tutorial.html" title="Tutorial">Tutorial</a>
|
||||
</h2></div></div></div>
|
||||
<div class="toc"><dl class="toc">
|
||||
<dt><span class="section"><a href="tutorial.html#boost_optional.tutorial.motivation">Motivation</a></span></dt>
|
||||
<dt><span class="section"><a href="../boost_optional/tutorial/design_overview.html">Design Overview</a></span></dt>
|
||||
<dt><span class="section"><a href="../boost_optional/tutorial/when_to_use_optional.html">When to
|
||||
use Optional</a></span></dt>
|
||||
<dt><span class="section"><a href="../boost_optional/tutorial/relational_operators.html">Relational
|
||||
operators</a></span></dt>
|
||||
<dt><span class="section"><a href="../boost_optional/tutorial/io_operators.html">IO operators</a></span></dt>
|
||||
<dt><span class="section"><a href="../boost_optional/tutorial/optional_references.html">Optional
|
||||
references</a></span></dt>
|
||||
<dt><span class="section"><a href="../boost_optional/tutorial/in_place_factories.html">In-Place
|
||||
Factories</a></span></dt>
|
||||
<dt><span class="section"><a href="../boost_optional/tutorial/gotchas.html">Gotchas</a></span></dt>
|
||||
<dt><span class="section"><a href="../boost_optional/tutorial/exception_safety_guarantees.html">Exception
|
||||
Safety Guarantees</a></span></dt>
|
||||
<dt><span class="section"><a href="../boost_optional/tutorial/type_requirements.html">Type requirements</a></span></dt>
|
||||
<dt><span class="section"><a href="../boost_optional/tutorial/performance_considerations.html">Performance
|
||||
considerations</a></span></dt>
|
||||
</dl></div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.tutorial.motivation"></a><a class="link" href="tutorial.html#boost_optional.tutorial.motivation" title="Motivation">Motivation</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
Consider these functions which should return a value but which might not
|
||||
have a value to return:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
(A) <code class="computeroutput"><span class="keyword">double</span> <span class="identifier">sqrt</span><span class="special">(</span><span class="keyword">double</span> <span class="identifier">n</span> <span class="special">);</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
(B) <code class="computeroutput"><span class="keyword">char</span> <span class="identifier">get_async_input</span><span class="special">();</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
(C) <code class="computeroutput"><span class="identifier">point</span> <span class="identifier">polygon</span><span class="special">::</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span></code>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
There are different approaches to the issue of not having a value to return.
|
||||
</p>
|
||||
<p>
|
||||
A typical approach is to consider the existence of a valid return value as
|
||||
a postcondition, so that if the function cannot compute the value to return,
|
||||
it has either undefined behavior (and can use assert in a debug build) or
|
||||
uses a runtime check and throws an exception if the postcondition is violated.
|
||||
This is a reasonable choice for example, for function (A), because the lack
|
||||
of a proper return value is directly related to an invalid parameter (out
|
||||
of domain argument), so it is appropriate to require the callee to supply
|
||||
only parameters in a valid domain for execution to continue normally.
|
||||
</p>
|
||||
<p>
|
||||
However, function (B), because of its asynchronous nature, does not fail
|
||||
just because it can't find a value to return; so it is incorrect to consider
|
||||
such a situation an error and assert or throw an exception. This function
|
||||
must return, and somehow, must tell the callee that it is not returning a
|
||||
meaningful value.
|
||||
</p>
|
||||
<p>
|
||||
A similar situation occurs with function (C): it is conceptually an error
|
||||
to ask a <span class="emphasis"><em>null-area</em></span> polygon to return a point inside
|
||||
itself, but in many applications, it is just impractical for performance
|
||||
reasons to treat this as an error (because detecting that the polygon has
|
||||
no area might be too expensive to be required to be tested previously), and
|
||||
either an arbitrary point (typically at infinity) is returned, or some efficient
|
||||
way to tell the callee that there is no such point is used.
|
||||
</p>
|
||||
<p>
|
||||
There are various mechanisms to let functions communicate that the returned
|
||||
value is not valid. One such mechanism, which is quite common since it has
|
||||
zero or negligible overhead, is to use a special value which is reserved
|
||||
to communicate this. Classical examples of such special values are <code class="computeroutput"><span class="identifier">EOF</span></code>, <code class="computeroutput"><span class="identifier">string</span><span class="special">::</span><span class="identifier">npos</span></code>,
|
||||
points at infinity, etc...
|
||||
</p>
|
||||
<p>
|
||||
When those values exist, i.e. the return type can hold all meaningful values
|
||||
<span class="emphasis"><em>plus</em></span> the <span class="emphasis"><em>signal</em></span> value, this mechanism
|
||||
is quite appropriate and well known. Unfortunately, there are cases when
|
||||
such values do not exist. In these cases, the usual alternative is either
|
||||
to use a wider type, such as <code class="computeroutput"><span class="keyword">int</span></code>
|
||||
in place of <code class="computeroutput"><span class="keyword">char</span></code>; or a compound
|
||||
type, such as <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span></code>.
|
||||
</p>
|
||||
<p>
|
||||
Returning a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span></code>, thus attaching a boolean flag to the
|
||||
result which indicates if the result is meaningful, has the advantage that
|
||||
can be turned into a consistent idiom since the first element of the pair
|
||||
can be whatever the function would conceptually return. For example, the
|
||||
last two functions could have the following interface:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="keyword">char</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">get_async_input</span><span class="special">();</span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">polygon</span><span class="special">::</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span>
|
||||
</pre>
|
||||
<p>
|
||||
These functions use a consistent interface for dealing with possibly nonexistent
|
||||
results:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">poly</span><span class="special">.</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">second</span> <span class="special">)</span>
|
||||
<span class="identifier">flood_fill</span><span class="special">(</span><span class="identifier">p</span><span class="special">.</span><span class="identifier">first</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
However, not only is this quite a burden syntactically, it is also error
|
||||
prone since the user can easily use the function result (first element of
|
||||
the pair) without ever checking if it has a valid value.
|
||||
</p>
|
||||
<p>
|
||||
Clearly, we need a better idiom.
|
||||
</p>
|
||||
</div>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
|
||||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||||
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
|
||||
</p>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="../boost_optional/quick_start/storage_in_containers.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="../boost_optional/tutorial/design_overview.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
|
||||
</div>
|
||||
</body>
|
||||
</html>
|
||||
|
Before Width: | Height: | Size: 293 B After Width: | Height: | Size: 293 B |
|
Before Width: | Height: | Size: 81 B After Width: | Height: | Size: 81 B |
@@ -1,5 +1,5 @@
|
||||
// Copyright (C) 2003, Fernando Luis Cacciola Carballal.
|
||||
// Copyright (C) 2014, 2015 Andrzej Krzemienski.
|
||||
// Copyright (C) 2014, 2024 Andrzej Krzemienski.
|
||||
//
|
||||
// Use, modification, and distribution is subject to the Boost Software
|
||||
// License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
|
||||
@@ -14,12 +14,11 @@
|
||||
#define BOOST_NONE_T_17SEP2003_HPP
|
||||
|
||||
#include <boost/config.hpp>
|
||||
#include <boost/config/pragma_message.hpp>
|
||||
|
||||
#if defined (BOOST_NO_CXX11_RVALUE_REFERENCES) || defined(BOOST_NO_CXX11_VARIADIC_TEMPLATES) || defined(BOOST_NO_CXX11_REF_QUALIFIERS) \
|
||||
|| defined(BOOST_NO_CXX11_LAMBDAS) || defined(BOOST_NO_CXX11_DECLTYPE_N3276) || defined(BOOST_NO_CXX11_NOEXCEPT) || defined(BOOST_NO_CXX11_DELETED_FUNCTIONS) || defined(BOOST_NO_CXX11_DEFAULTED_FUNCTIONS) || defined(BOOST_NO_CXX11_DEFAULTED_MOVES) || defined(BOOST_NO_CXX11_EXPLICIT_CONVERSION_OPERATORS)
|
||||
|
||||
BOOST_PRAGMA_MESSAGE("C++03 support is deprecated in Boost.Optional 1.83 and will be removed in Boost.Optional 1.86.")
|
||||
#error "Boost.Optional requires C++11 or later. If you have an older C++ version use Boost.Optional version 1.86 or earlier."
|
||||
|
||||
#endif
|
||||
|
||||
|
||||
@@ -12,19 +12,14 @@
|
||||
#ifndef BOOST_OPTIONAL_DETAIL_EXPERIMENTAL_TRAITS_04NOV2017_HPP
|
||||
#define BOOST_OPTIONAL_DETAIL_EXPERIMENTAL_TRAITS_04NOV2017_HPP
|
||||
|
||||
#include <boost/config.hpp>
|
||||
#include <boost/detail/workaround.hpp>
|
||||
#include <boost/predef.h>
|
||||
#include <boost/type_traits.hpp>
|
||||
|
||||
|
||||
// The condition to use POD implementation
|
||||
|
||||
#ifdef BOOST_OPTIONAL_CONFIG_NO_POD_SPEC
|
||||
# define BOOST_OPTIONAL_DETAIL_NO_SPEC_FOR_TRIVIAL_TYPES
|
||||
#elif defined BOOST_OPTIONAL_CONFIG_NO_SPEC_FOR_TRIVIAL_TYPES
|
||||
# define BOOST_OPTIONAL_DETAIL_NO_SPEC_FOR_TRIVIAL_TYPES
|
||||
#elif !defined BOOST_HAS_TRIVIAL_CONSTRUCTOR
|
||||
# define BOOST_OPTIONAL_DETAIL_NO_SPEC_FOR_TRIVIAL_TYPES
|
||||
#elif !defined BOOST_HAS_TRIVIAL_MOVE_ASSIGN
|
||||
# define BOOST_OPTIONAL_DETAIL_NO_SPEC_FOR_TRIVIAL_TYPES
|
||||
#elif !defined BOOST_HAS_TRIVIAL_MOVE_CONSTRUCTOR
|
||||
@@ -35,37 +30,14 @@
|
||||
# define BOOST_OPTIONAL_DETAIL_NO_SPEC_FOR_TRIVIAL_TYPES
|
||||
#elif !defined BOOST_HAS_TRIVIAL_DESTRUCTOR
|
||||
# define BOOST_OPTIONAL_DETAIL_NO_SPEC_FOR_TRIVIAL_TYPES
|
||||
#elif BOOST_WORKAROUND(BOOST_GCC, < 50000)
|
||||
# define BOOST_OPTIONAL_DETAIL_NO_SPEC_FOR_TRIVIAL_TYPES
|
||||
#endif
|
||||
|
||||
// GCC 5 or higher, or clang with libc++ or clang with libstdc++ 5 or higher
|
||||
#if __cplusplus >= 201103L
|
||||
# if BOOST_WORKAROUND(BOOST_GCC, >= 50000)
|
||||
# define BOOST_OPTIONAL_DETAIL_USE_STD_TYPE_TRAITS
|
||||
# elif (defined BOOST_CLANG)
|
||||
# if BOOST_LIB_STD_CXX > 0
|
||||
# define BOOST_OPTIONAL_DETAIL_USE_STD_TYPE_TRAITS
|
||||
# elif BOOST_LIB_STD_GNU >= 441200023 && BOOST_LIB_STD_GNU != 450600023 && BOOST_LIB_STD_GNU != 450600026 && BOOST_LIB_STD_GNU != 460800003 && BOOST_LIB_STD_GNU != 450400026 && BOOST_LIB_STD_GNU != 460700026
|
||||
# define BOOST_OPTIONAL_DETAIL_USE_STD_TYPE_TRAITS
|
||||
# endif
|
||||
# endif
|
||||
#endif
|
||||
|
||||
|
||||
#ifndef BOOST_OPTIONAL_DETAIL_USE_STD_TYPE_TRAITS
|
||||
# define BOOST_OPTIONAL_DETAIL_HAS_TRIVIAL_CTOR(T) BOOST_HAS_TRIVIAL_CONSTRUCTOR(T)
|
||||
#else
|
||||
# include <type_traits>
|
||||
# define BOOST_OPTIONAL_DETAIL_HAS_TRIVIAL_CTOR(T) std::is_trivially_default_constructible<T>::value
|
||||
#endif
|
||||
|
||||
|
||||
namespace boost { namespace optional_detail {
|
||||
|
||||
|
||||
#ifndef BOOST_OPTIONAL_DETAIL_NO_SPEC_FOR_TRIVIAL_TYPES
|
||||
template <typename T>
|
||||
struct is_type_trivially_copyable
|
||||
struct is_trivially_semiregular
|
||||
: boost::conditional<(boost::has_trivial_copy_constructor<T>::value &&
|
||||
boost::has_trivial_move_constructor<T>::value &&
|
||||
boost::has_trivial_destructor<T>::value &&
|
||||
@@ -75,30 +47,13 @@ struct is_type_trivially_copyable
|
||||
{};
|
||||
#else
|
||||
template <typename T>
|
||||
struct is_type_trivially_copyable
|
||||
struct is_trivially_semiregular
|
||||
: boost::conditional<(boost::is_scalar<T>::value && !boost::is_const<T>::value && !boost::is_volatile<T>::value),
|
||||
boost::true_type, boost::false_type>::type
|
||||
{};
|
||||
#endif
|
||||
|
||||
|
||||
|
||||
#ifndef BOOST_OPTIONAL_DETAIL_NO_SPEC_FOR_TRIVIAL_TYPES
|
||||
template <typename T>
|
||||
struct optional_uses_direct_storage_for_
|
||||
: boost::conditional< (is_type_trivially_copyable<T>::value && BOOST_OPTIONAL_DETAIL_HAS_TRIVIAL_CTOR(T)) ||
|
||||
(boost::is_scalar<T>::value && !boost::is_const<T>::value && !boost::is_volatile<T>::value)
|
||||
, boost::true_type, boost::false_type>::type
|
||||
{};
|
||||
#else
|
||||
template <typename T>
|
||||
struct optional_uses_direct_storage_for_
|
||||
: boost::conditional<(boost::is_scalar<T>::value && !boost::is_const<T>::value && !boost::is_volatile<T>::value)
|
||||
, boost::true_type, boost::false_type>::type
|
||||
{};
|
||||
#endif
|
||||
|
||||
|
||||
}} // boost::optional_detail
|
||||
|
||||
#endif
|
||||
|
||||
File diff suppressed because it is too large
Load Diff
@@ -60,9 +60,8 @@ class aligned_storage
|
||||
T * ptr_ref() { return static_cast<T *> (address()); }
|
||||
#endif
|
||||
|
||||
T const& ref() const { return *ptr_ref(); }
|
||||
T & ref() { return *ptr_ref(); }
|
||||
|
||||
T const& ref() const { return *boost::core::launder(ptr_ref()); }
|
||||
T & ref() { return *boost::core::launder(ptr_ref()); }
|
||||
} ;
|
||||
|
||||
} // namespace optional_detail
|
||||
|
||||
@@ -14,7 +14,7 @@
|
||||
#define BOOST_OPTIONAL_DETAIL_OPTIONAL_CONFIG_AJK_28JAN2015_HPP
|
||||
|
||||
#include <boost/config.hpp>
|
||||
#include <boost/detail/workaround.hpp>
|
||||
#include <boost/config/workaround.hpp>
|
||||
|
||||
#if (defined BOOST_NO_CXX11_RVALUE_REFERENCES) || (defined BOOST_OPTIONAL_CONFIG_NO_RVALUE_REFERENCES)
|
||||
# define BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
|
||||
@@ -55,7 +55,7 @@
|
||||
#if (defined(_MSC_VER) && _MSC_VER <= 1800)
|
||||
// on MSVC 2013 and earlier an unwanted temporary is created when you assign from
|
||||
// a const lvalue of integral type. Thus we bind not to the original address but
|
||||
// to a temporary.
|
||||
// to a temporary.
|
||||
# define BOOST_OPTIONAL_CONFIG_NO_PROPER_ASSIGN_FROM_CONST_INT
|
||||
#endif
|
||||
|
||||
|
||||
@@ -1,4 +1,4 @@
|
||||
// Copyright (C) 2015-2018 Andrzej Krzemienski.
|
||||
// Copyright (C) 2015-2024 Andrzej Krzemienski.
|
||||
//
|
||||
// Use, modification, and distribution is subject to the Boost Software
|
||||
// License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
|
||||
@@ -29,15 +29,15 @@ template <class From>
|
||||
void prevent_binding_rvalue()
|
||||
{
|
||||
#ifndef BOOST_OPTIONAL_CONFIG_ALLOW_BINDING_TO_RVALUES
|
||||
BOOST_STATIC_ASSERT_MSG(boost::is_lvalue_reference<From>::value,
|
||||
BOOST_STATIC_ASSERT_MSG(boost::is_lvalue_reference<From>::value,
|
||||
"binding rvalue references to optional lvalue references is disallowed");
|
||||
#endif
|
||||
#endif
|
||||
}
|
||||
|
||||
template <class T>
|
||||
BOOST_DEDUCED_TYPENAME boost::remove_reference<T>::type& forward_reference(T&& r)
|
||||
{
|
||||
BOOST_STATIC_ASSERT_MSG(boost::is_lvalue_reference<T>::value,
|
||||
BOOST_STATIC_ASSERT_MSG(boost::is_lvalue_reference<T>::value,
|
||||
"binding rvalue references to optional lvalue references is disallowed");
|
||||
return boost::forward<T>(r);
|
||||
}
|
||||
@@ -68,10 +68,10 @@ void prevent_assignment_from_false_const_integral()
|
||||
#ifdef BOOST_OPTIONAL_CONFIG_NO_PROPER_ASSIGN_FROM_CONST_INT
|
||||
// MSVC compiler without rvalue references: we need to disable the assignment from
|
||||
// const integral lvalue reference, as it may be an invalid temporary
|
||||
BOOST_STATIC_ASSERT_MSG(!is_const_integral<From>::value,
|
||||
BOOST_STATIC_ASSERT_MSG(!is_const_integral<From>::value,
|
||||
"binding const lvalue references to integral types is disabled in this compiler");
|
||||
#endif
|
||||
#endif
|
||||
#endif
|
||||
}
|
||||
|
||||
|
||||
@@ -114,7 +114,7 @@ template <class T>
|
||||
class optional<T&> : public optional_detail::optional_tag
|
||||
{
|
||||
T* ptr_;
|
||||
|
||||
|
||||
public:
|
||||
typedef T& value_type;
|
||||
typedef T& reference_type;
|
||||
@@ -122,19 +122,19 @@ public:
|
||||
typedef T& rval_reference_type;
|
||||
typedef T* pointer_type;
|
||||
typedef T* pointer_const_type;
|
||||
|
||||
|
||||
optional() BOOST_NOEXCEPT : ptr_() {}
|
||||
optional(none_t) BOOST_NOEXCEPT : ptr_() {}
|
||||
optional(none_t) BOOST_NOEXCEPT : ptr_() {}
|
||||
|
||||
template <class U>
|
||||
explicit optional(const optional<U&>& rhs) BOOST_NOEXCEPT : ptr_(rhs.get_ptr()) {}
|
||||
optional(const optional& rhs) BOOST_NOEXCEPT : ptr_(rhs.get_ptr()) {}
|
||||
|
||||
|
||||
// the following two implement a 'conditionally explicit' constructor: condition is a hack for buggy compilers with screwed conversion construction from const int
|
||||
template <class U>
|
||||
explicit optional(U& rhs, BOOST_DEDUCED_TYPENAME boost::enable_if_c<detail::is_same_decayed<T, U>::value && detail::is_const_integral_bad_for_conversion<U>::value, bool>::type = true) BOOST_NOEXCEPT
|
||||
: ptr_(boost::addressof(rhs)) {}
|
||||
|
||||
|
||||
template <class U>
|
||||
optional(U& rhs, BOOST_DEDUCED_TYPENAME boost::enable_if_c<detail::is_same_decayed<T, U>::value && !detail::is_const_integral_bad_for_conversion<U>::value, bool>::type = true) BOOST_NOEXCEPT
|
||||
: ptr_(boost::addressof(rhs)) {}
|
||||
@@ -143,15 +143,15 @@ public:
|
||||
template <class U>
|
||||
optional& operator=(const optional<U&>& rhs) BOOST_NOEXCEPT { ptr_ = rhs.get_ptr(); return *this; }
|
||||
optional& operator=(none_t) BOOST_NOEXCEPT { ptr_ = 0; return *this; }
|
||||
|
||||
|
||||
|
||||
|
||||
void swap(optional& rhs) BOOST_NOEXCEPT { std::swap(ptr_, rhs.ptr_); }
|
||||
T& get() const { BOOST_ASSERT(ptr_); return *ptr_; }
|
||||
|
||||
T* get_ptr() const BOOST_NOEXCEPT { return ptr_; }
|
||||
T* operator->() const { BOOST_ASSERT(ptr_); return ptr_; }
|
||||
T& operator*() const { BOOST_ASSERT(ptr_); return *ptr_; }
|
||||
|
||||
|
||||
T& value() const
|
||||
{
|
||||
if (this->is_initialized())
|
||||
@@ -159,17 +159,17 @@ public:
|
||||
else
|
||||
throw_exception(bad_optional_access());
|
||||
}
|
||||
|
||||
bool operator!() const BOOST_NOEXCEPT { return ptr_ == 0; }
|
||||
BOOST_EXPLICIT_OPERATOR_BOOL_NOEXCEPT()
|
||||
|
||||
|
||||
explicit operator bool() const BOOST_NOEXCEPT { return ptr_ != 0; }
|
||||
|
||||
void reset() BOOST_NOEXCEPT { ptr_ = 0; }
|
||||
|
||||
bool is_initialized() const BOOST_NOEXCEPT { return ptr_ != 0; }
|
||||
bool has_value() const BOOST_NOEXCEPT { return ptr_ != 0; }
|
||||
|
||||
|
||||
template <typename F>
|
||||
optional<typename boost::result_of<F(T&)>::type> map(F f) const
|
||||
optional<typename optional_detail::result_of<F, reference_const_type>::type>
|
||||
map(F f) const
|
||||
{
|
||||
if (this->has_value())
|
||||
return f(this->get());
|
||||
@@ -178,61 +178,62 @@ public:
|
||||
}
|
||||
|
||||
template <typename F>
|
||||
optional<typename optional_detail::optional_value_type<typename boost::result_of<F(T&)>::type>::type> flat_map(F f) const
|
||||
optional<typename optional_detail::result_value_type<F, reference_const_type>::type>
|
||||
flat_map(F f) const
|
||||
{
|
||||
if (this->has_value())
|
||||
return f(get());
|
||||
else
|
||||
return none;
|
||||
}
|
||||
|
||||
#ifndef BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
|
||||
|
||||
|
||||
#ifndef BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
|
||||
|
||||
optional(T&& /* rhs */) BOOST_NOEXCEPT { detail::prevent_binding_rvalue<T&&>(); }
|
||||
|
||||
|
||||
template <class R>
|
||||
optional(R&& r, BOOST_DEDUCED_TYPENAME boost::enable_if<detail::no_unboxing_cond<T, R>, bool>::type = true) BOOST_NOEXCEPT
|
||||
: ptr_(boost::addressof(r)) { detail::prevent_binding_rvalue<R>(); }
|
||||
|
||||
|
||||
template <class R>
|
||||
optional(bool cond, R&& r, BOOST_DEDUCED_TYPENAME boost::enable_if<detail::is_no_optional<R>, bool>::type = true) BOOST_NOEXCEPT
|
||||
: ptr_(cond ? boost::addressof(r) : 0) { detail::prevent_binding_rvalue<R>(); }
|
||||
|
||||
|
||||
template <class R>
|
||||
BOOST_DEDUCED_TYPENAME boost::enable_if<detail::is_no_optional<R>, optional<T&>&>::type
|
||||
operator=(R&& r) BOOST_NOEXCEPT { detail::prevent_binding_rvalue<R>(); ptr_ = boost::addressof(r); return *this; }
|
||||
|
||||
|
||||
template <class R>
|
||||
void emplace(R&& r, BOOST_DEDUCED_TYPENAME boost::enable_if<detail::is_no_optional<R>, bool>::type = true) BOOST_NOEXCEPT
|
||||
{ detail::prevent_binding_rvalue<R>(); ptr_ = boost::addressof(r); }
|
||||
|
||||
|
||||
template <class R>
|
||||
T& get_value_or(R&& r, BOOST_DEDUCED_TYPENAME boost::enable_if<detail::is_no_optional<R>, bool>::type = true) const BOOST_NOEXCEPT
|
||||
{ detail::prevent_binding_rvalue<R>(); return ptr_ ? *ptr_ : r; }
|
||||
|
||||
|
||||
template <class R>
|
||||
T& value_or(R&& r, BOOST_DEDUCED_TYPENAME boost::enable_if<detail::is_no_optional<R>, bool>::type = true) const BOOST_NOEXCEPT
|
||||
{ detail::prevent_binding_rvalue<R>(); return ptr_ ? *ptr_ : r; }
|
||||
|
||||
|
||||
template <class R>
|
||||
void reset(R&& r, BOOST_DEDUCED_TYPENAME boost::enable_if<detail::is_no_optional<R>, bool>::type = true) BOOST_NOEXCEPT
|
||||
{ detail::prevent_binding_rvalue<R>(); ptr_ = boost::addressof(r); }
|
||||
|
||||
|
||||
template <class F>
|
||||
T& value_or_eval(F f) const { return ptr_ ? *ptr_ : detail::forward_reference(f()); }
|
||||
|
||||
|
||||
#else // BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
|
||||
|
||||
|
||||
|
||||
// the following two implement a 'conditionally explicit' constructor
|
||||
template <class U>
|
||||
explicit optional(U& v, BOOST_DEDUCED_TYPENAME boost::enable_if_c<detail::no_unboxing_cond<T, U>::value && detail::is_const_integral_bad_for_conversion<U>::value, bool>::type = true) BOOST_NOEXCEPT
|
||||
: ptr_(boost::addressof(v)) { }
|
||||
|
||||
|
||||
template <class U>
|
||||
optional(U& v, BOOST_DEDUCED_TYPENAME boost::enable_if_c<detail::no_unboxing_cond<T, U>::value && !detail::is_const_integral_bad_for_conversion<U>::value, bool>::type = true) BOOST_NOEXCEPT
|
||||
: ptr_(boost::addressof(v)) { }
|
||||
|
||||
|
||||
template <class U>
|
||||
optional(bool cond, U& v, BOOST_DEDUCED_TYPENAME boost::enable_if<detail::is_no_optional<U>, bool>::type = true) BOOST_NOEXCEPT : ptr_(cond ? boost::addressof(v) : 0) {}
|
||||
|
||||
@@ -247,26 +248,26 @@ public:
|
||||
template <class U>
|
||||
void emplace(U& v, BOOST_DEDUCED_TYPENAME boost::enable_if<detail::is_no_optional<U>, bool>::type = true) BOOST_NOEXCEPT
|
||||
{ ptr_ = boost::addressof(v); }
|
||||
|
||||
|
||||
template <class U>
|
||||
T& get_value_or(U& v, BOOST_DEDUCED_TYPENAME boost::enable_if<detail::is_no_optional<U>, bool>::type = true) const BOOST_NOEXCEPT
|
||||
{ return ptr_ ? *ptr_ : v; }
|
||||
|
||||
|
||||
template <class U>
|
||||
T& value_or(U& v, BOOST_DEDUCED_TYPENAME boost::enable_if<detail::is_no_optional<U>, bool>::type = true) const BOOST_NOEXCEPT
|
||||
{ return ptr_ ? *ptr_ : v; }
|
||||
|
||||
|
||||
template <class U>
|
||||
void reset(U& v, BOOST_DEDUCED_TYPENAME boost::enable_if<detail::is_no_optional<U>, bool>::type = true) BOOST_NOEXCEPT
|
||||
{ ptr_ = boost::addressof(v); }
|
||||
|
||||
|
||||
template <class F>
|
||||
T& value_or_eval(F f) const { return ptr_ ? *ptr_ : f(); }
|
||||
|
||||
|
||||
#endif // BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
|
||||
};
|
||||
|
||||
template <class T>
|
||||
template <class T>
|
||||
void swap ( optional<T&>& x, optional<T&>& y) BOOST_NOEXCEPT
|
||||
{
|
||||
x.swap(y);
|
||||
|
||||
@@ -1,5 +1,5 @@
|
||||
// Copyright (C) 2003, 2008 Fernando Luis Cacciola Carballal.
|
||||
// Copyright (C) 2015 Andrzej Krzemienski.
|
||||
// Copyright (C) 2015, 2024 Andrzej Krzemienski.
|
||||
//
|
||||
// Use, modification, and distribution is subject to the Boost Software
|
||||
// License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
|
||||
@@ -16,7 +16,8 @@
|
||||
namespace boost {
|
||||
|
||||
// optional's relational operators ( ==, !=, <, >, <=, >= ) have deep-semantics (compare values).
|
||||
// WARNING: This is UNLIKE pointers. Use equal_pointees()/less_pointees() in generic code instead.
|
||||
// WARNING: This is UNLIKE pointers. Use equal_pointees()/less_pointees() in generic code instead,
|
||||
// to obtain the same semantic for pointers.
|
||||
|
||||
|
||||
//
|
||||
@@ -31,7 +32,7 @@ bool operator == ( optional<T> const& x, optional<T> const& y )
|
||||
template<class T>
|
||||
inline
|
||||
bool operator < ( optional<T> const& x, optional<T> const& y )
|
||||
{ return less_pointees(x,y); }
|
||||
{ return !y ? false : (!x ? true : (*x) < (*y)); }
|
||||
|
||||
template<class T>
|
||||
inline
|
||||
@@ -60,12 +61,12 @@ bool operator >= ( optional<T> const& x, optional<T> const& y )
|
||||
template<class T>
|
||||
inline
|
||||
bool operator == ( optional<T> const& x, T const& y )
|
||||
{ return equal_pointees(x, optional<T>(y)); }
|
||||
{ return x && (*x == y); }
|
||||
|
||||
template<class T>
|
||||
inline
|
||||
bool operator < ( optional<T> const& x, T const& y )
|
||||
{ return less_pointees(x, optional<T>(y)); }
|
||||
{ return (!x) || (*x < y); }
|
||||
|
||||
template<class T>
|
||||
inline
|
||||
@@ -94,12 +95,12 @@ bool operator >= ( optional<T> const& x, T const& y )
|
||||
template<class T>
|
||||
inline
|
||||
bool operator == ( T const& x, optional<T> const& y )
|
||||
{ return equal_pointees( optional<T>(x), y ); }
|
||||
{ return y && (x == *y); }
|
||||
|
||||
template<class T>
|
||||
inline
|
||||
bool operator < ( T const& x, optional<T> const& y )
|
||||
{ return less_pointees( optional<T>(x), y ); }
|
||||
{ return y && (x < *y); }
|
||||
|
||||
template<class T>
|
||||
inline
|
||||
@@ -133,8 +134,8 @@ bool operator == ( optional<T> const& x, none_t ) BOOST_NOEXCEPT
|
||||
|
||||
template<class T>
|
||||
inline
|
||||
bool operator < ( optional<T> const& x, none_t )
|
||||
{ return less_pointees(x,optional<T>() ); }
|
||||
bool operator < ( optional<T> const&, none_t )
|
||||
{ return false; }
|
||||
|
||||
template<class T>
|
||||
inline
|
||||
@@ -168,7 +169,7 @@ bool operator == ( none_t , optional<T> const& y ) BOOST_NOEXCEPT
|
||||
template<class T>
|
||||
inline
|
||||
bool operator < ( none_t , optional<T> const& y )
|
||||
{ return less_pointees(optional<T>() ,y); }
|
||||
{ return bool(y); }
|
||||
|
||||
template<class T>
|
||||
inline
|
||||
@@ -193,4 +194,3 @@ bool operator >= ( none_t x, optional<T> const& y )
|
||||
} // namespace boost
|
||||
|
||||
#endif // header guard
|
||||
|
||||
|
||||
@@ -35,11 +35,11 @@ class tc_optional_base : public optional_tag
|
||||
|
||||
tc_optional_base()
|
||||
:
|
||||
m_initialized(false) {}
|
||||
m_initialized(false), m_storage() {}
|
||||
|
||||
tc_optional_base ( none_t )
|
||||
:
|
||||
m_initialized(false) {}
|
||||
m_initialized(false), m_storage() {}
|
||||
|
||||
tc_optional_base ( init_value_tag, argument_type val )
|
||||
:
|
||||
|
||||
@@ -22,19 +22,14 @@
|
||||
#include <iosfwd>
|
||||
#endif // BOOST_NO_IOSTREAM
|
||||
|
||||
#ifdef BOOST_OPTIONAL_DETAIL_USE_STD_TYPE_TRAITS
|
||||
# include <type_traits>
|
||||
#endif
|
||||
|
||||
#include <boost/assert.hpp>
|
||||
#include <boost/core/addressof.hpp>
|
||||
#include <boost/core/enable_if.hpp>
|
||||
#include <boost/core/explicit_operator_bool.hpp>
|
||||
#include <boost/core/invoke_swap.hpp>
|
||||
#include <boost/core/launder.hpp>
|
||||
#include <boost/optional/bad_optional_access.hpp>
|
||||
#include <boost/static_assert.hpp>
|
||||
#include <boost/throw_exception.hpp>
|
||||
#include <boost/type.hpp>
|
||||
#include <boost/type_traits/alignment_of.hpp>
|
||||
#include <boost/type_traits/conditional.hpp>
|
||||
#include <boost/type_traits/conjunction.hpp>
|
||||
@@ -58,8 +53,6 @@
|
||||
#include <boost/type_traits/is_scalar.hpp>
|
||||
#include <boost/move/utility.hpp>
|
||||
#include <boost/none.hpp>
|
||||
#include <boost/utility/compare_pointees.hpp>
|
||||
#include <boost/utility/result_of.hpp>
|
||||
|
||||
#include <boost/optional/optional_fwd.hpp>
|
||||
#include <boost/optional/detail/optional_config.hpp>
|
||||
@@ -74,17 +67,34 @@ struct optional_value_type
|
||||
{
|
||||
};
|
||||
|
||||
template <typename T>
|
||||
struct optional_value_type< ::boost::optional<T> >
|
||||
template <typename U>
|
||||
struct optional_value_type< ::boost::optional<U> >
|
||||
{
|
||||
typedef T type;
|
||||
typedef U type;
|
||||
};
|
||||
|
||||
template <typename T>
|
||||
T declval();
|
||||
|
||||
|
||||
// implementing my own result_of so that it works for C++11 (std::result_of)
|
||||
// and in C++20 (std::invoke_result).
|
||||
template <typename F, typename Ref, typename Rslt = decltype(declval<F>()(declval<Ref>()))>
|
||||
struct result_of
|
||||
{
|
||||
typedef Rslt type;
|
||||
};
|
||||
|
||||
template <typename F, typename Ref, typename Rslt = typename optional_value_type<typename result_of<F, Ref>::type>::type>
|
||||
struct result_value_type
|
||||
{
|
||||
typedef Rslt type;
|
||||
};
|
||||
|
||||
// optional<typename optional_detail::optional_value_type<decltype(optional_detail::declval<F>()(optional_detail::declval<reference_type>()))>::type>
|
||||
|
||||
}} // namespace boost::optional_detail
|
||||
|
||||
#ifdef BOOST_OPTIONAL_CONFIG_USE_OLD_IMPLEMENTATION_OF_OPTIONAL
|
||||
#include <boost/optional/detail/old_optional_implementation.hpp>
|
||||
#else
|
||||
namespace boost {
|
||||
|
||||
namespace optional_ns {
|
||||
@@ -1383,7 +1393,6 @@ class optional
|
||||
}
|
||||
#endif
|
||||
|
||||
|
||||
#if (!defined BOOST_NO_CXX11_REF_QUALIFIERS) && (!defined BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES)
|
||||
template <typename F>
|
||||
value_type value_or_eval ( F f ) const&
|
||||
@@ -1404,7 +1413,7 @@ class optional
|
||||
}
|
||||
|
||||
template <typename F>
|
||||
optional<typename boost::result_of<F(reference_type)>::type> map(F f) &
|
||||
optional<typename optional_detail::result_of<F, reference_type>::type> map(F f) &
|
||||
{
|
||||
if (this->has_value())
|
||||
return f(get());
|
||||
@@ -1413,7 +1422,7 @@ class optional
|
||||
}
|
||||
|
||||
template <typename F>
|
||||
optional<typename boost::result_of<F(reference_const_type)>::type> map(F f) const&
|
||||
optional<typename optional_detail::result_of<F, reference_const_type>::type> map(F f) const&
|
||||
{
|
||||
if (this->has_value())
|
||||
return f(get());
|
||||
@@ -1422,7 +1431,7 @@ class optional
|
||||
}
|
||||
|
||||
template <typename F>
|
||||
optional<typename boost::result_of<F(reference_type_of_temporary_wrapper)>::type> map(F f) &&
|
||||
optional<typename optional_detail::result_of<F, reference_type_of_temporary_wrapper>::type> map(F f) &&
|
||||
{
|
||||
if (this->has_value())
|
||||
return f(boost::move(this->get()));
|
||||
@@ -1431,7 +1440,8 @@ class optional
|
||||
}
|
||||
|
||||
template <typename F>
|
||||
optional<typename optional_detail::optional_value_type<typename boost::result_of<F(reference_type)>::type>::type> flat_map(F f) &
|
||||
optional<typename optional_detail::result_value_type<F, reference_type>::type>
|
||||
flat_map(F f) &
|
||||
{
|
||||
if (this->has_value())
|
||||
return f(get());
|
||||
@@ -1440,7 +1450,8 @@ class optional
|
||||
}
|
||||
|
||||
template <typename F>
|
||||
optional<typename optional_detail::optional_value_type<typename boost::result_of<F(reference_const_type)>::type>::type> flat_map(F f) const&
|
||||
optional<typename optional_detail::result_value_type<F, reference_const_type>::type>
|
||||
flat_map(F f) const&
|
||||
{
|
||||
if (this->has_value())
|
||||
return f(get());
|
||||
@@ -1449,7 +1460,8 @@ class optional
|
||||
}
|
||||
|
||||
template <typename F>
|
||||
optional<typename optional_detail::optional_value_type<typename boost::result_of<F(reference_type_of_temporary_wrapper)>::type>::type> flat_map(F f) &&
|
||||
optional<typename optional_detail::result_value_type<F, reference_type_of_temporary_wrapper>::type>
|
||||
flat_map(F f) &&
|
||||
{
|
||||
if (this->has_value())
|
||||
return f(boost::move(get()));
|
||||
@@ -1468,7 +1480,8 @@ class optional
|
||||
}
|
||||
|
||||
template <typename F>
|
||||
optional<typename boost::result_of<F(reference_type)>::type> map(F f)
|
||||
optional<typename optional_detail::result_of<F, reference_type>::type>
|
||||
map(F f)
|
||||
{
|
||||
if (this->has_value())
|
||||
return f(get());
|
||||
@@ -1477,7 +1490,8 @@ class optional
|
||||
}
|
||||
|
||||
template <typename F>
|
||||
optional<typename boost::result_of<F(reference_const_type)>::type> map(F f) const
|
||||
optional<typename optional_detail::result_of<F, reference_const_type>::type>
|
||||
map(F f) const
|
||||
{
|
||||
if (this->has_value())
|
||||
return f(get());
|
||||
@@ -1486,7 +1500,8 @@ class optional
|
||||
}
|
||||
|
||||
template <typename F>
|
||||
optional<typename optional_detail::optional_value_type<typename boost::result_of<F(reference_type)>::type>::type> flat_map(F f)
|
||||
optional<typename optional_detail::result_value_type<F, reference_type>::type>
|
||||
flat_map(F f)
|
||||
{
|
||||
if (this->has_value())
|
||||
return f(get());
|
||||
@@ -1495,7 +1510,8 @@ class optional
|
||||
}
|
||||
|
||||
template <typename F>
|
||||
optional<typename optional_detail::optional_value_type<typename boost::result_of<F(reference_const_type)>::type>::type> flat_map(F f) const
|
||||
optional<typename optional_detail::result_value_type<F, reference_const_type>::type>
|
||||
flat_map(F f) const
|
||||
{
|
||||
if (this->has_value())
|
||||
return f(get());
|
||||
@@ -1507,16 +1523,9 @@ class optional
|
||||
|
||||
bool has_value() const BOOST_NOEXCEPT { return this->is_initialized() ; }
|
||||
|
||||
bool operator!() const BOOST_NOEXCEPT { return !this->is_initialized() ; }
|
||||
|
||||
BOOST_EXPLICIT_OPERATOR_BOOL_NOEXCEPT()
|
||||
explicit operator bool() const BOOST_NOEXCEPT { return this->has_value() ; }
|
||||
} ;
|
||||
|
||||
} // namespace boost
|
||||
|
||||
#endif // BOOST_OPTIONAL_CONFIG_USE_OLD_IMPLEMENTATION_OF_OPTIONAL
|
||||
|
||||
namespace boost {
|
||||
|
||||
#ifndef BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
|
||||
template<class T>
|
||||
|
||||
+1
-1
@@ -13,5 +13,5 @@
|
||||
"Fernando Cacciola <fernando_cacciola -at- ciudad.com.ar>",
|
||||
"Andrzej Krzemienski <akrzemi1 -at- gmail.com>"
|
||||
],
|
||||
"cxxstd": "03"
|
||||
"cxxstd": "11"
|
||||
}
|
||||
|
||||
@@ -0,0 +1,11 @@
|
||||
# Copyright 2018, 2019 Peter Dimov
|
||||
# Distributed under the Boost Software License, Version 1.0.
|
||||
# See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt
|
||||
|
||||
include(BoostTestJamfile OPTIONAL RESULT_VARIABLE HAVE_BOOST_TEST)
|
||||
|
||||
if(HAVE_BOOST_TEST)
|
||||
|
||||
boost_test_jamfile(FILE Jamfile.v2 LINK_LIBRARIES Boost::optional Boost::core Boost::bind Boost::mpl Boost::tuple)
|
||||
|
||||
endif()
|
||||
+88
-74
@@ -2,6 +2,7 @@
|
||||
#
|
||||
# Copyright (C) 2003, Fernando Luis Cacciola Carballal.
|
||||
# Copyright (C) 2014 - 2017 Andrzej Krzemienski
|
||||
# Copyright (C) 2024 Alexander Grund
|
||||
#
|
||||
# Use, modification, and distribution is subject to the Boost Software
|
||||
# License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
|
||||
@@ -13,79 +14,92 @@
|
||||
# akrzemi1@gmail.com
|
||||
#
|
||||
|
||||
import config : requires ;
|
||||
import testing ;
|
||||
|
||||
{
|
||||
test-suite optional :
|
||||
[ run optional_test.cpp ]
|
||||
[ run optional_test_assign.cpp ]
|
||||
[ run optional_test_swap.cpp ]
|
||||
[ run optional_test_conversions_from_U.cpp ]
|
||||
[ run optional_test_convert_from_T.cpp ]
|
||||
[ run optional_test_convert_assign.cpp ]
|
||||
[ run optional_test_empty_braces.cpp ]
|
||||
[ run optional_test_make_optional.cpp ]
|
||||
[ run optional_test_flat_map.cpp ]
|
||||
[ run optional_test_hash.cpp ]
|
||||
[ run optional_test_map.cpp ]
|
||||
[ run optional_test_tie.cpp ]
|
||||
[ run optional_test_ref_assign_portable_minimum.cpp ]
|
||||
[ run optional_test_ref_assign_mutable_int.cpp ]
|
||||
[ run optional_test_ref_assign_const_int.cpp ]
|
||||
[ run optional_test_ref_converting_ctor.cpp ]
|
||||
[ run optional_test_ref_convert_assign_non_int.cpp ]
|
||||
[ run optional_test_ref_convert_assign_mutable_int.cpp ]
|
||||
[ run optional_test_ref_convert_assign_const_int.cpp ]
|
||||
[ run optional_test_ref_portable_minimum.cpp ]
|
||||
[ run optional_test_ref_move.cpp ]
|
||||
[ run optional_test_ref_to_val.cpp ]
|
||||
[ run optional_test_inplace_factory.cpp ]
|
||||
[ run optional_test_io.cpp ]
|
||||
[ run optional_test_move.cpp ]
|
||||
[ run optional_test_noexcept_move.cpp ]
|
||||
[ run optional_test_old_impl.cpp ]
|
||||
[ run optional_test_equals_none.cpp ]
|
||||
[ run optional_test_value_access.cpp ]
|
||||
[ run optional_test_emplace.cpp ]
|
||||
[ run optional_test_minimum_requirements.cpp ]
|
||||
[ run optional_test_msvc_bug_workaround.cpp ]
|
||||
[ run optional_test_member_T.cpp ]
|
||||
[ run optional_test_tc_base.cpp ]
|
||||
[ compile optional_test_sfinae_friendly_ctor.cpp ]
|
||||
[ compile optional_test_path_assignment.cpp ]
|
||||
[ compile-fail optional_test_fail_const_swap.cpp ]
|
||||
[ compile-fail optional_test_ref_convert_assign_const_int_prevented.cpp ]
|
||||
[ compile-fail optional_test_fail1.cpp ]
|
||||
[ compile-fail optional_test_fail3a.cpp ]
|
||||
[ compile-fail optional_test_fail3b.cpp ]
|
||||
[ compile-fail optional_test_ref_fail1.cpp ]
|
||||
[ compile-fail optional_test_ref_fail3.cpp ]
|
||||
[ compile-fail optional_test_ref_fail4.cpp ]
|
||||
[ compile-fail optional_test_inplace_fail.cpp ]
|
||||
[ compile-fail optional_test_inplace_fail2.cpp ]
|
||||
[ compile-fail optional_test_fail_implicit_bool_convert.cpp ]
|
||||
[ compile-fail optional_test_fail_copying_a_moveable_type.cpp ]
|
||||
[ compile-fail optional_test_fail_optional_rvalue_ref.cpp ]
|
||||
[ compile-fail optional_test_ref_fail_init_from_Trefref.cpp ]
|
||||
[ compile-fail optional_test_ref_fail_init_from_Urefref.cpp ]
|
||||
[ compile-fail optional_test_ref_fail_assign_from_Trefref.cpp ]
|
||||
[ compile-fail optional_test_ref_fail_assign_from_Urefref.cpp ]
|
||||
[ compile-fail optional_test_fail_convert_from_null.cpp ]
|
||||
[ compile-fail optional_test_fail_explicit_convert_in_value_or.cpp ]
|
||||
[ compile-fail optional_test_fail_explicit_convert_in_value_or_call.cpp ]
|
||||
[ compile-fail optional_test_fail_io_without_io.cpp ]
|
||||
[ compile-fail optional_test_fail_none_io_without_io.cpp ]
|
||||
[ compile-fail optional_test_fail_convert_assign_of_enums.cpp ]
|
||||
[ run optional_test_static_properties.cpp ]
|
||||
[ compile optional_test_maybe_uninitialized_warning.cpp ]
|
||||
[ compile optional_test_deleted_default_ctor.cpp ]
|
||||
[ compile optional_test_constructible_from_other.cpp ]
|
||||
#[ run optional_xconfig_HACK_TO_LIST_PREDEFINED_MACROS.cpp ]
|
||||
[ run optional_xconfig_NO_PROPER_ASSIGN_FROM_CONST_INT_pass.cpp ]
|
||||
[ run-fail optional_xconfig_NO_PROPER_ASSIGN_FROM_CONST_INT_fail.cpp ]
|
||||
[ run optional_xconfig_NO_PROPER_CONVERT_FROM_CONST_INT_pass.cpp ]
|
||||
[ run-fail optional_xconfig_NO_PROPER_CONVERT_FROM_CONST_INT_fail.cpp ]
|
||||
[ run optional_xconfig_NO_LEGAL_CONVERT_FROM_REF_pass.cpp ]
|
||||
[ compile-fail optional_xconfig_NO_LEGAL_CONVERT_FROM_REF_fail.cpp ]
|
||||
;
|
||||
}
|
||||
project
|
||||
: requirements
|
||||
<library>/boost/optional//boost_optional
|
||||
[ requires
|
||||
cxx11_decltype
|
||||
cxx11_defaulted_functions
|
||||
cxx11_defaulted_moves
|
||||
cxx11_deleted_functions
|
||||
cxx11_explicit_conversion_operators
|
||||
cxx11_noexcept
|
||||
cxx11_rvalue_references
|
||||
cxx11_variadic_templates
|
||||
]
|
||||
;
|
||||
|
||||
|
||||
run optional_test.cpp : : : <library>/boost/bind//boost_bind <library>/boost/mpl//boost_mpl ;
|
||||
run optional_test_assign.cpp ;
|
||||
run optional_test_swap.cpp ;
|
||||
compile optional_test_wuninitialized.cpp ;
|
||||
run optional_test_conversions_from_U.cpp ;
|
||||
run optional_test_convert_from_T.cpp ;
|
||||
run optional_test_convert_assign.cpp ;
|
||||
run optional_test_empty_braces.cpp ;
|
||||
run optional_test_make_optional.cpp ;
|
||||
run optional_test_flat_map.cpp ;
|
||||
run optional_test_hash.cpp ;
|
||||
run optional_test_map.cpp ;
|
||||
run optional_test_tie.cpp : : : <library>/boost/tuple//boost_tuple ;
|
||||
run optional_test_ref_assign_portable_minimum.cpp ;
|
||||
run optional_test_ref_assign_mutable_int.cpp ;
|
||||
run optional_test_ref_assign_const_int.cpp ;
|
||||
run optional_test_ref_converting_ctor.cpp ;
|
||||
run optional_test_ref_convert_assign_non_int.cpp ;
|
||||
run optional_test_ref_convert_assign_mutable_int.cpp ;
|
||||
run optional_test_ref_convert_assign_const_int.cpp ;
|
||||
run optional_test_ref_portable_minimum.cpp ;
|
||||
run optional_test_ref_move.cpp ;
|
||||
run optional_test_ref_to_val.cpp ;
|
||||
run optional_test_inplace_factory.cpp ;
|
||||
run optional_test_io.cpp ;
|
||||
run optional_test_move.cpp ;
|
||||
run optional_test_noexcept_move.cpp ;
|
||||
run optional_test_cmp_none.cpp ;
|
||||
run optional_test_value_access.cpp ;
|
||||
run optional_test_emplace.cpp ;
|
||||
run optional_test_minimum_requirements.cpp ;
|
||||
run optional_test_msvc_bug_workaround.cpp ;
|
||||
run optional_test_member_T.cpp ;
|
||||
run optional_test_tc_base.cpp ;
|
||||
compile optional_test_sfinae_friendly_ctor.cpp ;
|
||||
compile optional_test_path_assignment.cpp ;
|
||||
compile-fail optional_test_fail_const_swap.cpp ;
|
||||
compile-fail optional_test_ref_convert_assign_const_int_prevented.cpp ;
|
||||
compile-fail optional_test_fail1.cpp ;
|
||||
compile-fail optional_test_fail3a.cpp ;
|
||||
compile-fail optional_test_fail3b.cpp ;
|
||||
compile-fail optional_test_ref_fail1.cpp ;
|
||||
compile-fail optional_test_ref_fail3.cpp ;
|
||||
compile-fail optional_test_ref_fail4.cpp ;
|
||||
compile-fail optional_test_inplace_fail.cpp ;
|
||||
compile-fail optional_test_inplace_fail2.cpp ;
|
||||
compile-fail optional_test_fail_implicit_bool_convert.cpp ;
|
||||
compile-fail optional_test_fail_copying_a_moveable_type.cpp ;
|
||||
compile-fail optional_test_fail_optional_rvalue_ref.cpp ;
|
||||
compile-fail optional_test_ref_fail_init_from_Trefref.cpp ;
|
||||
compile-fail optional_test_ref_fail_init_from_Urefref.cpp ;
|
||||
compile-fail optional_test_ref_fail_assign_from_Trefref.cpp ;
|
||||
compile-fail optional_test_ref_fail_assign_from_Urefref.cpp ;
|
||||
compile-fail optional_test_fail_convert_from_null.cpp ;
|
||||
compile-fail optional_test_fail_explicit_convert_in_value_or.cpp ;
|
||||
compile-fail optional_test_fail_explicit_convert_in_value_or_call.cpp ;
|
||||
compile-fail optional_test_fail_io_without_io.cpp ;
|
||||
compile-fail optional_test_fail_none_io_without_io.cpp ;
|
||||
compile-fail optional_test_fail_convert_assign_of_enums.cpp ;
|
||||
run optional_test_static_properties.cpp ;
|
||||
compile optional_test_maybe_uninitialized_warning.cpp ;
|
||||
compile optional_test_deleted_default_ctor.cpp ;
|
||||
compile optional_test_constructible_from_other.cpp ;
|
||||
#run optional_xconfig_HACK_TO_LIST_PREDEFINED_MACROS.cpp ;
|
||||
run optional_xconfig_NO_PROPER_ASSIGN_FROM_CONST_INT_pass.cpp ;
|
||||
run-fail optional_xconfig_NO_PROPER_ASSIGN_FROM_CONST_INT_fail.cpp ;
|
||||
run optional_xconfig_NO_PROPER_CONVERT_FROM_CONST_INT_pass.cpp ;
|
||||
run-fail optional_xconfig_NO_PROPER_CONVERT_FROM_CONST_INT_fail.cpp ;
|
||||
run optional_xconfig_NO_LEGAL_CONVERT_FROM_REF_pass.cpp ;
|
||||
compile-fail optional_xconfig_NO_LEGAL_CONVERT_FROM_REF_fail.cpp ;
|
||||
|
||||
@@ -0,0 +1,17 @@
|
||||
# Copyright 2018, 2019 Peter Dimov
|
||||
# Distributed under the Boost Software License, Version 1.0.
|
||||
# See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt
|
||||
|
||||
cmake_minimum_required(VERSION 3.5...3.16)
|
||||
|
||||
project(cmake_subdir_test LANGUAGES CXX)
|
||||
|
||||
find_package(boost_optional REQUIRED)
|
||||
|
||||
add_executable(main main.cpp)
|
||||
target_link_libraries(main Boost::optional)
|
||||
|
||||
enable_testing()
|
||||
add_test(main main)
|
||||
|
||||
add_custom_target(check COMMAND ${CMAKE_CTEST_COMMAND} --output-on-failure -C $<CONFIG>)
|
||||
@@ -0,0 +1,13 @@
|
||||
// Copyright 2024 Peter Dimov
|
||||
// Distributed under the Boost Software License, Version 1.0.
|
||||
// https://www.boost.org/LICENSE_1_0.txt
|
||||
|
||||
#include <boost/optional/optional.hpp>
|
||||
|
||||
int main()
|
||||
{
|
||||
boost::optional<int> o1;
|
||||
boost::optional<int> o2( 0 );
|
||||
o1 = o2;
|
||||
return *o1;
|
||||
}
|
||||
@@ -0,0 +1,18 @@
|
||||
# Copyright 2018, 2019 Peter Dimov
|
||||
# Distributed under the Boost Software License, Version 1.0.
|
||||
# See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt
|
||||
|
||||
cmake_minimum_required(VERSION 3.5...3.16)
|
||||
|
||||
project(cmake_subdir_test LANGUAGES CXX)
|
||||
|
||||
set(BOOST_INCLUDE_LIBRARIES optional)
|
||||
add_subdirectory(../../../.. boostorg/boost)
|
||||
|
||||
add_executable(main main.cpp)
|
||||
target_link_libraries(main Boost::optional)
|
||||
|
||||
enable_testing()
|
||||
add_test(main main)
|
||||
|
||||
add_custom_target(check COMMAND ${CMAKE_CTEST_COMMAND} --output-on-failure -C $<CONFIG>)
|
||||
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Reference in New Issue
Block a user