mp-units/docs/getting_started/installation_and_usage.md

Installation And Usage

This chapter provides all the necessary information to obtain and build the code using mp-units. It also describes how to build or distribute the library and generate its documentation.

C++ compiler support

!!! info

**mp-units** library tries to provide the best user experience possible with the C++ language.
To achieve that, it extensively uses C++20 features and the
[explicit object parameter](https://en.cppreference.com/w/cpp/language/member_functions#Explicit_object_parameter)
from C++23.

Even though the library benefits from C++23 (if available), C++20 is enough to compile and
use all of the library's functionality. C++23 features are hidden behind
a [preprocessor macro](../users_guide/framework_basics/systems_of_quantities.md#defining-quantities)
providing a backward-compatible way to use it.

The below table provides the minimum compiler version required to compile the code using the specific feature:

Feature	gcc	clang	apple-clang	MSVC
Minimum support	12	16	15	None
std::format	13	17	None	None
C++ modules	14	17	None	None
C++23 extensions	14	18	None	None

More requirements for C++ modules support can be found in the CMake's documentation.

Modules

The mp-units library provides the following C++ modules.

flowchart TD
    mp_units --- mp_units.systems --- mp_units.core

C++ Module	CMake Target	Contents
mp_units.core	mp-units::core	Core library framework and systems-independent utilities
mp_units.systems	mp-units::systems	All the systems of quantities and units
mp_units	mp-units::mp-units	Core + Systems

!!! note

C++ modules are provided within the package only when [`cxx_modules`](#cxx_modules) Conan
option is set to `True`.

Repository structure and dependencies

This repository contains three independent CMake-based projects:

• ./src

  • header-only project containing whole mp-units library

  • ./src/CMakeList.txt file is intended as an entry point for library users

  • in case this library becomes part of the C++ standard, it will have no external dependencies but until then, it depends on the following:

    • gsl-lite to verify runtime contracts with the gsl_Expects macro,
    • {fmt} to provide text formatting of quantities (if std::format is not supported yet on a specific compiler).

• .

  • project used as an entry point for library development and CI/CD

  • it wraps ./src project together with usage examples and tests

  • additionally to the dependencies of ./src project, it uses:

    • Catch2 library as a unit tests framework,
    • linear algebra library based on proposal P1385 used in some examples and tests.

• ./test_package

  • CMake library installation and Conan package verification.

!!! important "Important: Library users should not use the top-level CMake file"

Top level _CMakeLists.txt_ file should only be used by **mp-units** developers and contributors
as an entry point for the project's development. We want to ensure that everyone will build **ALL**
the code correctly before pushing a commit. Having such options would allow unintended issues to
leak to PRs and CI.

This is why our projects have two entry points:

- _./CMakeLists.txt_ is **to be used by projects developers** to build **ALL** the project code
  with really restrictive compilation flags,
- _./src/CMakeLists.txt_ contains only a pure library definition and **should be used by the
  customers** that prefer to use CMake's
  [`add_subdirectory()`](https://cmake.org/cmake/help/latest/command/add_subdirectory.html) to
  handle the dependencies.

To learn more about the rationale, please check our
[FAQ](faq.md#why-dont-we-have-cmake-options-to-disable-building-of-tests-and-examples).

Obtaining dependencies

This library assumes that most of the dependencies will be provided by the Conan Package Manager. If you want to obtain required dependencies by other means, some modifications to the library's CMake files might be needed. The rest of the dependencies responsible for documentation generation are provided by python3-pip.

Conan quick intro

In case you are not familiar with Conan, to install it (or upgrade) just do:

pip3 install -U conan

After that, you might need to add a custom profile file for your development environment in ~/.conan2/profiles directory. An example profile can look as follows:

[settings]
arch=x86_64
build_type=Release
compiler=gcc
compiler.cppstd=20
compiler.libcxx=libstdc++11
compiler.version=12
os=Linux

[conf]
tools.build:compiler_executables={"c": "gcc-12", "cpp": "g++-12"}

!!! tip "Setting the language version"

Please note that the **mp-units** library requires at least C++20 to be set in a Conan profile
or forced via the Conan command line. If we do the former, we will not need to provide
`-s compiler.cppstd=20` every time we run a Conan command line (as provided in the command
line instructions below).

!!! tip "Using Ninja as a CMake generator for Conan"

It is highly recommended to set Ninja as a CMake generator for Conan. To do so, we should
create a _~/.conan2/global.conf_ file that will set `tools.cmake.cmaketoolchain:generator`
to one of the Ninja generators. For example:

```text title="~/.conan2/global.conf"
tools.cmake.cmaketoolchain:generator="Ninja Multi-Config"
```

!!! tip "Separate build folders for different configurations"

_~/.conan2/global.conf_ file may also set `tools.cmake.cmake_layout:build_folder_vars` which
[makes working with several compilers or build configurations easier](https://docs.conan.io/2/reference/tools/cmake/cmake_layout.html#multi-setting-option-cmake-layout).
For example, the below line will force Conan to generate separate CMake presets and folders for
each compiler and C++ standard version:

```text title="~/.conan2/global.conf"
tools.cmake.cmake_layout:build_folder_vars=["settings.compiler", "settings.compiler.version", "settings.compiler.cppstd"]
```

In such a case, we will need to use a configuration-specific preset name in the Conan instructions
provided below rather than just `conan-default` and `conan-release`
(e.g. `conan-gcc-13-23` and `conan-gcc-13-23-release`)

Build options

Conan options

cxx_modules{ #cxx_modules }

:octicons-tag-24: 2.2.0 · :octicons-milestone-24: True/False (Default: False)

Configures CMake to add C++ modules to the list of default targets.

use_fmtlib{ #use_fmtlib }

:octicons-tag-24: 2.2.0 · :octicons-milestone-24: True/False (Default: False)

Forces usage of {fmt} library instead of the C++20 Standard Library features.

Conan configuration properties

user.mp-units.build:all{ #user.mp-units.build-all }

:octicons-tag-24: 2.2.0 · :octicons-milestone-24: True/False (Default: False)

Enables compilation of all the source code, including tests and examples. To support this, it requires some additional Conan build dependencies described in Repository Structure and Dependencies. It also runs unit tests during Conan build (unless tools.build:skip_test configuration property is set to True).

user.mp-units.build:skip_la{ #user-skip-la }

:octicons-tag-24: 2.2.0 · :octicons-milestone-24: True/False (Default: False)

If user.mp-units.build:all is enabled, among others, Conan installs the external wg21-linear_algebra dependency and enables the compilation of linear algebra-based tests and usage examples. Such behavior can be disabled with this option.

CMake options

MP_UNITS_BUILD_CXX_MODULES{ #MP_UNITS_BUILD_CXX_MODULES }

:octicons-tag-24: 2.2.0 · :octicons-milestone-24: ON/OFF (Default: OFF)

Adds C++ modules to the list of default targets.

MP_UNITS_USE_FMTLIB{ #MP_UNITS_USE_FMTLIB }

:octicons-tag-24: 2.2.0 · :octicons-milestone-24: ON/OFF (Default: OFF)

Forces usage of {fmt} library instead of the C++20 Standard Library features.

MP_UNITS_BUILD_LA{ #MP_UNITS_BUILD_LA }

:octicons-tag-24: 2.0.0 · :octicons-milestone-24: ON/OFF (Default: ON)

Enables building code depending on the linear algebra library.

MP_UNITS_IWYU{ #MP_UNITS_IWYU }

:octicons-tag-24: 2.0.0 · :octicons-milestone-24: ON/OFF (Default: OFF)

Enables include-what-you-use when compiling with a clang compiler. Additionally turns on MP_UNITS_AS_SYSTEM_HEADERS.

CMake with presets support

It is recommended to use at least CMake 3.23 to build this project as this version introduced support for CMake Presets schema version 4, used now by Conan to generate presets files. All build instructions below assume that you have such support. If not, your CMake invocations have to be replaced with something like:

mkdir build && cd build
cmake .. -G "Ninja Multi-Config" -DCMAKE_TOOLCHAIN_FILE=<path_to_generators_dir>/conan_toolchain.cmake
cmake --build . --config Release

!!! tip

In case you can't use CMake 3.23 but you have access to CMake 3.20 or later, you can append
`-c tools.cmake.cmaketoolchain.presets:max_schema_version=2` to the `conan install` command
which will force Conan to use an older version of the CMake Presets schema.

Installation and reuse

There are many different ways of installing/reusing mp-units in your project. Below we mention only a few of many options possible.

!!! important "Important: Prefer using Conan if possible"

The easiest and most recommended way to obtain **mp-units** is with the Conan package manager.
See [Conan + CMake (release)](#conan-cmake-release) for a detailed instruction.

Copy

As mp-units is a C++ header-only library you can simply copy all needed src/*/include subdirectories to your source tree.

!!! note

In such a case, you are on your own to ensure all the dependencies are installed and their header
files can be located during the build. Please also note that some compiler-specific flags are needed
to make the code compile without issues.

Copy + CMake

If you copy the whole mp-units repository to your project's file tree, you can reuse CMake targets defined by the library. To do so, you should use CMakeLists.txt file from the ./src directory:

add_subdirectory(<path_to_units_folder>/src)
# ...
target_link_libraries(<your_target> <PUBLIC|PRIVATE|INTERFACE> mp-units::mp-units)

!!! note

You are still on your own to make sure all the dependencies are installed and their header and CMake
configuration files can be located during the build.

Conan + CMake (release)

!!! tip

If you are new to the Conan package manager, it is highly recommended to read
[Obtaining Dependencies](#obtaining-dependencies) and refer to
[Consuming packages](https://docs.conan.io/2/tutorial/consuming_packages.html)
chapter of the official Conan documentation for more information.

mp-units releases are hosted on Conan-Center. The following steps may be performed to obtain an official library release:

1. Create Conan configuration file (either conanfile.txt or conanfile.py) in your project's top-level directory and add mp-units as a dependency of your project. For example, the simplest file may look as follows:

   [requires]
   mp-units/2.1.0
   
   [options]
   mp-units:cxx_modules=True
   
   [layout]
   cmake_layout
   
   [generators]
   CMakeToolchain
   CMakeDeps
   

2. Import mp-units and its dependencies definitions to your project's build procedure with find_package:

   find_package(mp-units REQUIRED)
   

3. Link your CMake targets with mp-units:

   target_link_libraries(<your_target> <PUBLIC|PRIVATE|INTERFACE> mp-units::mp-units)
   

4. Download, build, and install Conan dependencies before running the CMake configuration step:

   conan install . -pr <your_conan_profile> -s compiler.cppstd=20 -b=missing
   cmake --preset conan-default
   cmake --build --preset conan-release
   

Conan + CMake (Live At Head)

This chapter describes the procedure to Live At Head, which means using the latest stable version of mp-units all the time.

!!! note

Please note that even though the Conan packages that you will be using are generated **ONLY**
for builds that are considered stable (passed our CI tests), some minor regressions may happen
(our CI and C++20 build environment is not perfect yet). Also, please expect that the library
interface might, and probably will, change occasionally. Even though we do our best, such
changes might not be reflected in the project's documentation right away.

The procedure is similar to the one described in Conan + CMake (release) with the following differences:

1. Before starting the previous procedure, add mp-units remote to your Conan configuration:

   conan remote add conan-mpusz https://mpusz.jfrog.io/artifactory/api/conan/conan-oss
   

2. In your Conan configuration file, provide the package identifier of the mpusz/testing stream:

   [requires]
   mp-units/2.2.0@mpusz/testing
   
   [options]
   mp-units:cxx_modules=True
   
   [layout]
   cmake_layout
   
   [generators]
   CMakeToolchain
   CMakeDeps
   

   !!! tip

    The identifiers of the latest packages can always be found in
    [the project's README file](https://github.com/mpusz/mp-units/blob/master/README.md) or on
    [the project's Artifactory](https://mpusz.jfrog.io/ui/packages/conan:%2F%2Fmp-units).
   

3. Force Conan to check for updated recipes with -u:

   conan install . -pr <your_conan_profile> -s compiler.cppstd=20 -b=missing -u
   

Install

In case you don't want to use Conan in your project and just want to install the mp-units library on your file system and use find_package(mp-units) from another repository to find it; it is enough to perform the following steps:

conan install . -pr <your_conan_profile> -s compiler.cppstd=20 -b=missing
mv CMakeUserPresets.json src
cd src
cmake --preset conan-default -DCMAKE_INSTALL_PREFIX=<your_installation_path>
cmake --build --preset conan-release --target install

Contributing (or just building all the tests and examples)

In case you would like to build all the mp-units source code (with unit tests and examples), you should:

1. Use the CMakeLists.txt from the top-level directory.
2. Run Conan with user.mp-units.build:all = True.

git clone https://github.com/mpusz/mp-units.git && cd units
conan build . -pr <your_conan_profile> -s compiler.cppstd=23 -o cxx_modules=True -c user.mp-units.build:all=True -b missing

The above will download and install all of the dependencies needed for the development of the library, build all of the source code, and run unit tests.

If you prefer to build the project via CMake rather than Conan, then you should replace the conan build with conan install command and then follow with a regular CMake build:

cmake --preset conan-default
cmake --build --preset conan-release
cmake --build --preset conan-release --target all_verify_interface_header_sets
cmake --build --preset conan-release --target test

Building documentation

Starting from mp-units 2.0 we are using Material for MkDocs to build our documentation. The easiest way to install all the required dependencies is with pip:

pip install -U mkdocs-material mkdocs-rss-plugin

Additionally, a Cairo Graphics library is required by Material for MkDocs. Please follow the official MkDocs documentation to install it.

After that, you can either:

• easily start a live server to preview the documentation as you write

  mkdocs serve
  

• build the documentation

  mkdocs build
  

Packaging

To test CMake installation and Conan packaging or create a Conan package run:

conan create . --user <username> --channel <channel> -pr <your_conan_profile> -s compiler.cppstd=20 -o cxx_modules=True -c user.mp-units.build:all=True -b missing

The above will create a Conan package and run tests provided in ./test_package directory.

Uploading mp-units package to the Conan server

conan upload -r <remote-name> --all mp-units/2.1.0@<user>/<channel>