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Iterator Facade and Adaptor
+++++++++++++++++++++++++++++
:Author: David Abrahams, Jeremy Siek, Thomas Witt
:Contact: dave@boost-consulting.com, jsiek@osl.iu.edu, witt@ive.uni-hannover.de
:organization: `Boost Consulting`_, Indiana University `Open Systems
Lab`_, University of Hanover `Institute for Transport
Railway Operation and Construction`_
:date: $Date$
:Number: N1476=03-0059
:copyright: Copyright Dave Abrahams, Jeremy Siek, and Thomas Witt 2003. All rights reserved
.. _`Boost Consulting`: http://www.boost-consulting.com
.. _`Open Systems Lab`: http://www.osl.iu.edu
.. _`Institute for Transport Railway Operation and Construction`: http://www.ive.uni-hannover.de
:abstract: We propose a set of class templates that help programmers
build standard-conforming iterators, both from scratch and
by adapting other iterators.
.. contents:: Table of Contents
============
Motivation
============
Iterators play an important role in modern C++ programming. The
iterator is the central abstraction of the algorithms of the Standard
Library, allowing algorithms to be re-used in in a wide variety of
contexts. The C++ Standard Library contains a wide variety of useful
iterators. Every one of the standard containers comes with constant
and mutable iterators [#mutable]_, and also reverse versions of those
same iterators which traverse the container in the opposite direction.
The Standard also supplies ``istream_iterator`` and
``ostream_iterator`` for reading from and writing to streams,
``insert_iterator``, ``front_insert_iterator`` and
``back_insert_iterator`` for inserting elements into containers, and
``raw_storage_iterator`` for initializing raw memory [7].
Despite the many iterators supplied by the Standard Library, obvious
and useful iterators are missing, and creating new iterator types is
still a common task for C++ programmers. The literature documents
several of these, for example line_iterator [3] and Constant_iterator
[9]. The iterator abstraction is so powerful that we expect
programmers will always need to invent new iterator types.
Although it is easy to create iterators that *almost* conform to the
standard, the iterator requirements contain subtleties which can make
creating an iterator which *actually* conforms quite difficult.
Further, the iterator interface is rich, containing many operators
that are technically redundant and tedious to implement. To automate
the repetitive work of constructing iterators, we propose
``iterator_facade``, an iterator base class template which provides
the rich interface of standard iterators and delegates its
implementation to member functions of the derived class. In addition
to reducing the amount of code necessary to create an iterator, the
``iterator_facade`` also provides compile-time error detection.
Iterator implementation mistakes that often go unnoticed are turned
into compile-time errors because the derived class implementation must
match the expectations of the ``iterator_facade``.
A common pattern of iterator construction is the adaptation of one
iterator to form a new one. The functionality of an iterator is
composed of four orthogonal aspects: traversal, indirection, equality
comparison and distance measurement. Adapting an old iterator to
create a new one often saves work because one can reuse one aspect of
functionality while redefining the other. For example, the Standard
provides ``reverse_iterator``, which adapts any Bidirectional Iterator
by inverting its direction of traversal. As with plain iterators,
iterator adaptors defined outside the Standard have become commonplace
in the literature:
* Checked iter[13] adds bounds-checking to an existing iterator.
* The iterators of the View Template Library[14], which adapts
containers, are themselves adaptors over the underlying iterators.
* Smart iterators [5] adapt an iterator's dereferencing behavior by
applying a function object to the object being referenced and
returning the result.
* Custom iterators [4], in which a variety of adaptor types are enumerated.
* Compound iterators [1], which access a slice out of a container of containers.
* Several iterator adaptors from the MTL [12]. The MTL contains a
strided iterator, where each call to ``operator++()`` moves the
iterator ahead by some constant factor, and a scaled iterator, which
multiplies the dereferenced value by some constant.
.. [#concept] We use the term concept to mean a set of requirements
that a type must satisfy to be used with a particular template
parameter.
.. [#mutable] The term mutable iterator refers to iterators over objects that
can be changed by assigning to the dereferenced iterator, while
constant iterator refers to iterators over objects that cannot be
modified.
To fulfill the need for constructing adaptors, we propose the
``iterator_adaptor`` class template. Instantiations of
``iterator_adaptor`` serve as a base classes for new iterators,
providing the default behavior of forwarding all operations to the
underlying iterator. The user can selectively replace these features
in the derived iterator class. This proposal also includes a number
of more specialized adaptors, such as the ``transform_iterator`` that
applies some user-specified function during the dereference of the
iterator.
========================
Impact on the Standard
========================
This proposal is purely an addition to the C++ standard library.
However, note that this proposal relies on the proposal for New
Iterator Concepts.
========
Design
========
Iterator Concepts
=================
This proposal is formulated in terms of the new ``iterator concepts``
as proposed in `n1477`_, since user-defined and especially adapted
iterators suffer from the well known categorization problems that are
inherent to the current iterator categories.
.. _`n1477`: http://anubis.dkuug.dk/JTC1/SC22/WG21/docs/papers/2003/n1477.html
This proposal does not strictly depend on proposal `n1477`_, as there
is a direct mapping between new and old categories. This proposal
could be reformulated using this mapping if `n1477`_ was not accepted.
Interoperability
================
The question of iterator interoperability is poorly addressed in the
current standard. There are currently two defect reports that are
concerned with interoperability issues.
Issue `179`_ concerns the fact that mutable container iterator types
are only required to be convertible to the corresponding constant
iterator types, but objects of these types are not required to
interoperate in comparison or subtraction expressions. This situation
is tedious in practice and out of line with the way built in types
work. This proposal implements the proposed resolution to issue
`179`_, as most standard library implementations do nowadays. In other
words, if an iterator type A has an implicit or user defined
conversion to an iterator type B, the iterator types are interoperable
and the usual set of operators are available.
Issue `280`_ concerns the current lack of interoperability between
reverse iterator types. The proposed new reverse_iterator template
fixes the issues raised in 280. It provides the desired
interoperability without introducing unwanted overloads.
.. _`179`: http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/lwg-defects.html#179
.. _`280`: http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/lwg-active.html#280
Iterator Facade
===============
While the iterator interface is rich, there is a core subset of the
interface that is necessary for all the functionality. We have
identified the following core behaviors for iterators:
* dereferencing
* incrementing
* decrementing
* equality comparison
* random-access motion
* distance measurement
In addition to the behaviors listed above, the core interface elements
include the associated types exposed through iterator traits:
``value_type``, ``reference``, ``difference_type``, and
``iterator_category``.
Iterator facade uses the Curiously Recurring Template Pattern (CRTP)
[Cop95]_ so that the user can specify the behavior of
``iterator_facade`` in a derived class. Former designs used policy
objects to specify the behavior. The proposal does not use policy
objects for several reasons:
1. the creation and eventual copying of the policy object may create
overhead that can be avoided with the current approach.
2. The policy object approach does not allow for custom constructors
on the created iterator types, an essential feature if
``iterator_facade`` should be used in other library
implementations.
3. Without the use of CRTP, the standard requirement that an
iterator's ``operator++`` returns the iterator type itself means
that all iterators generated by ``iterator_facade`` would be
instantiations of ``iterator_facade``. Cumbersome type generator
metafunctions would be needed to build new parameterized
iterators, and a separate ``iterator_adaptor`` layer would be
impossible.
The user of ``iterator_facade`` derives his iterator class from an
instantiation of ``iterator_facade`` and defines member functions
implementing the core behaviors. The following table describes
expressions which are required to be valid depending on the category
of the derived iterator type. These member functions are described
briefly below and in more detail in the `iterator facade requirements`_.
+----------------------------------------+-------------------------------------------+
| Expression | Effects |
+========================================+===========================================+
| ``i.dereference()`` | Access the value referred to |
+----------------------------------------+-------------------------------------------+
| ``i.equal(j)`` | Compare for equality with ``j`` |
+----------------------------------------+-------------------------------------------+
| ``i.increment()`` | Advance by one position |
+----------------------------------------+-------------------------------------------+
| ``i.decrement()`` | Retreat by one position |
+----------------------------------------+-------------------------------------------+
| ``i.advance(n)`` | Advance by ``n`` positions |
+----------------------------------------+-------------------------------------------+
| ``i.distance_to(j)`` | Measure the distance to ``j`` |
+----------------------------------------+-------------------------------------------+
.. Should we add a comment that a zero overhead implementation of iterator_facade
is possible with proper inlining?
In addition to implementing the core interface functions, an iterator
derived from ``iterator_facade`` typically defines several
constructors. To model any of the standard iterator concepts, the
iterator must at least have a copy constructor. Also, if the iterator
type ``X`` is meant to be automatically interoperate with another
iterator type ``Y`` (as with constant and mutable iterators) then
there must be an implicit conversion from ``X`` to ``Y`` or from ``Y``
to ``X`` (but not both), typically implemented as a conversion
constructor. Also, if the iterator is to model Forward Traversal
Iterator, a default constructor is required.
Iterator Core Access
====================
``iterator_facade`` and the operator implementations need to be able
to access the core member functions in the derived class. Making the
core member functions public would expose an implementation detail to
the user. This proposal frees the public interface of the derived
iterator type from any implementation detail.
Preventing direct access to the core member functions has two
advantages. First, there is no possibility for the user to accidently
use a member function of the iterator when a member of the value_type
was intended. This has been an issue with smart pointer
implementations in the past. The second and main advantage is that
library implementers can freely exchange a hand-rolled iterator
implementation for one based on ``iterator_facade`` without fear of
breaking code that was accessing the public core member functions
directly.
In a naive implementation, keeping the derived class' core member
functions private would require it to grant friendship to
``iterator_facade`` and each of the seven operators. In order to
reduce the burden of limiting access, this proposal provides
``iterator_core_access``, a class that acts as a gateway to the core
member functions in the derived iterator class. The author of the
derived class only needs to grant friendship to
``iterator_core_access`` to make his core member functions available
to the library.
.. This is no long uptodate -thw
.. Yes it is; I made sure of it! -DWA
``iterator_core_access`` will be typically implemented as an empty
class containing only private static member functions which invoke the
iterator core member functions. There is, however, no need to
standardize the gateway protocol. Note that even if
``iterator_core_access`` used public member functions it would not
open a safety loophole, as every core member function preserves the
invariants of the iterator.
``operator[]``
================
The indexing operator for a generalized iterator presents special
challenges. A random access iterator's ``operator[]`` is only
required to return something convertible to its ``value_type``.
Requiring that it return an lvalue would rule out currently-legal
random-access iterators which hold the referenced value in a data
member (e.g. `counting_iterator`_), because ``*(p+n)`` is a reference
into the temporary iterator ``p+n``, which is destroyed when
``operator[]`` returns.
Writable iterators built with ``iterator_facade`` implement the
semantics required by the preferred resolution to `issue 299`_ and
adopted by proposal `n1477`_: the result of ``p[n]`` is a proxy object
containing a copy of ``p+n``, and ``p[n] = x`` is equivalent to ``*(p
+ n) = x``. This approach will work properly for any random-access
iterator regardless of the other details of its implementation. A
user who knows more about the implementation of her iterator is free
to implement an ``operator[]`` which returns an lvalue in the derived
iterator class; it will hide the one supplied by ``iterator_facade``
from clients of her iterator.
.. _issue 299: http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/lwg-active.html#299
``operator->``
==============
The ``reference`` type of a readable iterator (and today's input
iterator) need not in fact be a reference, so long as it is
convertible to the iterator's ``value_type``. When the ``value_type``
is a class, however, it must still be possible to access members
through ``operator->``. Therefore, an iterator whose ``reference``
type is not in fact a reference must return a proxy containing a copy
of the referenced value from its ``operator->``.
This proposal does not explicitly specify the return type for
``operator->`` and ``operator[]``. Instead it requires each
``iterator_facade`` instantiation to meet the requirements of its
``iterator_category``.
Iterator Adaptor
================
The ``iterator_adaptor`` class template adapts some ``Base`` [#base]_
type to create a new iterator. Instantiations of ``iterator_adaptor``
are derived from a corresponding instantiation of ``iterator_facade``
and implement the core behaviors in terms of the ``Base`` type. In
essence, ``iterator_adaptor`` merely forwards all operations to an
instance of the ``Base`` type, which it stores as a member.
.. [#base] The term "Base" here does not refer to a base class and is
not meant to imply the use of derivation. We have followed the lead
of the standard library, which provides a base() function to access
the underlying iterator object of a ``reverse_iterator`` adaptor.
The user of ``iterator_adaptor`` creates a class derived from an
instantiation of ``iterator_adaptor`` and then selectively
redefines some of the core member functions described in the table
above. The ``Base`` type need not meet the full requirements for an
iterator. It need only support the operations used by the core
interface functions of ``iterator_adaptor`` that have not been
redefined in the user's derived class.
Several of the template parameters of ``iterator_adaptor`` default to
``use_default``. This allows the user to make use of a default
parameter even when the user wants to specify a parameter later in the
parameter list. Also, the defaults for the corresponding associated
types are fairly complicated, so metaprogramming is required to
compute them, and ``use_default`` can help to simplify the
implementation. Finally, ``use_default`` is not left unspecified
because specification helps to highlight that the ``Reference``
template parameter may not always be identical to the iterator's
``reference`` type, and will keep users making mistakes based on that
assumtion.
Specialized Adaptors
====================
This proposal also contains several examples of specialized adaptors
which were easily implemented using ``iterator_adaptor``:
* ``indirect_iterator``, which iterates over iterators, pointers,
or smart pointers and applies an extra level of dereferencing.
* A new ``reverse_iterator``, which inverts the direction of a Base
iterator's motion, while allowing adapted constant and mutable
iterators to interact in the expected ways (unlike those in most
implementations of C++98).
* ``transform_iterator``, which applies a user-defined function object
to the underlying values when dereferenced.
* ``projection_iterator``, which is similar to ``transform_iterator``
except that when dereferenced it returns a reference instead of
a value.
* ``filter_iterator``, which provides a view of an iterator range in
which some elements of the underlying range are skipped.
.. _counting_iterator:
* ``counting_iterator``, which adapts any incrementable type
(e.g. integers, iterators) so that incrementing/decrementing the
adapted iterator and dereferencing it produces successive values of
the Base type.
* ``function_output_iterator``, which makes it easier to create custom
output iterators.
Based on examples in the Boost library, users have generated many new
adaptors, among them a permutation adaptor which applies some
permutation to a random access iterator, and a strided adaptor, which
adapts a random access iterator by multiplying its unit of motion by a
constant factor. In addition, the Boost Graph Library (BGL) uses
iterator adaptors to adapt other graph libraries, such as LEDA [10]
and Stanford GraphBase [8], to the BGL interface (which requires C++
Standard compliant iterators).
===============
Proposed Text
===============
Header ``<iterator_helper>`` synopsis [lib.iterator.helper.synopsis]
=======================================================================
::
struct use_default;
struct iterator_core_access { /* implementation detail */ };
template <
class Derived
, class Value
, class AccessCategory
, class TraversalCategory
, class Reference = Value&
, class Difference = ptrdiff_t
>
class iterator_facade;
template <
class Derived
, class Base
, class Value = use_default
, class Category = use_default
, class Reference = use_default
, class Difference = use_default
>
class iterator_adaptor;
template <
class Iterator
, class Value = use_default
, class Category = use_default
, class Reference = use_default
, class Difference = use_default
>
class indirect_iterator;
template <class Iterator>
class reverse_iterator;
template <
class UnaryFunction
, class Iterator
, class Reference = use_default
, class Value = use_default
>
class transform_iterator;
template <class Predicate, class Iterator>
class filter_iterator;
template <
class Incrementable
, class Category = use_default
, class Difference = use_default
>
class counting_iterator
template <class UnaryFunction>
class function_output_iterator;
Iterator facade [lib.iterator.facade]
=====================================
``iterator_facade`` is a base class template which implements the
interface of standard iterators in terms of a few core functions
and associated types, to be supplied by a derived iterator class.
Class template ``iterator_facade``
----------------------------------
::
template <
class Derived
, class Value
, class AccessCategory
, class TraversalCategory
, class Reference = /* see below */
, class Difference = ptrdiff_t
>
class iterator_facade {
public:
typedef remove_cv<Value>::type value_type;
typedef Reference reference;
typedef /* see description of operator-> */ pointer;
typedef Difference difference_type;
typedef iterator_tag<AccessCategory, TraversalCategory> iterator_category;
reference operator*() const;
/* see below */ operator->() const;
/* impl defined */ operator[](difference_type n) const;
Derived& operator++();
Derived operator++(int);
Derived& operator--();
Derived operator--(int);
Derived& operator+=(difference_type n);
Derived& operator-=(difference_type n);
Derived operator-(difference_type n) const;
};
// Comparison operators
template <class Dr1, class V1, class AC1, class TC1, class R1, class D1,
class Dr2, class V2, class AC2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1, Dr2, bool>::type // exposition
operator ==(iterator_facade<Dr1, V1, AC1, TC1, R1, D1> const& lhs,
iterator_facade<Dr2, V2, AC2, TC2, R2, D2> const& rhs);
template <class Dr1, class V1, class AC1, class TC1, class R1, class D1,
class Dr2, class V2, class AC2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1, Dr2, bool>::type
operator !=(iterator_facade<Dr1, V1, AC1, TC1, R1, D1> const& lhs,
iterator_facade<Dr2, V2, AC2, TC2, R2, D2> const& rhs);
template <class Dr1, class V1, class AC1, class TC1, class R1, class D1,
class Dr2, class V2, class AC2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1, Dr2, bool>::type
operator <(iterator_facade<Dr1, V1, AC1, TC1, R1, D1> const& lhs,
iterator_facade<Dr2, V2, AC2, TC2, R2, D2> const& rhs);
template <class Dr1, class V1, class AC1, class TC1, class R1, class D1,
class Dr2, class V2, class AC2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1, Dr2, bool>::type
operator <=(iterator_facade<Dr1, V1, AC1, TC1, R1, D1> const& lhs,
iterator_facade<Dr2, V2, AC2, TC2, R2, D2> const& rhs);
template <class Dr1, class V1, class AC1, class TC1, class R1, class D1,
class Dr2, class V2, class AC2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1, Dr2, bool>::type
operator >(iterator_facade<Dr1, V1, AC1, TC1, R1, D1> const& lhs,
iterator_facade<Dr2, V2, AC2, TC2, R2, D2> const& rhs);
template <class Dr1, class V1, class AC1, class TC1, class R1, class D1,
class Dr2, class V2, class AC2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1, Dr2, bool>::type
operator >=(iterator_facade<Dr1, V1, AC1, TC1, R1, D1> const& lhs,
iterator_facade<Dr2, V2, AC2, TC2, R2, D2> const& rhs);
template <class Dr1, class V1, class AC1, class TC1, class R1, class D1,
class Dr2, class V2, class AC2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1, Dr2, bool>::type
operator >=(iterator_facade<Dr1, V1, AC1, TC1, R1, D1> const& lhs,
iterator_facade<Dr2, V2, AC2, TC2, R2, D2> const& rhs);
// Iterator difference
template <class Dr1, class V1, class AC1, class TC1, class R1, class D1,
class Dr2, class V2, class AC2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1, Dr2, bool>::type
operator -(iterator_facade<Dr1, V1, AC1, TC1, R1, D1> const& lhs,
iterator_facade<Dr2, V2, AC2, TC2, R2, D2> const& rhs);
// Iterator addition
template <class Derived, class V, class AC, class TC, class R, class D>
Derived operator+ (iterator_facade<Derived, V, AC, TC, R, D> const&,
typename Derived::difference_type n)
[*Note:* The ``enable_if_interoperable`` template used above is for exposition
purposes. The member operators should be only be in an overload set
provided the derived types ``Dr1`` and ``Dr2`` are interoperable, by
which we mean they are convertible to each other. The
``enable_if_interoperable`` approach uses SFINAE to take the operators
out of the overload set when the types are not interoperable.]
.. we need a new label here because the presence of markup in the
title prevents an automatic link from being generated
.. _iterator facade requirements:
``iterator_facade`` requirements
--------------------------------
The ``Derived`` template parameter must be a class derived from
``iterator_facade``.
The default for the ``Reference`` parameter is `Value&`` if the access
category for ``iterator_facade`` is implicitly convertible to
``writable_iterator_tag``, and ``const Value&`` otherwise.
The following table describes the other requirements on the
``Derived`` parameter. Depending on the resulting iterator's
``iterator_category``, a subset of the expressions listed in the table
are required to be valid. The operations in the first column must be
accessible to member functions of class ``iterator_core_access``.
In the table below, ``X`` is the derived iterator type, ``a`` is an
object of type ``X``, ``b`` and ``c`` are objects of type ``const X``,
``n`` is an object of ``X::difference_type``, ``y`` is a constant
object of a single pass iterator type interoperable with X, and ``z``
is a constant object of a random access traversal iterator type
interoperable with ``X``.
+----------------------------------------+----------------------------------------+-------------------------------------------------+-------------------------------------------+
| Expression | Return Type | Assertion/Note/Precondition/Postcondition | Required to implement Iterator Concept(s) |
| | | | |
+========================================+========================================+=================================================+===========================================+
| ``c.dereference()`` | ``X::reference`` | | Readable Iterator, Writable Iterator |
+----------------------------------------+----------------------------------------+-------------------------------------------------+-------------------------------------------+
| ``c.equal(b)`` | convertible to bool |true iff ``b`` and ``c`` are equivalent. | Single Pass Iterator |
+----------------------------------------+----------------------------------------+-------------------------------------------------+-------------------------------------------+
| ``c.equal(y)`` | convertible to bool |true iff ``c`` and ``y`` refer to the same | Single Pass Iterator |
| | |position. Implements ``c == y`` and ``c != y``. | |
+----------------------------------------+----------------------------------------+-------------------------------------------------+-------------------------------------------+
| ``a.advance(n)`` | unused | | Random Access Traversal Iterator |
+----------------------------------------+----------------------------------------+-------------------------------------------------+-------------------------------------------+
| ``a.increment()`` | unused | | Incrementable Iterator |
+----------------------------------------+----------------------------------------+-------------------------------------------------+-------------------------------------------+
| ``a.decrement()`` | unused | | Bidirectional Traversal Iterator |
+----------------------------------------+----------------------------------------+-------------------------------------------------+-------------------------------------------+
| ``c.distance_to(b)`` | convertible to X::difference_type | equivalent to ``distance(c, b)`` | Random Access Traversal Iterator |
+----------------------------------------+----------------------------------------+-------------------------------------------------+-------------------------------------------+
| ``c.distance_to(z)`` | convertible to X::difference_type |equivalent to ``distance(c, z)``. Implements ``c| Random Access Traversal Iterator |
| | |- z``, ``c < z``, ``c <= z``, ``c > z``, and ``c | |
| | |>= c``. | |
+----------------------------------------+----------------------------------------+-------------------------------------------------+-------------------------------------------+
.. We should explain more about how the
functions in the interface of iterator_facade
are there conditionally. -JGS
``iterator_facade`` operations
------------------------------
The operations in this section are described in terms of operations on
the core interface of ``Derived`` which may be inaccessible
(i.e. private). The implementation should access these operations
through member functions of class ``iterator_core_access``.
``reference operator*() const;``
:Returns: ``static_cast<Derived const*>(this)->dereference()``
*see below* ``operator->() const;``
:Returns: If ``X::reference`` is a reference type, returns an object
of type ``X::pointer`` equal to::
&static_cast<Derived const*>(this)->dereference()
Otherwise returns an object of unspecified type such that, given an
object ``a`` of type ``X``, ``a->m`` is equivalent to ``(w = *a,
w.m)`` for some temporary object ``w`` of type ``X::value_type``.
The type ``X::pointer`` is ``Value*`` if the access category for
``X`` is implicitly convertible to ``writable_iterator_tag``, and
``Value const*`` otherwise.
*unspecified* ``operator[](difference_type n) const;``
:Returns: an object convertible to ``X::reference`` and holding a copy
*p* of ``a+n`` such that, for a constant object ``v`` of type
``X::value_type``, ``X::reference(a[n] = v)`` is equivalent
to ``p = v``.
``Derived& operator++();``
:Effects:
::
static_cast<Derived*>(this)->increment();
return *this;
.. I realize that the committee is moving away from specifying things
like this in terms of code, but I worried about the imprecision of
saying that a core interface function is invoked without describing
the downcast. An alternative to what I did would be to mention it
above where we talk about accessibility.
``Derived operator++(int);``
:Effects:
::
Derived tmp(static_cast<Derived const*>(this));
++*this;
return tmp;
``Derived& operator--();``
:Effects:
::
static_cast<Derived*>(this)->decrement();
return *this;
``Derived operator--(int);``
:Effects:
::
Derived tmp(static_cast<Derived const*>(this));
--*this;
return tmp;
``Derived& operator+=(difference_type n);``
:Effects:
::
static_cast<Derived*>(this)->advance(n);
return *this;
``Derived& operator-=(difference_type n);``
:Effects:
::
static_cast<Derived*>(this)->advance(-n);
return *this;
``Derived operator-(difference_type n) const;``
:Effects:
Derived tmp(static_cast<Derived const*>(this));
return tmp -= n;
:Returns: ``static_cast<Derived const*>(this)->advance(-n);``
Iterator adaptor [lib.iterator.adaptor]
=======================================
The ``iterator_adaptor`` is a base class template derived from an
instantiation of ``iterator_facade``. The core interface functions
expected by ``iterator_facade`` are implemented in terms of the
``iterator_adaptor``\ 's ``Base`` template parameter. A class derived
from ``iterator_adaptor`` typically redefines some of the core
interface functions to adapt the behavior of the ``Base`` type.
Whether the derived class models any of the standard iterator concepts
depends on the operations supported by the ``Base`` type and which
core interface functions of ``iterator_facade`` are redefined in the
``Derived`` class.
Class template ``iterator_adaptor``
-----------------------------------
::
template <
class Derived
, class Base
, class Value = use_default
, class Category = use_default
, class Reference = use_default
, class Difference = use_default
>
class iterator_adaptor
: public iterator_facade<Derived, /* see details ...*/>
{
friend class iterator_core_access;
public:
iterator_adaptor();
explicit iterator_adaptor(Base iter);
Base base() const;
protected:
Base const& base_reference() const;
Base& base_reference();
private: // Core iterator interface for iterator_facade.
typename iterator_adaptor::reference dereference() const;
template <
class OtherDerived, class OtherIterator, class V, class C, class R, class D
>
bool equal(iterator_adaptor<OtherDerived, OtherIterator, V, C, R, D> const& x) const;
void advance(typename iterator_adaptor::difference_type n);
void increment();
void decrement();
template <
class OtherDerived, class OtherIterator, class V, class C, class R, class D
>
typename iterator_adaptor::difference_type distance_to(
iterator_adaptor<OtherDerived, OtherIterator, V, C, R, D> const& y) const;
private:
Base m_iterator;
};
``iterator_adaptor`` requirements
---------------------------------
The ``Derived`` template parameter must be a derived class of
``iterator_adaptor``. The ``Base`` type must implement the expressions
involving ``m_iterator`` in the specifications of those private member
functions of ``iterator_adaptor`` that are not redefined by the
``Derived`` class and that are needed to model the concept
corresponding to the chosen ``Category`` according to the requirements
of ``iterator_facade``. The rest of the template parameters specify
the types for the member typedefs in ``iterator_facade``. The
following pseudo-code specifies the traits types for
``iterator_adaptor``.
::
if (Value == use_default)
value_type = iterator_traits<Base>::value_type;
else
value_type = remove_cv<Value>::type;
if (Reference == use_default) {
if (Value == use_default)
reference = iterator_traits<Base>::reference;
else
reference = Value&;
} else
reference = Reference;
if (Distance == use_default)
difference_type = iterator_traits<Base>::difference_type;
else
difference_type = Distance;
if (Category == use_default)
iterator_category = iterator_tag<
access_category<
iterator< iterator_traits<Base>::iterator_category,
Value,
Distance,
Value*,
Reference > >,
traversal_category<
iterator< iterator_traits<Base>::iterator_category,
Value,
Distance,
Value*,
Reference > >
else
iterator_category = Category;
.. Replaced with new semantics --thw
if (Category == use_default)
iterator_category = iterator_traits<Base>::iterator_category;
else
iterator_category = Category;
``iterator_adaptor`` public operations
--------------------------------------
``iterator_adaptor();``
:Requires: The ``Base`` type must be Default Constructible.
:Returns: An instance of ``iterator_adaptor`` with
``m_iterator`` default constructed.
``explicit iterator_adaptor(Base iter);``
:Returns: An instance of ``iterator_adaptor`` with
``m_iterator`` copy constructed from ``iter``.
``Base base() const;``
:Returns: ``m_iterator``
``iterator_adaptor`` protected member functions
-----------------------------------------------
``Base const& base_reference() const;``
:Returns: A const reference to ``m_iterator``.
``Base& base_reference();``
:Returns: A non-const reference to ``m_iterator``.
``iterator_adaptor`` private member functions
---------------------------------------------
``typename iterator_adaptor::reference dereference() const;``
:Returns: ``*m_iterator``
::
template <
class OtherDerived, class OtherIterator, class V, class C, class R, class D
>
bool equal(iterator_adaptor<OtherDerived, OtherIterator, V, C, R, D> const& x) const;
:Returns: ``m_iterator == x.base()``
``void advance(typename iterator_adaptor::difference_type n);``
:Effects: ``m_iterator += n;``
``void increment();``
:Effects: ``++m_iterator;``
``void decrement();``
:Effects: ``--m_iterator;``
::
template <
class OtherDerived, class OtherIterator, class V, class C, class R, class D
>
typename iterator_adaptor::difference_type distance_to(
iterator_adaptor<OtherDerived, OtherIterator, V, C, R, D> const& y) const;
:Returns: ``y.base() - m_iterator``
Specialized adaptors [lib.iterator.special.adaptors]
====================================================
.. The requirements for all of these need to be written *much* more
formally -DWA
[*Note:* The ``enable_if_convertible<X,Y>::type`` expression used in
this section is for exposition purposes. The converting constructors
for specialized adaptors should be only be in an overload set provided
that an object of type ``X`` is implicitly convertible to an object of
type ``Y``. The ``enable_if_convertible`` approach uses SFINAE to
take the constructor out of the overload set when the types are not
implicitly convertible.]
Indirect iterator
-----------------
The indirect iterator adapts an iterator by applying an *extra*
dereference inside of ``operator*()``. For example, this iterator
adaptor makes it possible to view a container of pointers
(e.g. ``list<foo*>``) as if it were a container of the pointed-to type
(e.g. ``list<foo>``) .
.. At some point we should add the capability to handle
iterators over smart pointers, which the impl handles. -JGS
Class template ``indirect_iterator``
....................................
::
template <
class Iterator
, class Value = use_default
, class Category = use_default
, class Reference = use_default
, class Difference = use_default
>
class indirect_iterator
: public iterator_adaptor</* see discussion */>
{
friend class iterator_core_access;
public:
indirect_iterator();
indirect_iterator(Iterator x);
template <
class Iterator2, class Value2, class Category2
, class Reference2, class Difference2
>
indirect_iterator(
indirect_iterator<
Iterator2, Value2, Category2, Reference2, Difference2
> const& y
, typename enable_if_convertible<Iterator2, Iterator>::type* = 0 // exposition
);
private: // as-if specification
typename indirect_iterator::reference dereference() const
{
return **this->base();
}
};
``indirect_iterator`` requirements
..................................
The ``value_type`` of the ``Iterator`` template parameter should
itself be dereferenceable. The return type of the ``operator*`` for
the ``value_type`` must be the same type as the ``Reference`` template
parameter. The ``Value`` template parameter will be the ``value_type``
for the ``indirect_iterator``, unless ``Value`` is const. If ``Value``
is ``const X``, then ``value_type`` will be *non-* ``const X``. The
default for ``Value`` is
::
iterator_traits< iterator_traits<Iterator>::value_type >::value_type
If the default is used for ``Value``, then there must be a valid
specialization of ``iterator_traits`` for the value type of the base
iterator.
The ``Reference`` parameter will be the ``reference`` type of the
``indirect_iterator``. The default is ``Value&``.
The ``Category`` parameter is the ``iterator_category`` type for the
``indirect_iterator``. The default is
``iterator_traits<Iterator>::iterator_category``.
The indirect iterator will model the most refined standard traversal
concept that is modeled by the ``Iterator`` type. The indirect
iterator will model the most refined standard access concept that is
modeled by the value type of ``Iterator``.
``indirect_iterator`` operations
................................
``indirect_iterator();``
:Requires: ``Iterator`` must be Default Constructible.
:Returns: An instance of ``indirect_iterator`` with
a default constructed base object.
``indirect_iterator(Iterator x);``
:Returns: An instance of ``indirect_iterator`` with
the ``iterator_adaptor`` subobject copy constructed from ``x``.
::
template <
class Iterator2, class Value2, class Category2
, class Reference2, class Difference2
>
indirect_iterator(
indirect_iterator<
Iterator2, Value2, Category2, Reference2, Difference2
> const& y
, typename enable_if_convertible<Iterator2, Iterator>::type* = 0 // exposition
);
:Requires: ``Iterator2`` is implicitly convertible to ``Iterator``.
:Returns: An instance of ``indirect_iterator`` that is a copy of ``y``.
Reverse iterator
----------------
.. I think we'd better strike the old reverse_iterator text from the standard, eh?
The reverse iterator adaptor flips the direction of a base iterator's
motion. Invoking ``operator++()`` moves the base iterator backward and
invoking ``operator--()`` moves the base iterator forward.
Class template ``reverse_iterator``
...................................
::
template <class Iterator>
class reverse_iterator :
public iterator_adaptor< reverse_iterator<Iterator>, Iterator >
{
friend class iterator_core_access;
public:
reverse_iterator() {}
explicit reverse_iterator(Iterator x) ;
template<class OtherIterator>
reverse_iterator(
reverse_iterator<OtherIterator> const& r
, typename enable_if_convertible<OtherIterator, Iterator>::type* = 0 // exposition
);
private: // as-if specification
typename reverse_iterator::reference dereference() const { return *prior(this->base()); }
void increment() { --this->base_reference(); }
void decrement() { ++this->base_reference(); }
void advance(typename reverse_iterator::difference_type n)
{
this->base_reference() += -n;
}
template <class OtherIterator>
typename reverse_iterator::difference_type
distance_to(reverse_iterator<OtherIterator> const& y) const
{
return this->base_reference() - y.base();
}
};
``reverse_iterator`` requirements
.................................
The base ``Iterator`` must be a model of Bidirectional Traversal
Iterator. The resulting ``reverse_iterator`` will be a model of the
most refined standard traversal and access concepts that are modeled
by ``Iterator``.
``reverse_iterator();``
:Requires: ``Iterator`` must be Default Constructible.
:Returns: An instance of ``reverse_iterator`` with a
default constructed base object.
``explicit reverse_iterator(Iterator x);``
:Returns: An instance of ``reverse_iterator`` with a
base object copy constructed from ``x``.
::
template<class OtherIterator>
reverse_iterator(
reverse_iterator<OtherIterator> const& r
, typename enable_if_convertible<OtherIterator, Iterator>::type* = 0 // exposition
);
:Requires: ``OtherIterator`` is implicitly convertible to ``Iterator``.
:Returns: An instance of ``reverse_iterator`` that is a copy of ``r``.
Transform iterator
------------------
The transform iterator adapts an iterator by applying some function
object to the result of dereferencing the iterator. In other words,
the ``operator*`` of the transform iterator first dereferences the
base iterator, passes the result of this to the function object, and
then returns the result.
Class template ``transform_iterator``
.....................................
::
template <class AdaptableUnaryFunction,
class Iterator,
class Reference = use_default,
class Value = use_default>
class transform_iterator
: public iterator_adaptor</* see discussion */>
{
friend class iterator_core_access;
public:
transform_iterator();
transform_iterator(Iterator const& x, AdaptableUnaryFunction f);
template<class OtherIterator, class R2, class V2>
transform_iterator(
transform_iterator<AdaptableUnaryFunction, OtherIterator, R2, V2> const& t
, typename enable_if_convertible<OtherIterator, Iterator>::type* = 0 // exposition
);
AdaptableUnaryFunction functor() const;
private:
typename transform_iterator::value_type dereference() const;
AdaptableUnaryFunction m_f;
};
``transform_iterator`` requirements
...................................
The type ``AdaptableUnaryFunction`` must be Assignable, Copy
Constructible, and the expression ``f(x)`` must be valid where ``f``
is an object of type ``AdaptableUnaryFunction``, ``x`` is an object of
type ``AdaptableUnaryFunction::argument_type``, and where the type of
``f(x)`` must be ``AdaptableUnaryFunction::result_type``.
The type ``Iterator`` must at least model Readable Iterator. The
resulting ``transform_iterator`` models the most refined of the
following options that is also modeled by ``Iterator``.
* Writable Lvalue Iterator if the ``result_type`` of the
``AdaptableUnaryFunction`` is a non-const reference.
* Readable Lvalue Iterator if the ``result_type`` is a const
reference.
* Readable Iterator otherwise.
The ``transform_iterator`` models the most refined standard traversal
concept that is modeled by ``Iterator``.
The ``value_type`` of ``transform_iterator`` is
``remove_reference<result_type>::type``. The ``reference`` type is
``result_type``.
``transform_iterator`` public operations
........................................
``transform_iterator();``
:Returns: An instance of ``transform_iterator`` with ``m_f``
and ``m_iterator`` default constructed.
``transform_iterator(Iterator const& x, AdaptableUnaryFunction f);``
:Returns: An instance of ``transform_iterator`` with ``m_f``
initialized to ``f`` and ``m_iterator`` initialized to ``x``.
::
template<class OtherIterator, class R2, class V2>
transform_iterator(
transform_iterator<AdaptableUnaryFunction, OtherIterator, R2, V2> const& t
, typename enable_if_convertible<OtherIterator, Iterator>::type* = 0 // exposition
);
:Returns: An instance of ``transform_iterator`` that is a copy of ``t``.
:Requires: ``OtherIterator`` is implicitly convertible to ``Iterator``.
``AdaptableUnaryFunction functor() const;``
:Returns: ``m_f``
``transform_iterator`` private operations
.........................................
``typename transform_iterator::value_type dereference() const;``
:Returns: ``m_f(transform_iterator::dereference());``
Filter iterator
---------------
The filter iterator adaptor creates a view of an iterator range in
which some elements of the range are skipped over. A predicate
function object controls which elements are skipped. When the
predicate is applied to an element, if it returns ``true`` then the
element is retained and if it returns ``false`` then the element is
skipped over. When skipping over elements, it is necessary for the
filter adaptor to know when to stop so as to avoid going past the end
of the underlying range. Therefore the constructor of the filter
iterator takes two iterator parameters: the position for the filtered
iterator and the end of the range.
Class template ``filter_iterator``
..................................
::
template <class Predicate, class Iterator>
class filter_iterator
: public iterator_adaptor<
filter_iterator<Predicate, Iterator>, Iterator
, use_default
, /* see details */
>
{
public:
filter_iterator();
filter_iterator(Predicate f, Iterator x, Iterator end = Iterator());
filter_iterator(Iterator x, Iterator end = Iterator());
template<class OtherIterator>
filter_iterator(
filter_iterator<Predicate, OtherIterator> const& t
, typename enable_if_convertible<OtherIterator, Iterator>::type* = 0 // exposition
);
Predicate predicate() const;
Iterator end() const;
private: // as-if specification
void increment()
{
++(this->base_reference());
satisfy_predicate();
}
void satisfy_predicate()
{
while (this->base() != this->m_end && !this->m_predicate(*this->base()))
++(this->base_reference());
}
Predicate m_predicate;
Iterator m_end;
};
``filter_iterator`` requirements
--------------------------------
The base ``Iterator`` parameter must be a model of Readable Iterator
and Single Pass Iterator. The resulting ``filter_iterator`` will be a
model of Forward Traversal Iterator if ``Iterator`` is, otherwise the
``filter_iterator`` will be a model of Single Pass Iterator. The
access category of the ``filter_iterator`` will be the most refined
standard access category that is modeled by ``Iterator``.
.. Thomas is going to try implementing filter_iterator so that
it will be bidirectional if the underlying iterator is. -JGS
The ``Predicate`` must be Assignable, Copy Constructible, and the
expression ``p(x)`` must be valid where ``p`` is an object of type
``Predicate``, ``x`` is an object of type
``iterator_traits<Iterator>::value_type``, and where the type of
``p(x)`` must be convertible to ``bool``.
``filter_iterator`` operations
------------------------------
``filter_iterator();``
:Requires: ``Predicate`` and ``Iterator`` must be Default Constructible.
:Returns: a ``filter_iterator`` whose
predicate is a default constructed ``Predicate`` and
whose ``end`` is a default constructed ``Iterator``.
``filter_iterator(Predicate f, Iterator x, Iterator end = Iterator());``
:Returns: A ``filter_iterator`` at position ``x`` that filters according
to predicate ``f`` and that will not increment past ``end``.
``filter_iterator(Iterator x, Iterator end = Iterator());``
:Requires: ``Predicate`` must be Default Constructible.
:Returns: A ``filter_iterator`` at position ``x`` that filters
according to a default constructed ``Predicate``
and that will not increment past ``end``.
::
template <class OtherIterator>
filter_iterator(
filter_iterator<Predicate, OtherIterator> const& t
, typename enable_if_convertible<OtherIterator, Iterator>::type* = 0 // exposition
);``
:Requires: ``OtherIterator`` is implicitly convertible to ``Iterator``.
:Returns: A copy of iterator ``t``.
``Predicate predicate() const;``
:Returns: A copy of the predicate object used to construct ``*this``.
``Iterator end() const;``
:Returns: The object ``end`` used to construct ``*this``.
Counting iterator
-----------------
The counting iterator adaptor implements dereference by returning a
reference to the base object. The other operations are implemented by
the base ``m_iterator``, as per the inheritance from
``iterator_adaptor``.
Class template ``counting_iterator``
....................................
::
template <class Incrementable, class Category = use_default, class Difference = use_default>
class counting_iterator
: public iterator_adaptor<
counting_iterator<Incrementable, Category, Difference>
, Incrementable
, Incrementable
, /* see details for category */
, Incrementable const&
, Incrementable const*
, /* distance = Difference or a signed integral type */>
{
friend class iterator_core_access;
public:
counting_iterator();
counting_iterator(counting_iterator const& rhs);
counting_iterator(Incrementable x);
private:
typename counting_iterator::reference dereference() const
{
return this->base_reference();
}
};
[*Note:* implementers are encouraged to provide an implementation of
``distance_to`` and a ``difference_type`` that avoids overflows in
the cases when the ``Incrementable`` type is a numeric type.]
``counting_iterator`` requirements
----------------------------------
The ``Incrementable`` type must be Default Constructible, Copy
Constructible, and Assignable. The default distance is
an implementation defined signed integegral type.
The resulting ``counting_iterator`` models Readable Lvalue Iterator.
Furthermore, if you wish to create a counting iterator that is a Forward
Traversal Iterator, then the following expressions must be valid:
::
Incrementable i, j;
++i // pre-increment
i == j // operator equal
If you wish to create a counting iterator that is a
Bidirectional Traversal Iterator, then pre-decrement is also required:
::
--i
If you wish to create a counting iterator that is a Random Access
Traversal Iterator, then these additional expressions are also
required:
::
counting_iterator::difference_type n;
i += n
n = i - j
i < j
``counting_iterator`` operations
--------------------------------
``counting_iterator();``
:Returns: A default constructed instance of ``counting_iterator``.
``counting_iterator(counting_iterator const& rhs);``
:Returns: An instance of ``counting_iterator`` that is a copy of ``rhs``.
``counting_iterator(Incrementable x);``
:Returns: An instance of ``counting_iterator`` with its base
object copy constructed from ``x``.
Function output iterator
------------------------
The function output iterator adaptor makes it easier to create custom
output iterators. The adaptor takes a unary function and creates a
model of Output Iterator. Each item assigned to the output iterator is
passed as an argument to the unary function. The motivation for this
iterator is that creating a conforming output iterator is non-trivial,
particularly because the proper implementation usually requires a
proxy object.
Class template ``function_output_iterator``
...........................................
::
template <class UnaryFunction>
class function_output_iterator {
public:
typedef iterator_tag<
writable_iterator_tag
, incrementable_iterator_tag
> iterator_category;
typedef void value_type;
typedef void difference_type;
typedef void pointer;
typedef void reference;
explicit function_output_iterator(const UnaryFunction& f = UnaryFunction());
struct output_proxy {
output_proxy(UnaryFunction& f);
template <class T> output_proxy& operator=(const T& value);
};
output_proxy operator*();
function_output_iterator& operator++();
function_output_iterator& operator++(int);
};
``function_output_iterator`` requirements
-----------------------------------------
The ``UnaryFunction`` must be Assignable, Copy Constructible, and the
expression ``f(x)`` must be valid, where ``f`` is an object of type
``UnaryFunction`` and ``x`` is an object of a type accepted by ``f``.
The resulting ``function_output_iterator`` is a model of the Writable
and Incrementable Iterator concepts.
``function_output_iterator`` operations
---------------------------------------
``explicit function_output_iterator(const UnaryFunction& f = UnaryFunction());``
:Returns: An instance of ``function_output_iterator`` with
``f`` stored as a data member.
``output_proxy operator*();``
:Returns: An instance of ``output_proxy`` constructed with
a copy of the unary function ``f``.
``function_output_iterator& operator++();``
:Returns: ``*this``
``function_output_iterator& operator++(int);``
:Returns: ``*this``
``function_output_iterator::output_proxy`` operations
-----------------------------------------------------
``output_proxy(UnaryFunction& f);``
:Returns: An instance of ``output_proxy`` with ``f`` stored as
a data member.
``template <class T> output_proxy& operator=(const T& value);``
:Effects:
::
m_f(value);
return *this;
.. [Cop95] [Coplien, 1995] Coplien, J., Curiously Recurring Template
Patterns, C++ Report, February 1995, pp. 24-27.
..
LocalWords: Abrahams Siek Witt istream ostream iter MTL strided interoperate
LocalWords: CRTP metafunctions inlining lvalue JGS incrementable BGL LEDA cv
LocalWords: GraphBase struct ptrdiff UnaryFunction const int typename bool pp
LocalWords: lhs rhs SFINAE markup iff tmp OtherDerived OtherIterator DWA foo
LocalWords: dereferenceable subobject AdaptableUnaryFunction impl pre ifdef'd
LocalWords: OtherIncrementable Coplien
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