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template<classI>conceptforward_iterator=std::input_iterator<I>&&std::derived_from</*ITER_CONCEPT*/<I>,std::forward_iterator_tag>&&std::incrementable<I>&&std::sentinel_for<I,I>;(since C++20)This concept refines
by requiring that I also model
(thereby making it suitable for multi-pass algorithms), and guaranteeing that two iterators to the same range can be compared against each other.
Iterator concept determination
Definition of this concept is specified via an exposition-only alias template /*ITER_CONCEPT*/.
In order to determine /*ITER_CONCEPT*/<I>, let ITER_TRAITS<I> denote I if the specialization std::iterator_traits<I> is generated from the primary template, or std::iterator_traits<I> otherwise:
If ITER_TRAITS<I>::iterator_concept is valid and names a type, /*ITER_CONCEPT*/<I> denotes the type.
Otherwise, if ITER_TRAITS<I>::iterator_category is valid and names a type, /*ITER_CONCEPT*/<I> denotes the type.
Otherwise, if std::iterator_traits<I> is generated from the primary template, /*ITER_CONCEPT*/<I> denotes
std::random_access_iterator_tag
.
(That is, std::derived_from</*ITER_CONCEPT*/<I>,std::forward_iterator_tag> is assumed to be true.)
Otherwise, /*ITER_CONCEPT*/<I> does not denote a type and results in a substitution failure.
Semantic requirements
I models std::forward_iterator if, and only if I models all the concepts it subsumes, and given objects i and j of type I:
Comparison between iterators i and j has a defined result if
i and j are iterators to the same underlying sequence, or
both i and j are value-initialized, in which case they compare equal.
Pointers and references obtained from a forward iterator into a range remain valid while the range exists.
If i and j are dereferenceable, they offer the multi-pass guarantee, that is:
i==j implies ++i==++j, and
((void)[](autox){++x;}(i),*i) is equivalent to *i.
Notes
Unlike the
requirements, the forward_iterator concept does not require dereference to return a reference.
Example
A minimum forward iterator.
#include<cstddef>#include<iterator>structSimpleForwardIterator{usingdifference_type=std::ptrdiff_t;usingvalue_type=int;intoperator*()const;SimpleForwardIterator&operator++();SimpleForwardIteratoroperator++(int){autotmp=*this;++*this;returntmp;}booloperator==(constSimpleForwardIterator&)const;};static_assert(std::forward_iterator<SimpleForwardIterator>);See also