NAME

std::forward_iterator - std::forward_iterator

Synopsis

Defined in header <iterator>
template<class I>

concept forward_iterator =
std::input_iterator<I> && (since C++20)
std::derived_from</*ITER_CONCEPT*/<I>, std::forward_iterator_tag> &&
std::incrementable<I> &&

std::sentinel_for<I, I>;

This concept refines std::input_iterator by requiring that I also model
std::incrementable (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.
* 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)[](auto x){ ++x; }(i), *i) is equivalent to *i.

Notes

Unlike the LegacyForwardIterator requirements, the forward_iterator concept does not
require dereference to return an lvalue.

See also

input_iterator specifies that a type is an input iterator, that is, its referenced
(C++20) values can be read and it can be both pre- and post-incremented
(concept)