< cpp‎ | iterator
Iterator library
Iterator concepts

Iterator primitives
Algorithm concepts and utilities
Indirect callable concepts
Common algorithm requirements
Iterator adaptors
Range access
Defined in header <iterator>
template< class I >

    concept forward_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 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.


[edit] 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.

[edit] 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.

[edit] Notes

Unlike the LegacyForwardIterator requirements, the forward_iterator concept does not require dereference to return a reference.

[edit] Example

A minimum forward iterator.

#include <cstddef>
#include <iterator>
struct SimpleForwardIterator
    using difference_type = std::ptrdiff_t;
    using value_type = int;
    int operator*() const;
    SimpleForwardIterator& operator++();
    SimpleForwardIterator operator++(int)
        auto tmp = *this;
        return tmp;
    bool operator==(const SimpleForwardIterator&) const;

[edit] See also

specifies that a type is an input iterator, that is, its referenced values can be read and it can be both pre- and post-incremented
(concept) [edit]
specifies that a forward_iterator is a bidirectional iterator, supporting movement backwards
(concept) [edit]