std::forward_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>;

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 a reference.

See also