std::forward_iterator - cppreference.com (original) (raw)

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.

Contents

• 1 Iterator concept determination
• 2 Semantic requirements
• 3 Notes
• 4 Example
• 5 See also

[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.
  (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.

[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 #include   struct SimpleForwardIterator { using difference_type = std::ptrdiff_t; using value_type = int;   int operator*() const;   SimpleForwardIterator& operator++();   SimpleForwardIterator operator++(int) { auto tmp = *this; ++*this; return tmp; }   bool operator==(const SimpleForwardIterator&) const; };   static_assert(std::forward_iterator);

[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] |