< cpp‎ | experimental‎ | ranges
Technical specifications
Filesystem library (filesystem TS)
Library fundamentals (library fundamentals TS)
Library fundamentals 2 (library fundamentals 2 TS)
Extensions for parallelism (parallelism TS)
Extensions for parallelism 2 (parallelism TS v2)
Extensions for concurrency (concurrency TS)
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Ranges (ranges TS)
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template < class I >

concept bool ForwardIterator =
  InputIterator<I> &&
  DerivedFrom<ranges::iterator_category_t<I>, ranges::forward_iterator_tag> &&
  Incrementable<I> &&

  Sentinel<I, I>;
(ranges TS)

The concept ForwardIterator<I> refines InputIterator by adding equality comparison and the multi-pass guarantee.

The domain of == for forward iterators is that of iterators over the same underlying sequence, except that value-initialized iterators of the same type may be compared with each other and shall compare equal.

Pointers and references obtained from a forward iterator into a range [i, s) must remain valid while [i, s) continues to denote a range.

Let a and b be two deferenceable iterators of type I. ForwardIterator<I> is satisfied only if:

  • a == b implies ++a == ++b; and
  • The expression ([](X x){++x;}(a), *a) is equivalent to *a, i.e., incrementing a copy of a has no effect on the result of dereferencing a.

This is known as the multi-pass guarantee.


A forward iterator cannot be "stashing": it cannot return a reference to an object within itself, because such references would be invalidated by the destruction of the forward iterator.