25 Iterators library [iterators]

25.3 Iterator requirements [iterator.requirements]

25.3.4 Iterator concepts [iterator.concepts] Concept forward_iterator [iterator.concept.forward]

The forward_iterator concept adds copyability, equality comparison, and the multi-pass guarantee, specified below.
template<class I> concept forward_iterator = input_iterator<I> && derived_from<ITER_CONCEPT(I), forward_iterator_tag> && incrementable<I> && sentinel_for<I, I>;
The domain of == for forward iterators is that of iterators over the same underlying sequence.
However, value-initialized iterators of the same type may be compared and shall compare equal to other value-initialized iterators of the same type.
[Note 1: 
Value-initialized iterators behave as if they refer past the end of the same empty sequence.
— end note]
Pointers and references obtained from a forward iterator into a range [i, s) shall remain valid while [i, s) continues to denote a range.
Two dereferenceable iterators a and b of type X offer the multi-pass guarantee if:
  • a == b implies ++a == ++b and
  • the expression ((void)[](X x){++x;}(a), *a) is equivalent to the expression *a.
[Note 2: 
The requirement that a == b implies ++a == ++b and the removal of the restrictions on the number of assignments through a mutable iterator (which applies to output iterators) allow the use of multi-pass one-directional algorithms with forward iterators.
— end note]