25 Iterators library [iterators]

25.3 Iterator requirements [iterator.requirements]

25.3.4 Iterator concepts [iterator.concepts]

25.3.4.11 Concept forward_iterator [iterator.concept.forward]

1
#
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>;
2
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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]
3
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Pointers and references obtained from a forward iterator into a range [i, s) shall remain valid while [i, s) continues to denote a range.
4
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Two dereferenceable iterators a and b of type X offer the multi-pass guarantee if:
  • (4.1)
    a == b implies ++a == ++b and
  • (4.2)
    the expression ((void)[](X x){++x;}(a), *a) is equivalent to the expression *a.
5
#
[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]