[iterator.concept.forward] (original) (raw)

23 Iterators library [iterators]

23.3 Iterator requirements [iterator.requirements]

23.3.4 Iterator concepts [iterator.concepts]

23.3.4.11 Concept forward_­iterator [iterator.concept.forward]

The forward_­iterator concept adds copyability, equality comparison, and the multi-pass guarantee, specified below.

template concept forward_iterator = input_iterator && derived_from<ITER_CONCEPT(I), forward_iterator_tag> && incrementable && 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

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Value-initialized iterators behave as if they refer past the end of the same empty sequence.

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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.

Two dereferenceable iterators a and b of type Xoffer 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

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The requirement thata == bimplies++a == ++band 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.

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]