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

23 Iterators library [iterators]

23.3 Iterator requirements [iterator.requirements]

23.3.4 Iterator concepts [iterator.concepts]

23.3.4.12 Concept bidirectional_­iterator [iterator.concept.bidir]

The bidirectional_­iterator concept adds the ability to move an iterator backward as well as forward.

template concept bidirectional_iterator = forward_iterator && derived_from<ITER_CONCEPT(I), bidirectional_iterator_tag> && requires(I i) { { --i } -> same_as<I&>; { i-- } -> same_as; };

A bidirectional iterator r is decrementable if and only if there exists some q such that++q == r.

Decrementable iterators r shall be in the domain of the expressions--r and r--.

Let a and b be equal objects of type I.

I models bidirectional_­iterator only if:

• If a and b are decrementable, then all of the following are true:
  • addressof(--a) == addressof(a)
  • bool(a-- == b)
  • after evaluating both a-- and --b,bool(a == b) is still true
  • bool(++(--a) == b)
• If a and b are incrementable, thenbool(--(++a) == b).