23 Iterators library [iterators]

23.3 Iterator requirements [iterator.requirements]

23.3.4 Iterator concepts [iterator.concepts] Concept bidirectional_­iterator [iterator.concept.bidir]

The bidirectional_­iterator concept adds the ability to move an iterator backward as well as forward.
template<class I> concept bidirectional_­iterator = forward_­iterator<I> && derived_­from<ITER_CONCEPT(I), bidirectional_iterator_tag> && requires(I i) { { --i } -> same_­as<I&>; { i-- } -> same_­as<I>; };
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, then bool(--(++a) == b).