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]

1
#
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>;
    };
2
#
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--.
3
#
Let a and b be equal objects of type I.
I models bidirectional_­iterator only if:
  • (3.1)
    If a and b are decrementable, then all of the following are true:
    • (3.1.1)
      addressof(--a) == addressof(a)
    • (3.1.2)
      bool(a-- == b)
    • (3.1.3)
      after evaluating both a-- and --b, bool(a == b) is still true
    • (3.1.4)
      bool(++(--a) == b)
  • (3.2)
    If a and b are incrementable, then bool(--(++a) == b).