25 Iterators library [iterators]

25.3 Iterator requirements [iterator.requirements]

25.3.4 Iterator concepts [iterator.concepts] Concept incrementable [iterator.concept.inc]

The incrementable concept specifies requirements on types that can be incremented with the pre- and post-increment operators.
The increment operations are required to be equality-preserving, and the type is required to be equality_comparable.
[Note 1: 
This supersedes the annotations on the increment expressions in the definition of weakly_incrementable.
— end note]
template<class I> concept incrementable = regular<I> && weakly_incrementable<I> && requires(I i) { { i++ } -> same_as<I>; };
Let a and b be incrementable objects of type I.
I models incrementable only if
  • If bool(a == b) then bool(a++ == b).
  • If bool(a == b) then bool(((void)a++, a) == ++b).
[Note 2: 
The requirement that a equals b implies ++a equals ++b (which is not true for weakly incrementable types) allows the use of multi-pass one-directional algorithms with types that model incrementable.
— end note]