23 Iterators library [iterators]

23.3 Iterator requirements [iterator.requirements]

23.3.4 Iterator concepts [iterator.concepts]

23.3.4.5 Concept incrementable [iterator.concept.inc]

1
#
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
:
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>;
    };
2
#
Let a and b be incrementable objects of type I.
I models incrementable only if
  • (2.1)
    If bool(a == b) then bool(a++ == b).
  • (2.2)
    If bool(a == b) then bool(((void)a++, a) == ++b).
3
#
[Note
:
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
]