[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 “not required to be equality-preserving” comments 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:

(2.1)
If bool(a == b) then bool(a++ == b).
(2.2)
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]