25 Iterators library [iterators]

25.3 Iterator requirements [iterator.requirements]

25.3.4 Iterator concepts [iterator.concepts] Concept sized_sentinel_for [iterator.concept.sizedsentinel]

The sized_sentinel_for concept specifies requirements on an input_or_output_iterator type I and a corresponding sentinel_for<I> that allow the use of the - operator to compute the distance between them in constant time.
template<class S, class I> concept sized_sentinel_for = sentinel_for<S, I> && !disable_sized_sentinel_for<remove_cv_t<S>, remove_cv_t<I>> && requires(const I& i, const S& s) { { s - i } -> same_as<iter_difference_t<I>>; { i - s } -> same_as<iter_difference_t<I>>; };
Let i be an iterator of type I, and s a sentinel of type S such that [i, s) denotes a range.
Let N be the smallest number of applications of ++i necessary to make bool(i == s) be true.
S and I model sized_sentinel_for<S, I> only if
  • If N is representable by iter_difference_t<I>, then s - i is well-defined and equals N.
  • If is representable by iter_difference_t<I>, then i - s is well-defined and equals .
template<class S, class I> constexpr bool disable_sized_sentinel_for = false;
Remarks: Pursuant to [namespace.std], users may specialize disable_sized_sentinel_for for cv-unqualified non-array object types S and I if S and/or I is a program-defined type.
Such specializations shall be usable in constant expressions ([expr.const]) and have type const bool.
[Note 1: 
disable_sized_sentinel_for allows use of sentinels and iterators with the library that satisfy but do not in fact model sized_sentinel_for.
— end note]
[Example 1: 
The sized_sentinel_for concept is modeled by pairs of random_access_iterators ([iterator.concept.random.access]) and by counted iterators and their sentinels ([counted.iterator]).
— end example]