25 Iterators library [iterators]

25.3 Iterator requirements [iterator.requirements]

25.3.4 Iterator concepts [iterator.concepts]

25.3.4.8 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

(2.1)
If N is representable by iter_difference_t<I>, then s - i is well-defined and equals N.
(2.2)
If $- N$ is representable by iter_difference_t<I>, then i - s is well-defined and equals $- N$ .

🔗

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]