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std::sized_sentinel_for, std::disable_sized_sentinel_for

Defined in header <iterator> 
template< class S, class I >
    concept sized_sentinel_for =
        std::sentinel_for<S, I> &&
        !std::disable_sized_sentinel_for<std::remove_cv_t<S>, std::remove_cv_t<I>> &&
        requires(const I& i, const S& s) {
            { s - i } -> std::same_as<std::iter_difference_t<I>>;
            { i - s } -> std::same_as<std::iter_difference_t<I>>;
        };
 (1) (since C++20) 
template< class S, class I >
    inline constexpr bool disable_sized_sentinel_for = false;
 (2) (since C++20)
1) The sized_sentinel_for concept specifies that an object of the iterator type I and an object of the sentinel type S can be subtracted to compute the distance between them in constant time.
2) The disable_sized_sentinel_for variable template can be used to prevent iterators and sentinels that can be subtracted but do not actually model sized_sentinel_for from satisfying the concept.
The variable template is allowed to be specialized for cv-unqualified non-array object type S and I, as long as at least one of which is a program-defined type. Such specializations shall be usable in constant expressions and have type const bool.

Semantic requirements

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. I and S model sized_sentinel_for<S, I> only if:

  • If n is representable by std::iter_difference_t<I>, then s - i is well-defined and equals n; and
  • If -n is representable by std::iter_difference_t<I>, then i - s is well-defined and equals -n.
  • Subtraction between i and s has constant time complexity.

Equality preservation

Expressions declared in requires expressions of the standard library concepts are required to be equality-preserving (except where stated otherwise).

Implicit expression variations

A requires expression that uses an expression that is non-modifying for some constant lvalue operand also requires implicit expression variations.

See also

(C++20)
specifies that a range knows its size in constant time
(concept)
(C++20)
returns an integer equal to the size of a range
(customization point object)

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