# std::sized_sentinel_for, std::disable_sized_sentinel_for

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< cpp‎ | iterator

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 implicitly requires additional variations of that expression that accept a non-constant lvalue or (possibly constant) rvalue for the given operand unless such an expression variation is explicitly required with differing semantics. These implicit expression variations must meet the same semantic requirements of the declared expression. The extent to which an implementation validates the syntax of the variations is unspecified.

### See also

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

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