std::sized_sentinel_for, std::disable_sized_sentinel_for

 

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istream_iterator ostream_iterator	istreambuf_iterator ostreambuf_iterator
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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

An expression is equality preserving if it results in equal outputs given equal inputs.

• The inputs to an expression consist of its operands.
• The outputs of an expression consist of its result and all operands modified by the expression (if any).

In specification of standard concepts, operands are defined as the largest subexpressions that include only:

• an id-expression, and
• invocations of std::move, std::forward, and std::declval.

The cv-qualification and value category of each operand is determined by assuming that each template type parameter denotes a cv-unqualified complete non-array object type.

Every expression required to be equality preserving is further required to be stable: two evaluations of such an expression with the same input objects must have equal outputs absent any explicit intervening modification of those input objects.

Unless noted otherwise, every expression used in a requires-expression is required to be equality preserving and stable, and the evaluation of the expression may modify only its non-constant operands. Operands that are constant must not be modified.

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)