std::sized_sentinel_for, std::disable_sized_sentinel_for

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>>;
    };

template<class S, class I>
  inline constexpr bool disable_sized_sentinel_for = false;

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