26 Ranges library [ranges]

26.4 Range requirements [range.req]

26.4.3 Sized ranges [range.sized]

The sized_range concept refines range with the requirement that the number of elements in the range can be determined in amortized constant time using ranges::size.

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template<class T>
  concept sized_range =
    range<T> && requires(T& t) { ranges::size(t); };

Given an lvalue t of type remove_reference_t<T>, T models sized_range only if

(2.1)
ranges::size(t) is amortized $O (1)$ , does not modify t, and is equal to ranges::distance(ranges::begin(t), ranges::end(t)), and
(2.2)
if iterator_t<T> models forward_iterator, ranges::size(t) is well-defined regardless of the evaluation of ranges::begin(t).
[Note 1:
ranges::size(t) is otherwise not required to be well-defined after evaluating ranges::begin(t).

For example, it is possible for ranges::size(t) to be well-defined for a sized_range whose iterator type does not model forward_iterator only if evaluated before the first call to ranges::begin(t).
— end note]

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template<class>
  constexpr bool disable_sized_range = false;

Remarks: Pursuant to [namespace.std], users may specialize disable_sized_range for cv-unqualified program-defined types.

Such specializations shall be usable in constant expressions ([expr.const]) and have type const bool.

[Note 2:

disable_sized_range allows use of range types with the library that satisfy but do not in fact model sized_range .

— end note]