std::ranges::approximately_sized_range

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

The approximately_sized_range concept specifies the requirements of a range type that can estimate its size in constant time with the reserve_hint function.

Semantic requirements

Given an lvalue t of type std::remove_reference_t<T>, T models approximately_sized_range only if

• ranges::reserve_hint(t)
• has amortized constant-time complexity,
• does not alter the value of t in a manner observable to equality-preserving expressions, and
• has a value that is not negative and is representable in ranges::range_difference_t<T>, and
• if ranges::iterator_t<T> models forward_iterator, ranges::reserve_hint(t) is well-defined regardless of the evaluation of ranges::begin(t) (in other words, a single-pass approximately sized range may support a call to reserve_hint only before the first call to begin, but a forward range must support size at all times).

Notes

Example

See also