std::ranges:: approximately_sized_range

Defined in header <ranges>
template < class T > concept approximately_sized_range = ranges:: range < T > && requires ( T & t ) { ranges :: reserve_hint ( t ) ; } ;		(since C++26)

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.

Contents 1 Semantic requirements 2 Notes 3 Example 4 See also

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

Feature-test macro	Value	Std	Feature
__cpp_lib_ranges_reserve_hint	202502L	(C++26)	ranges::approximately_sized_range

Example

This section is incomplete Reason: no example

See also

ranges::sized_range (C++20)

specifies that a range knows its size in constant time (concept)