std::ranges:: reserve_hint

Defined in header <ranges>
Defined in header <iterator>
inline namespace /* unspecified */ { inline constexpr auto reserve_hint = /* unspecified */ ; }		(since C++26) (customization point object)
Call signature
template < class T > requires /* see below */ constexpr auto reserve_hint ( T && t ) ;		(since C++26)

Estimates the number of elements in t in constant time.

Given the subexpression of which t denotes the (possibly materialized ) result object as E , and the type of E as T :

• If ranges:: size ( E ) is a valid expression, ranges :: reserve_hint ( E ) is expression-equivalent to ranges:: size ( E ) .
• Otherwise, if auto ( t. reserve_hint ( ) ) is a valid expression of integer-like type , ranges :: reserve_hint ( E ) is expression-equivalent to auto ( t. reserve_hint ( ) ) .
• Otherwise, if all following conditions are satisfied, then ranges :: reserve_hint ( E ) is expression-equivalent to auto ( reserve_hint ( t ) ) :
  • T is a class or enumeration type.
  • auto ( reserve_hint ( t ) ) is a valid expression of integer-like type, where the meaning of reserve_hint is established as if by performing argument-dependent lookup only.
• Otherwise, ranges :: reserve_hint ( E ) is ill-formed.

Diagnosable ill-formed cases above result in substitution failure when ranges :: reserve_hint ( E ) appears in the immediate context of a template instantiation.

Contents 1 Customization point objects 2 Notes 3 Example 4 See also

Customization point objects

The name ranges::reserve_hint denotes a customization point object , which is a const function object of a literal semiregular class type. See CustomizationPointObject for details.

Notes

Whenever ranges :: reserve_hint ( e ) is valid for an expression e , the return type is integer-like .

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

Example

This section is incomplete Reason: no example

See also

ranges::size (C++20)	returns an integer equal to the size of a range (customization point object)
ranges::ssize (C++20)	returns a signed integer equal to the size of a range (customization point object)
ranges::approximately_sized_range (C++26)	specifies that a range can estimate its size in constant time (concept)