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Variants

std::ranges:: sized_range, std::ranges:: disable_sized_range

From cppreference.net
C++
Ranges library
Range adaptors
Defined in header <ranges>
(1)
template < class T >

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

} ;
(since C++20)
(until C++26)
template < class T >

concept sized_range = ranges :: approximately_sized_range < T > &&
requires ( T & t ) {
ranges:: size ( t ) ;

} ;
(since C++26)
template < class >
constexpr bool disable_sized_range = false ;
(2) (since C++20)
1) The sized_range concept specifies the requirements of a range (until C++26) approximately_sized_range (since C++26) type that knows its size in constant time with the size function.
2) The disable_sized_range exists to allow use of range types that provide a size function (either as a member or as a non-member) but do not in fact model sized_range . Users may specialize disable_sized_range for cv-unqualified program-defined types. Such specializations shall be usable in constant expressions and have type const bool .

Contents

Semantic requirements

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

Notes

disable_sized_range cannot be used to opt-out a range whose iterator and sentinel satisfy sized_sentinel_for ; std::disable_sized_sentinel_for must be used instead.

disable_sized_range cannot be specialized for array types or reference types.

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

Example

Run this code 
#include <forward_list>
#include <list>
#include <ranges>
static_assert
(
    std::ranges::sized_range<std::list<int>> and
    not std::ranges::sized_range<std::forward_list<int>>
);
int main() {}

See also

(C++20)
specifies a range whose iterator type satisfies random_access_iterator
(concept)
(C++20)
specifies a range whose iterator type satisfies contiguous_iterator
(concept)