std::ranges:: size

Calculates 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 T is an array of unknown bound, ranges :: size ( E ) is ill-formed.
• Otherwise, if T is an array type, ranges :: size ( E ) is expression-equivalent to decay-copy  ( std:: extent_v < T > ) (until C++23) auto ( std:: extent_v < T > ) (since C++23) .
• Otherwise, if all following conditions are satisfied, ranges :: size ( E ) is expression-equivalent to decay-copy  ( t. size ( ) ) (until C++23) auto ( t. size ( ) ) (since C++23) :
  • ranges:: disable_sized_range < std:: remove_cv_t < T >> is false .
  • decay-copy  ( t. size ( ) ) (until C++23) auto ( t. size ( ) ) (since C++23) is a valid expression of integer-like type .
• Otherwise, if all following conditions are satisfied, ranges :: size ( E ) is expression-equivalent to decay-copy  ( size ( t ) ) (until C++23) auto ( size ( t ) ) (since C++23) :
  • T is a class or enumeration type.
  • ranges:: disable_sized_range < std:: remove_cv_t < T >> is false .
  • decay-copy  ( size ( t ) ) (until C++23) auto ( size ( t ) ) (since C++23) is a valid expression of integer-like type, where the meaning of size is established as if by performing argument-dependent lookup only.
• Otherwise, if all following conditions are satisfied, ranges :: size ( E ) is expression-equivalent to to-unsigned-like  ( ranges:: end ( t ) - ranges:: begin ( t ) ) :
  • T models forward_range .
  • Given the type of ranges:: begin ( t ) as I and the type of ranges:: end ( t ) as S , both sized_sentinel_for < S, I > and forward_iterator < I > are modeled.
  • to-unsigned-like  ( ranges:: end ( t ) - ranges:: begin ( t ) ) is a valid expression.
• Otherwise, ranges :: size ( E ) is ill-formed.

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

Customization point objects

The name ranges::size denotes a customization point object , which is a const function object of a literal semiregular class type. For exposition purposes, the cv-unqualified version of its type is denoted as __size_fn .

All instances of __size_fn are equal. The effects of invoking different instances of type __size_fn on the same arguments are equivalent, regardless of whether the expression denoting the instance is an lvalue or rvalue, and is const-qualified or not (however, a volatile-qualified instance is not required to be invocable). Thus, ranges::size can be copied freely and its copies can be used interchangeably.

Given a set of types Args... , if std:: declval < Args > ( ) ... meet the requirements for arguments to ranges::size above, __size_fn models

• std:: invocable < __size_fn, Args... > ,
• std:: invocable < const __size_fn, Args... > ,
• std:: invocable < __size_fn & , Args... > , and
• std:: invocable < const __size_fn & , Args... > .
  

Otherwise, no function call operator of __size_fn participates in overload resolution.

Notes

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

The C++20 standard requires that if the underlying size function call returns a prvalue, the return value is move-constructed from the materialized temporary object. All implementations directly return the prvalue instead. The requirement is corrected by the post-C++20 proposal P0849R8 to match the implementations.

The expression ranges:: distance ( e ) can also be used to determine the size of a range e . Unlike ranges :: size ( e ) , ranges:: distance ( e ) works even if e is an unsized range, at the cost of having linear complexity in that case.

Example

#include <iostream>
#include <ranges>
#include <type_traits>
#include <vector>
 
int main()
{
    auto v = std::vector<int>{};
    std::cout << "ranges::size(v) == " << std::ranges::size(v) << '\n';
 
    auto il = {7};     // std::initializer_list
    std::cout << "ranges::size(il) == " << std::ranges::size(il) << '\n';
 
    int array[]{4, 5}; // array has a known bound
    std::cout << "ranges::size(array) == " << std::ranges::size(array) << '\n';
 
    static_assert(std::is_signed_v<decltype(std::ranges::size(v))> == false);
}

ranges::size(v) == 0
ranges::size(il) == 1
ranges::size(array) == 2

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

See also