std::ranges::size

Header: <ranges>

Calculates the number of elements in t in constant time.

# Declarations

inline namespace /* unspecified */ {
inline constexpr auto size = /* unspecified */;
}

(since C++20) (customization point object)

Call signature

template< class T >
requires /* see below */
constexpr auto size( T&& t );

(since C++20)

# 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);
}

# Defect reports

# See also

• ranges::ssize
• ranges::sized_range
• ranges::distance
• sizessize