# std::ranges::size

< cpp‎ | ranges
 Defined in header  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)

Calculates the number of elements in t in constant time.

Let t be an object of type T. A call to ranges::size is expression-equivalent to:

1. , if T is an array type with a known bound.
2. Otherwise, t.size() converted to its decayed type, if is false, and the converted expression is valid and has an integer-like type.
3. Otherwise, size(t) converted to its decayed type, if is false, and the converted expression is valid and has an integer-like type, where the overload resolution is performed with the following candidates:
• void size(auto&) = delete;
• void size(const auto&) = delete;
4. Otherwise, /*to-unsigned-like*/(ranges::end(t) - ranges::begin(t)), if T models ranges::forward_range and models ,
where /*to-unsigned-like*/ denotes an explicit conversion to an unsigned-integer-like type.

In all other cases, a call to ranges::size is ill-formed, which can result in substitution failure when ranges::size(t) appears in the immediate context of a template instantiation.

## Contents

### Expression-equivalent

Expression e is expression-equivalent to expression f, if

• e and f have the same effects, and
• either both are constant subexpressions or else neither is a constant subexpression, and
• either both are potentially-throwing or else neither is potentially-throwing (i.e. noexcept(e) == noexcept(f)).

###  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

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.

### 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::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';

std::cout << std::boolalpha << "is_signed: "
<< std::is_signed_v<decltype(std::ranges::size(v))> << '\n';
}

Output:

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