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std::ranges::size(3) C++ Standard Libary std::ranges::size(3)


std::ranges::size - std::ranges::size


Defined in header <ranges>
inline namespace /*unspecified*/ {
(since C++20)
inline constexpr auto size = /*unspecified*/; (customization point object)

Call signature
template< class T >

requires /* see below */ (since C++20)

constexpr auto size( T&& t );

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. std::extent_v<T>, if T is an array type with a known bound.
2. Otherwise, t.size() converted to its decayed type, if
ranges::disable_sized_range<std::remove_cv_t<T>> is false, and the converted
expression is valid and has an integer-like type.
3. Otherwise, size(t) converted to its decayed type, if
ranges::disable_sized_range<std::remove_cv_t<T>> 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 ranges::sentinel_t<T> 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.


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

* 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


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

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.


// Run this code

#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';


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

See also

ranges::ssize returns a signed integer equal to the size of a range
(C++20) (customization point object)
ranges::sized_range specifies that a range knows its size in constant time
(C++20) (concept)
ssize returns the size of a container or array
(C++17) (function template)