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std::ranges::size

From cppreference.com
< cpp‎ | ranges
 
 
 
Defined in header <ranges>
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);

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, std::forward<T>(t).size(), if not ranges::disable_sized_range<std::remove_cv_t<T>>, and std::forward<T>(t).size() is valid and returns a type that is /*integer-like*/.
    where /*integer-like*/ is a type that models std::integral or is a class that behaves like an integer type, including all operators, implicit conversions, and std::numeric_limits specializations.
  3. Otherwise, size(std::forward<T>(t)), if not ranges::disable_sized_range<std::remove_cv_t<T>>, and size(std::forward<T>(t)) is valid and returns a type that is /*integer-like*/, where the overload resolution is performed with the following candidates:
    • void begin(auto&) = delete;
    • void begin(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 std::sized_sentinel_for<ranges::iterator_t<T>>.
    where /*to-unsigned-like*/ denotes an explicit conversion to an /*integer-like*/ type that behaves like an unsigned integer type, including all operators, implicit conversions, and std::numeric_limits specializations.

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

[edit] Expression-equivalent

Expression e is expression-equivalent to expression f, if e and f have the same effects, either are both potentially-throwing or are both not potentially-throwing (i.e. noexcept(e) == noexcept(f)), and either are both constant subexpressions or are both not constant subexpressions.

[edit] Customization point objects

The name ranges::size denotes a customization point object, which is a const function object of a literal semiregular class type (denoted, for exposition purposes, as size_ftor). All instances of size_ftor are equal. 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_ftor will satisfy std::invocable<const size_ftor&, Args...>. Otherwise, no function call operator of size_ftor participates in overload resolution.

[edit] 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

[edit] See also

obtains the size of a range whose size can be calculated in constant time and converts it to signed integral
(customization point object) [edit]
(C++17)(C++20)
returns the size of a container or array
(function template) [edit]