std::size,std::ssize (3) Linux Manual Page

std::size,std::ssize – std::size,std::ssize

Synopsis

Defined in header<iterator>

    template <class C>
    (1)(since C++ 17)

        constexpr auto size(const C &c) -> decltype(c.size());

template <class C>

constexpr auto ssize(const C &c)(2)(since C++ 20)

    -> std::common_type_t<std::ptrdiff_t,

                          std::make_signed_t<decltype(c.size())>>;

template <class T, std::size_t N>
(3)(since C++ 17)

    constexpr std::size_t size(const T (&array)[N]) noexcept;

template <class T, std::ptrdiff_t N>
(4)(since C++ 20)

    constexpr std::ptrdiff_t ssize(const T (&array)[N]) noexcept;
Returns the size of the given container c or array array.

1-2) Returns c.size(), converted to the return type if necessary.

3-4) Returns N.

Parameters

c – a container with a size method
array – an array of arbitrary type

Return value

The size of c or array

Notes

In addition to being included in <iterator>, std::size and std::ssize are guaranteed to become available if any of the following headers are included: <array>, <deque>, <forward_list>, <list>, <map>, <regex>, <set>
, <span>
(since C++20), <string>, <string_view>, <unordered_map>, <unordered_set>, and <vector>.

Possible implementation

First version

  template <class C>
  constexpr auto size(const C& c) -> decltype(c.size())
  {
      return c.size();
  }

Second version

  template <class C>
  constexpr auto ssize(const C& c)
      -> std::common_type_t<std::ptrdiff_t,
                            std::make_signed_t<decltype(c.size())>> {
      using R = std::common_type_t<std::ptrdiff_t,
                                   std::make_signed_t<decltype(c.size())>>;
      return static_cast<R>(c.size());
  }

Third version

  template <class T, std::size_t N>
  constexpr std::size_t size(const T (&array)[N]) noexcept
  {
      return N;
  }

Fourth version

  template <class T, std::ptrdiff_t N>
  constexpr std::ptrdiff_t ssize(const T (&array)[N]) noexcept
  {
      return N;
  }

Example

// Run this code

#include <iostream>

#include <vector>

#include <iterator>
int main()

{
    std::vector<int> v = {3, 1, 4};
    std::cout << std::size(v) << '\n';
    int a[] = {-5, 10, 15};
    std::cout << std::size(a) << '\n';
}

Output:

  3
  3