std::size,std::ssize (3) - Linux Manuals

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

NAME

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