std::size, std::ssize

Defined in header <iterator> 
template <class C>
constexpr auto size(const C& c) -> decltype(c.size());
 (1) (since C++17) 
template <class C>
constexpr auto ssize(const C& c) 
    -> std::common_type_t<std::ptrdiff_t,
                          std::make_signed_t<decltype(c.size())>>;
 (2) (since C++20) 
template <class T, std::size_t N>
constexpr std::size_t size(const T (&array)[N]) noexcept;
 (3) (since C++17) 
template <class T, std::ptrdiff_t N>
constexpr std::ptrdiff_t ssize(const T (&array)[N]) noexcept;
 (4) (since C++20)

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

#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

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