std::size

From cppreference.com
< cpplrm; | iterator
Defined in header <iterator>
template <class C>
constexpr auto size(const C& c) -> decltype(c.size());
(1) (since C++17)
template <class T, std::size_t N>
constexpr std::size_t size(const T (&array)[N]) noexcept;
(2) (since C++17)

Returns the size of the given container c or array array.

1) Returns c.size().
2) 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 is 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 T, std::size_t N>
constexpr std::size_t size(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