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std::size

From cppreference.com
< cpp‎ | iterator
Defined in header <iterator>
template < class C >
constexpr auto size( const C& c ) -> decltype(c.size());
(1) (since C++17)
template < class T, size_t N >
constexpr 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.

Contents

[edit] Parameters

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

[edit] Return value

The size of c or array

[edit] 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>, <string>, <unordered_map>, <unordered_set>, and <vector>.

[edit] 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;
}

[edit] 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