std::size

C++

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Language support library

Concepts library (C++20)

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Iterators library

Numerics library

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Technical Specifications

Iterator library

Iterator primitives
iterator_traits
input_iterator_tagoutput_iterator_tagforward_iterator_tagbidirectional_iterator_tagrandom_access_iterator_tag
iterator
Iterator adaptors
reverse_iterator move_iterator (C++11) back_insert_iterator front_insert_iterator insert_iterator	make_reverse_iterator (C++14) make_move_iterator (C++11) back_inserter front_inserter inserter
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istream_iterator
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ostreambuf_iterator
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advance
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begincbegin (C++11)(C++14)
endcend (C++11)(C++14)
rbegincrbegin (C++14)(C++14)
rendcrend (C++14)(C++14)
Container access
size (C++17)
empty (C++17)
data (C++17)

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

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