NAME

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

Synopsis

Defined in header <array>
Defined in header <deque>
Defined in header <forward_list>
Defined in header <iterator>
Defined in header <list>
Defined in header <map>
Defined in header <regex>
Defined in header <set>
Defined in header <span> (since C++20)
Defined in header <string>
Defined in header <string_view>
Defined in header <unordered_map>
Defined in header <unordered_set>
Defined in header <vector>
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 range.

1-2) Returns c.size(), converted to the return type if necessary.
3-4) Returns N.

Parameters

c - a container or view with a size member function
array - an array of arbitrary type

Return value

The size of c or array.

Exceptions

1-2) May throw implementation-defined exceptions.

Overloads

Custom overloads of size may be provided for classes and enumerations that do not
expose a suitable size() member function, yet can be detected.

Overloads of size found by argument-dependent lookup can be used to
customize the behavior of std::ranges::size, std::ranges::ssize, and (since C++20)
std::ranges::empty.

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;
}

Notes

Feature-test macro: __cpp_lib_nonmember_container_access (for std::size())

Feature-test macro: __cpp_lib_ssize (for std::ssize())

Example

// Run this code

#include <iostream>
#include <vector>

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';

// since C++20 the signed size (ssize) is available
auto i = std::ssize(v);
for (--i; i != -1; --i) {
std::cout << v[i] << (i ? ' ' : '\n');
}
std::cout << "i = " << i << '\n';
}

Output:

3
3
4 1 3
i = -1