std::size, std::ssize

< cpp‎ | iterator
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>
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 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 std::ranges::empty.

 (since C++20)

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';
 
    // since C++20 the signed size (ssize) can avail
    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

See also

 signed integer type returned when subtracting two pointers
(typedef)
unsigned integer type returned by the sizeof operator
(typedef)
(C++20)
 returns an unsigned integer equal to the size of a range
(customization point object)
(C++20)
 returns a signed integer equal to the size of a range
(customization point object)