# std::size, std::ssize

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< cpp‎ | iterator

Defined in header `<iterator>` |  |
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 container `c` or array `array`.

1-2) Returns `c.size()`, converted to the return type if necessary.
3-4) 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` and `std::ssize` are 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 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

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

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