# std::extent

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< cpp‎ | types

Defined in header `<type_traits>` |  |
template< class T, unsigned N = 0 > struct extent; |  | (since C++11)

If `T` is an array type, provides the member constant `value` equal to the number of elements along the `N`th dimension of the array, if `N` is in `[`​0​`,` std::rank<T>::value`)`. For any other type, or if `T` is an array of unknown bound along its first dimension and `N` is 0, `value` is 0.

The behavior of a program that adds specializations for `std::extent` or `std::extent_v` (since C++17) is undefined.

### Helper variable template

template< class T, unsigned N = 0 > inline constexpr std::size_t extent_v = extent<T, N>::value; |  | (since C++17)

## Inherited from std::integral_constant

### Member constants

value [static] | the number of elements along the `N`th dimension of `T` (public static member constant)

### Member functions

operator std::size_t | converts the object to std::size_t, returns value (public member function)
operator() (C++14) | returns value (public member function)

### Member types

Type | Definition
`value_type` | std::size_t
`type` | std::integral_constant<std::size_t, value>

### Possible implementation

    template<class T, unsigned N = 0>
    struct extent : std::integral_constant<std::size_t, 0> {};

    template<class T>
    struct extent<T[], 0> : std::integral_constant<std::size_t, 0> {};

    template<class T, unsigned N>
    struct extent<T[], N> : std::extent<T, N - 1> {};

    template<class T, std::size_t I>
    struct extent<T[I], 0> : std::integral_constant<std::size_t, I> {};

    template<class T, std::size_t I, unsigned N>
    struct extent<T[I], N> : std::extent<T, N - 1> {};

### Example

Run this code

    #include <iostream>
    #include <type_traits>

    int main()
    {
        std::cout << std::extent<int[3]>::value << '\n'; //< default dimension is 0
        std::cout << std::extent<int[3][4], 0>::value << '\n';
        std::cout << std::extent<int[3][4], 1>::value << '\n';
        std::cout << std::extent<int[3][4], 2>::value << '\n';
        std::cout << std::extent<int[]>::value << '\n';

        const auto ext = std::extent<int[9]>{};
        std::cout << ext << '\n'; //< implicit conversion to std::size_t

        const int ints[] = {1, 2, 3, 4};
        std::cout << std::extent<decltype(ints)>::value << '\n'; //< array size

        [[maybe_unused]] int ary[][3] = { {1, 2, 3} };

        // ary[0] is type of reference of 'int[3]', so, extent
        // cannot calculate correctly and return 0
        static_assert(std::is_same_v<decltype(ary[0]), int(&)[3]>);
        std::cout << std::extent<decltype(ary[0])>::value << '\n';
        // removing reference will give correct extent value 3
        std::cout << std::extent<std::remove_cvref_t<decltype(ary[0])>>::value << '\n';
    }

Output:

    3
    3
    4
    0
    0
    9
    4
    0
    3

### See also

is_array (C++11) | checks if a type is an array type (class template)
rank (C++11) | obtains the number of dimensions of an array type (class template)
remove_extent (C++11) | removes one extent from the given array type (class template)
remove_all_extents (C++11) | removes all extents from the given array type (class template)

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