# 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​.

If the program adds specializations for `std::extent` or `std::extent_v`(since C++17), the behavior is undefined.

### Helper variable template

template< class T, unsigned N = 0 > 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 <type_traits>

    static_assert(
        std::extent_v<int[3]> == 3 && // default dimension is 0
        std::extent_v<int[3], 0> == 3 && // the same as above
        std::extent_v<int[3][4], 0> == 3 &&
        std::extent_v<int[3][4], 1> == 4 &&
        std::extent_v<int[3][4], 2> == 0 &&
        std::extent_v<int[]> == 0
    );

    int main()
    {
        const auto ext = std::extent<int['*']>{};
        static_assert(ext == 42); // with implicit conversion to std::size_t

        const int ints[]{1, 2, 3, 4};
        static_assert(std::extent_v<decltype(ints)> == 4); // array size

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

        // ary[0] is of type reference to 'int[3]', so, the extent
        // cannot be calculated correctly and it returns 0
        static_assert(std::is_same_v<decltype(ary[0]), int(&)[3]>);
        static_assert(std::extent_v<decltype(ary[0])> == 0);

        // removing reference gives correct extent value 3
        static_assert(std::extent_v<std::remove_cvref_t<decltype(ary[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)
extents (C++23) | a descriptor of a multidimensional index space of some rank (class template)

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