std::extent - cppreference.com

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`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

the number of elements along the Nth dimension of T
(public static member constant)

Member functions

	converts the object to `std::size_t`, returns `value` (public member function)
	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

#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);
}

	checks if a type is an array type (class template) [edit]
	obtains the number of dimensions of an array type (class template) [edit]
	removes one extent from the given array type (class template) [edit]
	removes all extents from the given array type (class template) [edit]
	a descriptor of a multidimensional index space of some rank (class template) [edit]