std:: extent

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

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);
}

See also