std::extent(3)

NAME
std::extent - std::extent

Synopsis
   Defined in header <type_traits>
   template< class T, unsigned N = 0> (since C++11)
   struct extent;
   If T is an array type, provides the member constant value equal to the number of
   elements along the Nth 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 extent
   or extent_v
   (since C++17) is undefined.
  Helper variable template
   template< class T, unsigned N = 0 > (since C++17)
   inline constexpr std::size_t extent_v = extent<T, N>::value; Inherited from std::integral_constant

Member constants
   value the number of elements along the Nth dimension of T
   [static] (public static member constant)

Member functions
   operator std::size_t converts the object to std::size_t, returns value
                        (public member function)
   operator() returns value
   (C++14) (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 checks if a type is an array type
   (C++11) (class template)
   rank obtains the number of dimensions of an array type
   (C++11) (class template)
   remove_extent removes one extent from the given array type
   (C++11) (class template)
   remove_all_extents removes all extents from the given array type
   (C++11) (class template)