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std::extent(3)		       C++ Standard Libary		  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  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.

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()
      {
	  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
	  );

	  const auto ext = std::extent<int['*']>{};
	  std::cout << ext << '\n'; //< 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 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]>);
	  static_assert(std::extent_v<decltype(ary[0])> == 0);
	  // removing reference will give correct extent value 3
	  static_assert(std::extent_v<std::remove_cvref_t<decltype(ary[0])>>  ==
     3);
      }

Output:
      42

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)

http://cppreference.com 	   2024.06.10			  std::extent(3)
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