home | help 
std::rank(3)		       C++ Standard Libary		    std::rank(3)

NAME
     std::rank - std::rank

Synopsis
	Defined in header <type_traits>
	template< class T >		 (since C++11)
	struct rank;

	If  T  is an array type, provides the member constant value equal to the
     number of
	dimensions of the array. For any other type, value is 0.

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

Member constants
	value	 the number of dimensions of T or zero
	[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>
	struct rank : public std::integral_constant<std::size_t, 0> {};

	template<class T>
	struct	 rank<T[]>    :    public    std::integral_constant<std::size_t,
     rank<T>::value + 1> {};

	template<class T, std::size_t N>
	struct	  rank<T[N]>	:   public   std::integral_constant<std::size_t,
     rank<T>::value + 1> {};

Example
     // Run this code

      #include <type_traits>

      int main()
      {
	  static_assert(
		  std::rank<int>{} == 0
	      &&  std::rank<int[5]>{} == 1
	      &&  std::rank<int[5][5]>{} == 2
	      &&  std::rank<int[][5][5]>{} == 3 );

	  [[maybe_unused]] int ary[][3] = {{1, 2, 3}};
	  // The reason of rank of "ary[0]" is calculated as 0
	  static_assert(std::rank_v<decltype(ary[0])> == 0);
	  // is that rank cannot deal with reference type. i.e. int(&)[3]
	  static_assert(std::is_same_v<decltype(ary[0]), int(&)[3]>);
	  // The solution is to remove reference type
	  static_assert(std::rank_v<std::remove_cvref_t<decltype(ary[0])>>    ==
     1);
      }

See also
	is_array	   checks if a type is an array type
	(C++11) 	   (class template)
	extent		    obtains  the size of an array type along a specified
     dimension
	(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::rank(3)
home | help