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std::decay(3)		      C++ Standard Libary		 std::decay(3)

NAME
       std::decay - std::decay

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

	  Applies  lvalue-to-rvalue, array-to-pointer, and function-to-pointer
       implicit
	  conversions to the type T, removes cv-qualifiers,  and  defines  the
       resulting type as
	  the member typedef type. Formally:

	    *  If  T names the type "array of U" or "reference to array	of U",
       the member
	      typedef type is U*.

	    * Otherwise, if T is a function type F or a	reference thereto, the
       member typedef
	      type is std::add_pointer<F>::type.

	    * Otherwise, the member typedef type is
	      std::remove_cv<std::remove_reference<T>::type>::type.

	  These	conversions model the type conversion applied to all  function
       arguments when
	  passed by value.

	  The behavior of a program that adds specializations for decay	is un-
       defined.

Member types
	  Name Definition
	  type the result of applying the decay	type conversions to T

Helper types
	  template< class T >			    (since C++14)
	  using	decay_t	= typename decay<T>::type;

Possible implementation
	  template< class T >
	  struct decay {
	  private:
	      typedef typename std::remove_reference<T>::type U;
	  public:
	      typedef typename std::conditional<
		  std::is_array<U>::value,
		  typename std::remove_extent<U>::type*,
		  typename std::conditional<
		      std::is_function<U>::value,
		      typename std::add_pointer<U>::type,
		      typename std::remove_cv<U>::type
		  >::type
	      >::type type;
	  };

Example
       // Run this code

	#include <type_traits>

	template <typename T, typename U>
	struct decay_equiv :
	    std::is_same<typename std::decay<T>::type, U>::type
	{};
	template <typename T, typename U>
	constexpr bool is_decay_equ = decay_equiv<T, U>::value;

	int main()
	{
	    static_assert(
		is_decay_equ<int, int> &&
		! is_decay_equ<int, float> &&
		is_decay_equ<int&, int>	&&
		is_decay_equ<int&&, int> &&
		is_decay_equ<const int&, int> &&
		is_decay_equ<int[2], int*> &&
		! is_decay_equ<int[4][2], int*>	&&
		! is_decay_equ<int[4][2], int**> &&
		is_decay_equ<int[4][2],	int(*)[2]> &&
		is_decay_equ<int(int), int(*)(int)>
	    );
	}

See also
	  remove_cvref	       combines	 std::remove_cv	and std::remove_refer-
       ence
	  (C++20)	      (class template)
	  implicit conversion array-to-pointer,	 function-to-pointer,  lvalue-
       to-rvalue
			      conversions

http://cppreference.com		  2022.07.31			 std::decay(3)

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