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

NAME
       std::is_reference - std::is_reference

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

	  If  T	 is  a	reference type (lvalue reference or rvalue reference),
       provides	the member
	  constant value equal true. For any other type, value is false.

	  The behavior of a program that adds specializations for is_reference
	  or is_reference_v
	  (since C++17)	is undefined.

Template parameters
	  T - a	type to	check

	 Helper	variable template

	  template<		    class		  T		     >
       (since C++17)
	  inline constexpr bool	is_reference_v = is_reference<T>::value;

       Inherited from std::integral_constant

Member constants
	  value	   true	if T is	a reference type , false otherwise
	  [static] (public static member constant)

Member functions
	  operator bool	converts the object to bool, returns value
			(public	member function)
	  operator()	returns	value
	  (C++14)	(public	member function)

Member types
	  Type	     Definition
	  value_type bool
	  type	     std::integral_constant<bool, value>

Possible implementation
	  template <class T> struct is_reference      :	std::false_type	{};
	  template <class T> struct is_reference<T&>  :	std::true_type {};
	  template <class T> struct is_reference<T&&> :	std::true_type {};

Example
       // Run this code

	#include <iostream>
	#include <type_traits>

	class A	{};

	int main()
	{
	#   define REF(x) << #x	" ?: " << x << '\n'
	    std::cout << std::boolalpha
	    REF( std::is_reference_v<A>	)
	    REF( std::is_reference_v<A&> )
	    REF( std::is_reference_v<A&&> )
	    REF( std::is_reference_v<long> )
	    REF( std::is_reference_v<long&> )
	    REF( std::is_reference_v<long&&> )
	    REF( std::is_reference_v<double*> )
	    REF( std::is_reference_v<double*&> )
	    REF( std::is_reference_v<double*&&>	);
	#   undef REF
	}

Output:
	std::is_reference_v<A> ?: false
	std::is_reference_v<A&>	?: true
	std::is_reference_v<A&&> ?: true
	std::is_reference_v<long> ?: false
	std::is_reference_v<long&> ?: true
	std::is_reference_v<long&&> ?: true
	std::is_reference_v<double*> ?:	false
	std::is_reference_v<double*&> ?: true
	std::is_reference_v<double*&&> ?: true

See also
	  is_lvalue_reference checks if	a type is a lvalue reference
	  (C++11)	      (class template)
	  is_rvalue_reference checks if	a type is a rvalue reference
	  (C++11)	      (class template)

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

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