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

NAME
       std::declval - std::declval

Synopsis
	  Defined in header <utility>
	  template<		     class		   T		     >
       (since C++11)
	  typename std::add_rvalue_reference<T>::type declval()	noexcept;

	  Converts any type T to a reference type, making it possible  to  use
       member functions
	  in  the  operand  of	the  decltype specifier	without	the need to go
       through
	  constructors.

	  std::declval is commonly used	in templates where acceptable template
       parameters may
	  have no constructor in common, but have  the	same  member  function
       whose return type
	  is needed.

	  Note	that std::declval can only be used in unevaluated contexts and
       is not required
	  to be	defined; it is an error	to evaluate an	expression  that  con-
       tains this function.
	  Formally, the	program	is ill-formed if this function is odr-used.

Parameters
	  (none)

Return value
	  Cannot  be called and	thus never returns a value. The	return type is
       T&& unless T is
	  (possibly cv-qualified) void,	in which case the return type is T.

Possible implementation
	  template<typename T>
	  typename std::add_rvalue_reference<T>::type declval()	noexcept
	  {
	      static_assert(false, "declval not	allowed	in an  evaluated  con-
       text");
	  }

Example
       // Run this code

	#include <iostream>
	#include <utility>

	struct Default
	{
	    int	foo() const { return 1;	}
	};

	struct NonDefault
	{
	    NonDefault() = delete;
	    int	foo() const { return 1;	}
	};

	int main()
	{
	    decltype(Default().foo())  n1 = 1;			 // type of n1
       is int
	//  decltype(NonDefault().foo()) n2 = n1;		//  error:  no
       default constructor
	    decltype(std::declval<NonDefault>().foo())	n2 = n1; // type of n2
       is int
	    std::cout << "n1 = " << n1 << '\n'
		      << "n2 = " << n2 << '\n';
	}

Output:
	n1 = 1
	n2 = 1

See also
	  decltype specifier(C++11) obtains the	type of	an  expression	or  an
       entity
	  result_of		     deduces  the  result  type	 of invoking a
       callable	object with
	  invoke_result		    a set of arguments
	  (C++11)(removed in C++20) (class template)
	  (C++17)

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

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