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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  mem-
     ber 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 pa-
     rameters 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 contains
     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 context");
	}

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  de-
     fault 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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