universal_type_deduction.cc

//----------------------------------------------------------------------
// Type deduction class.
struct s_td { // Do not make this a class template, unless the template 
			  // is necessary for something other than echoval(...)
	template <typename K>
	void echoval(K&& t) // Universal reference.
	{
		std::cout << "t = " << t << '\n';
	}
};

//----------------------------------------------------------------------
// Universal type deduction.

void universal_type_deduction(void)
{
	// Trivially, 
	int x = 42;
	int& l_rx = x; // fine
	int&& r_rx1 = 42; // fine
	//int&& r_rx2 = x; // I know it will fail compile here.
	auto&& r_rx3 = 42; // fine, rvalue
	auto&& r_rx4 = x; // fine, type-deduced! Universal.
	auto&& r_rx5 = l_rx; // fine, type-deduced! Universal.
	
	s_td().echoval(42); // Do not specialize!
	s_td().echoval(x); 
}

gcc users compile with -std=c++11

This snippet illustrates briefly the concept of universal references, as defined by Scott Meyers. Note that this kind of universality of reference is true of templates as well.

• Note that when an rvalue is passed into a function / method, it WILL become an lvalue, since it has storage.