It’s a great question which foxes almost everyone in the beginning. template <class T> class A { template <class U> void foo(T&& t, U&& u); }; In this example, T is not deduced (you explicitly define it when you instanciate the template). U is deduced because it’s deduced from the argument u. Therefore, in almost … Read more
Overview It is known that since C++11, a parameter of type T&& is called an rvalue reference [ISO/IEC 14882:2011 ยง8.3.2/p2 References [dcl.ref] ]. That is, unless T is a template parameter type or auto or a typedef for some lvalue reference type. examples: template<typename T> void foo(T&& p) { // -> T is a template … Read more
When and if you should use auto&& in for loops has been explained very nicely by Howard Hinnant here. This leaves the question what x in auto &&x = …expr… actually is. And it is handled as if there there were a function template definition template <class U> void f(U&& u); and the type of … Read more
Do they mean the same thing? Universal reference was a term Scott Meyers coined to describe the concept of taking an rvalue reference to a cv-unqualified template parameter, which can then be deduced as either a value or an lvalue reference. At the time the C++ standard didn’t have a special term for this, which … Read more
A universal reference such as T&& can deduce T to be an “object type“, or a “reference type“ In your example it can deduce T as int when passed an rvalue, so the function parameter is int&&, or it can deduce T as int& when passed an lvalue, in which case the function parameter is … Read more
By using auto&& var = <initializer> you are saying: I will accept any initializer regardless of whether it is an lvalue or rvalue expression and I will preserve its constness. This is typically used for forwarding (usually with T&&). The reason this works is because a “universal reference”, auto&& or T&&, will bind to anything. … Read more