Trevor Hickey Trevor Hickey - 2 months ago 9
C++ Question

Is there a difference between universal references and forwarding referenes?

An argument to this function will bind to an rvalue reference:

void f(int && i);


However, an argument to this function will bind to either an rvalue or an lvalue reference:

template <typename T>
void f(T && t);


I've often heard this referred to as a universal reference.

I've also heard it been called a forwarding reference.

Do they mean the same thing?

Is it only a forwarding reference if the function body calls
std::forward
?

Answer

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 was an oversight in C++11 and makes it hard to teach. This oversight was remedied by N4164, which added the following definition to [temp.deduct]:

A forwarding reference is an rvalue reference to a cv-unqualified template parameter. If P is a forwarding reference and the argument is an lvalue, the type “lvalue reference to A” is used in place of A for type deduction.

Hence, the two mean the same thing, and the current C++ standard term is forwarding reference. The paper itself articulates why "forwarding reference" is a better term than "universal reference."

Is it only a forwarding reference if the function body calls std::forward?

Nope, what you do with a forwarding reference is irrelevant to the name. The concept forwarding reference simply refers to how the type T is deduced in:

template <class T> void foo(T&& ); // <== 

It does not need to be subsequently forwarded .

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