Understanding rvalue references

Question

I think there's something I'm not quite understanding about rvalue references. Why does the following fail to compile (VS2012) with the error 'foo' : cannot convert parameter 1 from 'int' to 'int &&'?

void foo(int &&) {} void bar(int &&x) { foo(x); }; 

I would have assumed that the type int && would be preserved when passed from bar into foo. Why does it get transformed into int once inside the function body?

I know the answer is to use std::forward:

void bar(int &&x) { foo(std::forward<int>(x)); } 

so maybe I just don't have a clear grasp on why. (Also, why not std::move?)

Answer 1

I always remember lvalue as a value that has a name or can be addressed. Since x has a name, it is passed as an lvalue. The purpose of reference to rvalue is to allow the function to completely clobber value in any way it sees fit. If we pass x by reference as in your example, then we have no way of knowing if is safe to do this:

void foo(int &&) {} void bar(int &&x) {      foo(x);      x.DoSomething();   // what could x be? }; 

Doing foo(std::move(x)); is explicitly telling the compiler that you are done with x and no longer need it. Without that move, bad things could happen to existing code. The std::move is a safeguard.

std::forward is used for perfect forwarding in templates.

Answer 2

Why does it get transformed into int once inside the function body?

It doesn't; it's still a reference to an rvalue.

When a name appears in an expression, it's an lvalue - even if it happens to be a reference to an rvalue. It can be converted into an rvalue if the expression requires that (i.e. if its value is needed); but it can't be bound to an rvalue reference.

So as you say, in order to bind it to another rvalue reference, you have to explicitly convert it to an unnamed rvalue. std::forward and std::move are convenient ways to do that.

Also, why not std::move?

Why not indeed? That would make more sense than std::forward, which is intended for templates that don't know whether the argument is a reference.