When to use std::forward to forward arguments?

Question

C++0x shows an example of using std::forward:

template<class T>
void foo(T&& arg) 
{
  bar(std::forward<T>(arg));
}

When is it advantageous to use std::forward, always?

Also, it requires to use && in the parameters declaration, is it valid in all cases? I thought you had to pass temporaries to a function if the function was declared with && in it, so can foo be called with any parameter?

Lastly, if I have a function call such as this:

template<int val, typename... Params>
void doSomething(Params... args) {
  doSomethingElse<val, Params...>(args...);
}

Should I use this instead:

template<int val, typename... Params>
void doSomething(Params&&... args) {
  doSomethingElse<val, Params...>(std::forward<Params>(args)...);
}

Also, if use the parameters twice in the function, i.e. forwarding to two functions at the same time, is it wise to use std::forward? Won't std::forward convert the same thing to a temporary twice, moving the memory and make it invalid for a second use? Would the following code be ok:

template<int val, typename... Params>
void doSomething(Params&&... args) {
  doSomethingElse<val, Params...>(std::forward<Params>(args)...);
  doSomethingWeird<val, Params...>(std::forward<Params>(args)...);
}

I'm a bit confused by std::forward, and I'd gladly use some clearing up.

Answer 1

Use it like your first example:

template <typename T> void f(T && x)
{
  g(std::forward<T>(x));
}

template <typename ...Args> void f(Args && ...args)
{
  g(std::forward<Args>(args)...);
}

That's because of the reference collapsing rules: If T = U&, then T&& = U&, but if T = U&&, then T&& = U&&, so you always end up with the correct type inside the function body. Finally, you need forward to turn the lvalue-turned x (because it has a name now!) back into an rvalue reference if it was one initially.

You should not forward something more than once however, because that usually does not make sense: Forwarding means that you're potentially moving the argument all the way through to the final caller, and once it's moved it's gone, so you cannot then use it again (in the way you probably meant to).