Page 7 of: C++ Rvalue References Explained (Perfect Forwarding)

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Page 7 of: Rvalue References Explained, by Thomas Becker about me

Perfect Forwarding: The Problem

The other problem besides move semantics that rvalue references were designed to solve is the perfect forwarding problem. Consider the following simple factory function:

template<typename T, typename Arg> 
shared_ptr<T> factory(Arg arg)
{ 
  return shared_ptr<T>(new T(arg));
}

Obviously, the intent here is to forward the argument arg from the factory function to T's constructor. Ideally, as far as arg is concerned, everything should behave just as if the factory function weren't there and the constructor were called directly in the client code: perfect forwarding. The code above fails miserably at that: it introduces an extra call by value, which is particularly bad if the constructor takes its argument by reference.

The most common solution, chosen e.g. by boost::bind, is to let the outer function take the argument by reference:

template<typename T, typename Arg> 
shared_ptr<T> factory(Arg& arg)
{ 
  return shared_ptr<T>(new T(arg));
}

That's better, but not perfect. The problem is that now, the factory function cannot be called on rvalues:

factory<X>(hoo()); // error if hoo returns by value
factory<X>(41); // error

This can be fixed by providing an overload which takes its argument by const reference:

template<typename T, typename Arg> 
shared_ptr<T> factory(Arg const & arg)
{ 
  return shared_ptr<T>(new T(arg));
}

There are two problems with this approach. Firstly, if factory had not one, but several arguments, you would have to provide overloads for all combinations of non-const and const reference for the various arguments. Thus, the solution scales extremely poorly to functions with several arguments.

Secondly, this kind of forwarding is less than perfect because it blocks out move semantics: the argument of the constructor of T in the body of factory is an lvalue. Therefore, move semantics can never happen even if it would without the wrapping function.

It turns out that rvalue references can be used to solve both these problems. They make it possible to achieve truly perfect forwarding without the use of overloads. In order to understand how, we need to look at two more rules for rvalue references.

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