Use of rvalue reference members?

Question

I was wondering what use an rvalue reference member has

class A {   // ...   // Is this one useful?   Foo &&f; }; 

Does it have any benefits or drawbacks compared to an lvalue reference member? What is a prime usecase of it?

Answer 1

I've seen one very motivating use case for rvalue reference data members, and it is in the C++0x draft:

template<class... Types> tuple<Types&&...> forward_as_tuple(Types&&... t) noexcept; 

Effects: Constructs a tuple of references to the arguments in t suitable for forwarding as arguments to a function. Because the result may contain references to temporary variables, a program shall ensure that the return value of this function does not outlive any of its arguments. (e.g., the program should typically not store the result in a named variable).

Returns: tuple<Types&&...>(std::forward<Types>(t)...)

The tuple has rvalue reference data members when rvalues are used as arguments to forward_as_tuple, and otherwise has lvalue reference data members.

I've found forward_as_tuple subsequently helpful when needing to catch variadic arguments, perfectly forward them packed as a tuple, and re-expand them later at the point of forwarding to a functor. I used forward_as_tuple in this style when implementing an enhanced version of tuple_cat proposed in LWG 1385:

http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#1385

Answer 2

According to Stephan T. Lavavej, rvalue reference data members have no use.

[at 31:00] The thing I've seen programmers do when they get hold of rvalue references is that, they start to go a little crazy, because they're so powerful. They start saying "Oh, I'm gonna have rvalue reference data members, I'm gonna have rvalue reference local variables, I'm gonna have rvalue reference return values!" And then they write code like this: [...]