C++0x rvalue references and temporaries

Question

(I asked a variation of this question on comp.std.c++ but didn\'t get an answer.)

Why does the call to f(arg) in this code call the const ref overload of

Answer 1

A lot of things in the current draft of the standard need clarification, if you ask me. And the compilers are still developing, so it's hard to trust their help.

It looks pretty clear that your intuition is right… temporaries of any kind are supposed to bind to rvalue references. For example, §3.10, the new "taxonomy" section, categorically defines temporaries as rvalues.

The problem may be that the RR argument specification is insufficient to invoke the creation of a temporary. §5.2.2/5: "Where a parameter is of const reference type a temporary object is introduced if needed." That sounds suspiciously exclusive.

Seems to slip through the cracks again at §13.3.3.1/6: (emphasis mine)

When the parameter type is not a reference, the implicit conversion sequence models a copy-initialization of the parameter from the argument expression. The implicit conversion sequence is the one required to convert the argument expression to a prvalue of the type of the parameter.

Note that copy-initialization string &&rr = "hello"; works fine in GCC.

EDIT: Actually the problem doesn't exist on my version of GCC. I'm still trying to figure out how the second standard conversion of the user-defined conversion sequence relates to forming an rvalue reference. (Is RR formation a conversion at all? Or is it dictated by scattered tidbits like 5.2.2/5?)