Re: [boost] decltype, LVALUE_TYPEOF, and binding a reference

From: Jaakko Järvi (jarvi_at_[hidden])
Date: 2006-02-11 18:08:45

On Feb 11, 2006, at 8:27 AM, Arkadiy Vertleyb wrote:

> Hi all.
>
> The primary purpose of decltype, according to the decltype/auto  
> proposal,
> seems to be to assist in creation forwarding functions, something  
> like:
>
> template<class F>
> decltype(F()()) forward(F f) {
>     return f();
> }
>
> For example, if f is declared to return const rvalue:
>
> const x f();
>
> using the real decltype (in the future, of course) would result in:
>
> const x forward(const x(*)()) {
>     return f();
> }
>
> Which is, of course, what we expect in this case.
>
> Since LVALUE_TYPEOF has a problem distinguishing const x from const  
> x&,
> using it in the above case would result in:
>
> const x& forward(const x(*)()) {
> ^^^^^^^
>     return f();
> }
>
> Now, my question is: isn't this fine?  The same rule (extending the  
> lifetime
> of const reference) that prevented LVALUE_TYPEOF from working  
> correctly,
> seems to now ensure that it is fine to use this "incorrect" result.

I think the extended lifetime rule for temporaries would make it work  
ok, but
the mere definition of the "forward" function above will at least  
trigger warnings about
returning a reference to a temporary.

(I'm assuming you meant "const x(*f)()" as the parameter of the  
forward function)

   Best, Jaakko