[boost] Re: Perfect Forwarding (almost)

From: David Abrahams (dave_at_[hidden])
Date: 2004-02-08 11:39:51

Brian McNamara <lorgon_at_[hidden]> writes:

> It seems you have a clever way to effectively "detect rvalue-ness"; the
> copy-constructor and the "pseudo-copy-constructor" (the thing with the
> enable_if) only swallow lvalues, whereas rvalues find a more attractive
> construction path via "ref".  Is that a correct summary?

I think so.

>> 12.2/1 says that the act of binding an rvalue to a (const) reference
>> creates a temporary (which may be elided).  The creation of the
>> temporary requires the existence of a copy ctor with a const&
>> argument.  As far as I know, that rule serves no useful purpose, but I
>> may have missed something.  
>
> I am not sure what portion of the rule you don't like.  Is it the fact
> that they point to a particular copy constructor (the const& one),
> rather than just any available/visible/something copy constructor?

No, it's the fact that any additional temporary at all (and thus, a
copy ctor) is needed.

> In test10, what do you 'want' to happen here:
>     sink2( <a temporary object, that is, an rvalue> );

It should bind a const reference to the temporary without complaining
that the temporary can't be copied.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com