Re: [boost] Re: Perfect Forwarding (almost)

From: Brian McNamara (lorgon_at_[hidden])
Date: 2004-02-08 15:20:30

On Sun, Feb 08, 2004 at 03:07:05PM -0500, David Abrahams wrote:
> Brian McNamara <lorgon_at_[hidden]> writes:
> > On Sun, Feb 08, 2004 at 11:39:51AM -0500, David Abrahams wrote:
> >> Brian McNamara <lorgon_at_[hidden]> writes:
> >> > David Abrahams writes: 
> >> >> 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.
> >
> > I see.  What about
> >
> >    const X& x = source();    // 12.2/5
> >
> > ?  Or something.  
> 
> That's exactly the case I'm talking about.
> 
> > I am trying to find the 'hard' cases where the
> > compiler might not be able to easily make the temporary have the right
> > 'lifetime', and thus require an extra copy.
> 
> I don't see any problem here.  There's no problem getting the rvalue
> out of source(), or sink(source()) would fail.  Furthermore, no

I think we're on the same page, but to clarify--this case
   const X& x = source();    // 12.2/5
seems harder than this one
   sink(source());
to me, since in the former, the "temporary" object must have a lifetime
that outlasts the full expression that created it.

> compiler I've ever seen will actually make the allowed temporary in
> that case.

But if that's true, then I agree with you that I don't see "the point of
the rule", so go lobby to change it and be the hero of many.  :)

-- 
-Brian McNamara (lorgon_at_[hidden])