Re: [boost] Perfect Forwarding (almost)

From: Brian McNamara (lorgon_at_[hidden])
Date: 2004-02-07 00:59:51

On Fri, Feb 06, 2004 at 07:00:49PM -0500, David Abrahams wrote:
> The example below shows a technique for perfect forwarding that seems
> to work with GCC 3.3.1.  rvalues are always moved automatically, and
> lvalues are copied.

I had to Google for "perfect forwarding" to discover
   http://std.dkuug.dk/jtc1/sc22/wg21/docs/papers/2002/n1385.htm
   http://std.dkuug.dk/jtc1/sc22/wg21/docs/papers/2002/n1377.htm
(which I'd read before but forgotten about).

It is unclear to me if/how your code solves the "perfect forwarding"
problem.  Instead it appears to show a way to make a particular type
"movable".  Have I misunderstood?  (How) does your generalize to a way
to write forwarding functions?

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?

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

Overall it looks clever, but I don't yet understand exactly what it
solves or the details of the 12.2/1 thing.

-- 
-Brian McNamara (lorgon_at_[hidden])