Re: [boost] Publishing results

From: Ross Smith (ross.s_at_[hidden])
Date: 2001-05-29 23:58:40

John Max Skaller wrote:
> 
> Ross Smith wrote:
> 
> > That would have helped, yes, but it wasn't the main problem. Consider a
> > slightly elaborated version of the above:
> >
> >     struct Y;
> >     struct X {
> >       Y* y;
> >       ~X() { delete y; } // ***
> >     };
> >     struct Y {
> >       X* x;
> >       ~Y() { delete x; }
> >     };
> >
> > That won't work, of course; the compiler rejects the line marked ***.
> 
>         Yes. But these are hand coded classes, and you can ..
> 
> > It will work if you put the body of ~X() out of line.
> 
>         But that solution is NOT available for templates.

Huh? X isn't a template. I don't see your point.

>         The problem isn't smart pointers per se, the problem
> is template instantiation.

In that case I must have completely missed your point. I thought you
were complaining about not being able to instantiate a smart pointer on
an incomplete type. If you were, I don't think that's a reasonable thing
to expect to be able to do. If you weren't, then I honestly have
absolutely no idea what problem you're talking about. Could you explain
further please? (Preferably with concrete examples; fragments with
uncertain context don't seem to be getting us anywhere.)

> > > But the example is a special case: just consider:
> > >
> > >         template<class T1, class T2>
> > >         struct X {
> > >                 smart_ptr<X<T2,T1> > ptr;
> > >         };
> > >
> > > to see that it isn't always possible to forward declare things.
> >
> > What's the problem there?
> 
>         The problem is that there is NO ordering
> of template instances for which lookup in 'expanded'
> classes can work:
> 
>         struct X<A,B> { smart_ptr<X<B,A> > ptr; };
> 
>         Woops! We cannot lookup X<B,A>, it isn't
> instantiated till after. With GCC, if smart_ptr
> contains
> 
>         p->incr()
> 
> g++ tries to lookup 'incr' .. and it can't find
> struct X<B,A> to lookup into. What should happen
> is that both 'struct X<A,B>' and 'struct X<B,A>'
> are layed down first, and THEN, if necessary,
> method bodies layed out. g++ incorrectly
> expands inline method bodies too early.

Could you please give an example of an actual complete program (not
fragment) that you claim should work but doesn't?

-- 
Ross Smith <ross.s_at_[hidden]> The Internet Group, Auckland, New Zealand
========================================================================
        "Hungarian notation is the tactical nuclear weapon of
         source code obfuscation techniques." -- Roedy Green