Re: [boost] Publishing results

From: Peter Dimov (pdimov_at_[hidden])
Date: 2001-05-30 06:48:39

From: "John Max Skaller" <skaller_at_[hidden]>

> > > 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; };

An interesting debate. This:

#include <boost/smart_ptr.hpp>

using boost::shared_ptr;

template<class T1, class T2> struct X
{
 shared_ptr< X<T2,T1> > ptr;
};

int main()
{
 X<int, long> x;
 return 0;
}

compiles with g++ 2.95.2. I don't know about .3.

If we get back to your original example:

class X;
class Y { shared_ptr<X> p; };

I believe that a compiler is within its rights to reject the code. Members
of class templates should not be instantiated unless used; however, Y is not
itself a template, so the compiler _may_ legitimately generate Y::~Y() at
the end of Y's definition, which in turn uses
shared_ptr<X>::~shared_ptr<X>().

--
Peter Dimov
Multi Media Ltd.