Re: [boost] Re: downcasting smart pointers (suggested code)

From: Greg Colvin (gcolvin_at_[hidden])
Date: 2001-07-25 09:40:31

From: Fernando Cacciola <fcacciola_at_[hidden]>
> 
> ----- Original Message -----
> From: Peter Dimov <pdimov_at_[hidden]>
> To: <boost_at_[hidden]>
> Sent: Tuesday, July 24, 2001 5:24 PM
> Subject: Re: [boost] Re: downcasting smart pointers (suggested code)
> 
> 
> > From: "Greg Colvin" <gcolvin_at_[hidden]>
> > [...]
> >
> > >     template<typename BasePtr, typename DerivedPtr>
> > >     DerivedPtr whatever(BasePtr in) {
> > >        ...
> > >        DerivedPtr p = do_dynamic_cast<DerivedPtr>(in);
> > >        ...
> > >        return p;
> > >     }
> >
> > Or something like
> >
> > template<class Ptr> void whatever(Ptr p)
> > {
> >    if(IDrawable q = do_dynamic_cast<IDrawable>(p)) q->draw();
> > }
> >
> > where IDrawable is typedef'ed somewhere to be, say, shared_ptr<Drawable>.
> >
> > (As an aside, people like to implement their own COM-like component
> systems;
> > it would be great if shared_ptr<> can be used for this. The two main
> > obstacles are (1) no 'queryInterface' support - this would hopefully be
> > addressed by do_dynamic_cast - and (2) a shared_ptr<> created in a DLL and
> > destroyed in the main program causes problems when different heaps are
> > used.)
> >
> shared_ptr<> is a non-intrusive rc smart pointer. It means that the
> reference count is held in the smart pointer itself, not the object.
> In COM-like systems, the reference counter is held by the object itself.
> The problem of using shared_ptr<> with objects that mantain their own
> reference count is that there are two different, unsyncronized counts
> managing the same life time. This creates a runtime mess.
> 
> It might be useful if shared_ptr<> could be either non-intrusive/intrusive,
> depending on the interface supported by the contained object.
> That is:
> 
> struct RefCountBase {  void AddRef() ; etc... /* details ommited*/. } ;
> 
> struct IFoo : public RefCountBase { ... } ;
> 
> class Fred {} ;
> 
> typedef shared_ptr<IFoo> ; // intrusive
> typedef shared_ptr<Fred> ; // non-intrusive

Are you suggesting that shared_ptr be specialized for RefCountBase?