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

From: Fernando Cacciola (fcacciola_at_[hidden])
Date: 2001-07-25 08:19:33

----- Original Message -----
From: Greg Colvin <gcolvin_at_[hidden]>
To: <boost_at_[hidden]>
Sent: Tuesday, July 24, 2001 4:42 PM
Subject: Re: [boost] Re: downcasting smart pointers (suggested code)

> From: Fernando Cacciola <fcacciola_at_[hidden]>
> > From: Greg Colvin <gcolvin_at_[hidden]>
> > > From: Peter Dimov <pdimov_at_[hidden]>
> > > > From: <greg_at_[hidden]>
> > > > > This looks like a good start, thanks.  I think do_dynamic_cast is
the
> > > > > name we previously agreed on for your shared_dynamic_cast, and
that
> > > > > we wanted a version that worked for raw pointers as well.
> > > >
> > > > If do_dynamic_cast is supposed to be a generalization of
dynamic_cast:
> > > >
> > > > template<class R, class T> R do_dynamic_cast(T /* const */ &);
> > > >
> > > > (dynamic_cast<U*> returns U*, dynamic_cast<U&> returns U&)
> > > >
> > > > then dynamic-casting a shared_ptr would look like
> > > >
> > > > do_dynamic_cast<shared_ptr<X> >(y); // 1
> > > >
> > > > whereas with a shared_ptr-specific cast the syntax would be
> > > >
> > > > shared_dynamic_cast<X>(y); // 2
> > > >
> > > > I prefer (2). (1) is more generic but I can't see how the genericity
> > would
> > > > be exploited.
> > >
> > > It could be exploited in generic code that didn't care whether
> > > it was operating on raw pointers or smart pointers?
> >
> > Hmmm. I'm not sure if this is a desired behaviour.
> > It is well known that mixing shared_ptr<T> and T* can lead to unexpected
> > behavior. You have to do that carefully.
> > Just as I like not being able to cast a shared_ptr<T> to a T* (I call
get()
> > if I really need to), I don't think I'd like a common
> > interface to downcasting pointers.
> > Don't get me wrong, I do support genericity as mush as possible, but in
this
> > area I'm not sure if both types can be generalized this much.
>
> With the signatures Peter gives I don't you can use these to mix smart
> pointers and raw pointers.
Oh yes, I see. Good then.

> And of course to use these in generic code
> you would have to have typedefs for the pointer types to keep the same
> syntax:
>
>     template<typename BasePtr, typename DerivedPtr>
>     DerivedPtr whatever(BasePtr in) {
>        ...
>        DerivedPtr p = do_dynamic_cast<DerivedPtr>(in);
>        ...
>        return p;
>     }
>
> An example might be a system that could use raw pointers and agarbage
> collector, or else use smart pointers on systems where such a collector
> was unavailable or inappropriate.
>  I suppose for completeness one should
> have do_static_cast and do_const_cast as well.
>
I understand how can we use do_const_cast, but I'm not sure about
do_static_cast.

Fernando Cacciola
Sierra s.r.l.
fcacciola_at_[hidden]
www.gosierra.com