RE: [Boost-Users] Re: shared_ptr and inheritance

From: Björn Karlsson (bjorn.karlsson_at_[hidden])
Date: 2001-12-10 05:10:12

> From: rz0 [mailto:rz0_at_[hidden]]
> > >
> > >VC++ 6.0 flags the above lines with a conversion error.
> > >
> > >Is there any way to resolve this?
> > 
> > Can you just define b and c as boost::shared_ptr<A>?
> 
> Hi Jon
> 
> Thank you very much for the response.  Unfortunately this will cause 
> the same problem that I ran into with std::auto_ptr and that is 
> methods funcB and funcC which are exclusive to B and C are not 
> allowed to be called.
> 
> Take care
> RZ
> 

It seems to me that the problem is in the design - you're trying to use
inheritance for the convenience of placeholding (and maybe insulation). That
won't work (as you've seen). Even with a compiler that allows the code to
compile (those that support member templates will) you will not get the
behaviour that you want - funcB and funcC can never be called (from client
code). The reason that the code compiles cleanly is that the conversion from
B* or C* to A* is valid - but there's no way you can dynamically change the
static type of "boost::shared_ptr<A> aptr;". 

If you really need to get this to work, it is still possible: You could use
boost::any, and any_cast through all the possible types. This, however, is
not very flexible, and IMO not very readable or maintainable.

Regards,

Bjorn Karlsson

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