Re: [cxx-abi-dev] trivial __dynamic_cast fails?

[Daniel Wallin <dalwan01@xxxxxxxxxxxxxx>]

David Baraff wrote:
> > David Baraff <deb@xxxxxxxxx> writes:
> >
> > > >
> > > David Abrahams wrote:
> > <snip>
> >
> > Incidentally, my system stores those offsets as an optimization for
> > all platforms that can handle it (which so far is everything) but if
> > you want to be strict about portable conformance you have to walk
> > through the cast graph.
> I do, so I end up calling the synthesized cast functions several times 
> in a single cast.
Maybe irrelevant but...

If you disregard strict conformance or are just interested in casting
up, it is possible to use the exception trick Dave A demonstrated in the
modified boost::any implementation to make the macro usage a little less
verbose and a bit more robust by omitting the bases.

  template<class T>
  T* cast_up(void* p, std::type_info const& type)
  {
      try
      {
          // will throw static_cast<U*>(p), where type = typeid(U)
          throw_registered_pointer(p, type);
      }
      catch (T* converted)
      {
          return converted;
      }
      catch (...)
      {
          return 0;
      }
  }

  // Now we just need to register the throw functions:
  struct A {};
  REGISTER_CLASS(A);

  struct B : A {};
  REGISTER_CLASS(B);

It should be possible to use the same trick to generate a cast graph
with byte offsets in the edges, if strict conformance isn't important.

--
Daniel Wallin