[cxx-abi-dev] trivial __dynamic_cast fails?

[Daniel Wallin]

David Baraff wrote:
>> David Baraff <deb at pixar.com> 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