Re: C++ Design question

This is the idea:
//better ways to do this but this is the essentials
//same as COM Interface IID
// better and faster than strings
enum{base1ID ,base2ID ,base2ID };
template< class T>struct interface2id;

struct iBase2{
   virtual tr1::shared_ptr<iBase2> GetInterface(int ,iBase2**) const=0;

//other stuff for internals
//add_ref, release, add_ref_copy, weak_ref, weak_release, etc
};
struct base1: iBase2{};
struct base2: iBase2{};
struct base3: iBase2{};
//a few helpers
template< class base1> struct interface2id{static const int id=base1ID
};
template< class base2> struct interface2id{static const int id=base2ID
};
template< class base3> struct interface2id{static const int id=base3ID
};

class myClass: public base1, public base2, public base3
,mystuff::enable_shared_from_this<myClass>//has counter mechanism
inside
{
                 tr1::shared_ptr<iBase2>
                   GetInterface( int arg,iBase2**_p)const{
                     //*_p ==0 precondition
                     switch (arg){
                           case interface2id<base1>::id;
                           case interface2id<base2>::id;
                           case interface2id<base3>::id;
                            *_p = this;
                            break;
                    }
                    return shared_from_this();

                }
};
template <class To>mystuff::shared_ptr<To>
interface_cast3( mystuff::shared_ptr<iBase2> const& _r ){
    iBase2*_a=0;
   //hold a pointer to the object while we construct
// a new pointer (needed for MT)
   mystuff::shared_ptr<iBase2> b=
          _r->GetInterface(interface2id<To>::id,&_a);
   //THIS ONLY WORKS WITH INTRUSIVE REFERENCE COUNTING
   //NOT STANDARD
    if (_a) return mystuff::shared_ptr< To>(static_cast<To>(_a));
    return mystuff::shared_ptr<To>();//empty
}

This would probably work with boost::instrusive_ptr, if you dont need
weak_ptrs support (probably not)

There are even more methods, however, this should be enough to chew on
for a while.

Hope this helps