Re: [GD-General] Multiple Inheritance and RTTI

[brian s. <pud...@po...>]

I think that might be too much of a simplification.  The ability to 
dynamic_cast implies object identity.  If I can dynamic_cast from 
InterfaceA * to InterfaceB *, I know that the pointer I'm holding 
points to an object that is both an InterfaceA * and a InterfaceB *.

QueryInterface doesn't mandate that.  If I query for InterfaceB* from 
an InterfaceA * pointer, I might get back a pointer that points to the 
same object as the InterfaceA *, like dynamic_cast would return.  But I 
could very well get back a pointer to a completely different object; 
the object pointed to by InterfaceA * could just as well return a 
pointer to a InterfaceB* member (see my previous post re: aggregation).

Why might I want to do that?  A few reasons:

1) I can inherit the behavior of an InterfaceB implementation without 
having that implementation exposed to me at all.  All I need is the 
definition of InterfaceB and a factory function that can create an 
object to implement InterfaceB for me.

2) I might want to delay creating an InterfaceB object until it's 
needed, if I know that it's heavyweight.

All that COM requires of QueryInterface is (1) consistency: if you can 
get an interface once, you can always get it and (2) identity: if you 
can get an InterfaceB * from an InterfaceA *, you can go back to the 
InterfaceA * from the InterfaceB *.  dynamic_cast goes further and 
implies an "is-a" relationship between InterfaceA and InterfaceB.

--brian

On Thursday, February 6, 2003, at 06:12  AM, Thatcher Ulrich wrote:

> To try to cut through the terminology a little:
>
> COM == RTTI