Thread: Ambiguity issue

Hi CornedBee,


Do you mean in COM, except downcast, other casts are not allowed, it is for COM provider developer for for COM client developer?

If you mean for COM client developer, I can understand.

If you mean for COM provider developer, I do not agree. You can see in COM aggregation model, in the inner component, we will use reinterpret_cast to convert unmanged unknown interface to IUnknown interface.

Originally Posted by CornedBee

Because the cast relies on knowledge you're not supposed to have.

regards,
George

Hi CornedBee,


1.

Originally Posted by CornedBee

The client developer.

Client developer only need to invoke QueryInterface. Why need to manually downcast manually?

2.

Originally Posted by CornedBee

I see no reason for any COM implementation ever to use reinterpret_cast.

I have attached the code for aggregation model from Inside COM book about how reinterpret_cast is used. Please refer to,

CB::NondelegatingQueryInterface (CMPNT2.cpp)


regards,
George

Hi CornedBee,


1.

In my knowledge of implementation of aggregation model, since a class can not give two different implementation of an interface, we have to name unmanaged IUnknown interface to some other value, like INondelegatingUnknown in this case, and using the same function table of interface IUnknown (QueryInterface/AddRef/Release).

To get the actual unmanaged interface (INondelegatingUnknown) as IUnknown interface, the only way we could do is,

IUnknown *pI = reinterpret_cast<IUnknown*>(static_cast<INondelega tingUnknown*>this);

If you have found better way, it is appreciated if you could share here.

Originally Posted by CornedBee

The cast there is
1) unnecessary (static_cast would suffice)
2) unnecessary (you could just cache the pointer)
3) unnecessary (the pointer is a copy of this anyway).

Well, except for the whole fake IUnknown business. Now that is ugly ...
I can't believe that's the best way to implement aggregation.

2.

Originally Posted by CornedBee

Not saying he needs to. Just that it's the only cast allowed.

Downcast means cast from base to derived, upcast means cast from derived to base. QueryInterface will always cast this to sub-objects, so I think it should be upcast. Right?


regards,
George

Thanks all the same, CornedBee!


When you happen to find in the future about both upcast/downcast samples in QueryInterface, please let me know here. :-)

Originally Posted by CornedBee

Could well be. I can never remember which is which.

regards,
George