CComVariant

[George2]

Hello everyone,


In the book ATL Internals, here is the description for CComVariant. My questions,

--------------------
COM permits an object to hand out different binary values each time a client queries it for a specific interface pointer (with the exception of a query for the IUnknown interface). Therefore, two VARIANTs containing IDispatch pointers referencing the same object might not compare for equality.
--------------------

1.

I think it means even if the pointer binary values are different, they may still be pointed to the same object? Right?

I think it happens only in the situation of multiple inheritance, and in single inheritance, there is no such issue. Right?

2.

I think the above statement describes the following scenario, in multiple inheritance, suppose we have interface A inherits from IDispose, and interface B inherits from IDispose, and have inteface C inherits from both A and B.

The queryInterface for A and B from interface C will return different result.


thanks in advance,
George

[iMalc]

Hello everyone,


In the book ATL Internals, here is the description for CComVariant. My questions,

--------------------
COM permits an object to hand out different binary values each time a client queries it for a specific interface pointer (with the exception of a query for the IUnknown interface). Therefore, two VARIANTs containing IDispatch pointers referencing the same object might not compare for equality.
--------------------

1.

I think it means even if the pointer binary values are different, they may still be pointed to the same object? Right?

I think it happens only in the situation of multiple inheritance, and in single inheritance, there is no such issue. Right?

2.

I think the above statement describes the following scenario, in multiple inheritance, suppose we have interface A inherits from IDispose, and interface B inherits from IDispose, and have inteface C inherits from both A and B.

The queryInterface for A and B from interface C will return different result.

Yep, you got it. One pointer might be to the A interface and the other might be to the B interface, and A != B. But the IUnknown interface can be obtained form either A of B and these will both be equal.
CComPtr::IsEqualObject comes in handy here!

[George2]

Thanks iMalc,


You mean C multi-inherits from A and B in your below description?

Yep, you got it. One pointer might be to the A interface and the other might be to the B interface, and A != B. But the IUnknown interface can be obtained form either A of B and these will both be equal.
CComPtr::IsEqualObject comes in handy here!

regards,
George

[iMalc]

You mean C multi-inherits from A and B in your below description?

Yes. It's very common in COM to inherrit (hence implement) multiple interfaces. I've worked with code that implemented over a dozen interfaces.

[George2]

Thanks iMalc,


Question answered.

Yes. It's very common in COM to inherrit (hence implement) multiple interfaces. I've worked with code that implemented over a dozen interfaces.

regards,
George