Re: CComVariant

"Igor Tandetnik" <>
Fri, 4 Apr 2008 11:17:13 -0400
George <> wrote:

In your C++ code, you can write a class that derives from two or more
COM interfaces (and often do, for implementation purposes). But this
class itself is not, by definition, a COM interface, since its binary
layout doesn't match the COM specification.

Why it is not? Any COM interface inherits from IUnknown, and I think
any derived class/interface from any COM interface should maintain
the same layout -- i.e. the first 3 methods are

So suppose I have these interfaces:

interface IA : IUnknown {
    HRESULT MethodA();

interface IB : IUnknown {
    HRESULT MethodB();

interface IC : IA, IB {
    HRESULT MethodC();

What should IC's vtable look like, in your opinion? List all methods in
the order they should appear. Note that the following should work:

IC* pC = ...; // initialized somehow
IA* pA = pC;
IB* pB = pC;

pA and pB should point to vtables compatible with IA and IB,
With best wishes,
    Igor Tandetnik

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