Difference in memory layout between nested classes and multiple inheritance in C ++?

Question

I am trying to understand how COM points out the layout of its objects so that the client who wants to use the COM object knows how to do this.

I read that a COM object that implements several interfaces can do this in different ways, including using nested classes or multiple inheritance.

I understand that both methods would have to create the same memory layout (corresponding to the COM specification) so that a client who wants to use a COM object (for example, in C) knows how to do this.

So, my specific question is: is there a difference in the memory layout for C ++ objects implemented using multiple inheritance compared to nested classes.

And can someone tell me where the layout of the COM object is indicated?

Answer 1

COM is completely agnostic of the memory layout of your object. All he wants and needs is a table of function pointers when it calls IUnknown::QueryInterface(). How you implement it is completely up to you. MFC uses nested classes, almost anything uses the built-in support for multiple inheritance in the C ++ compiler. The implementation method of the MSVC ++ compiler is fully compatible with what COM requires. This is no coincidence. Use the template code that you see in books about COM that shows how to implement IUnknown correctly.

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