When implementing several COM interfaces at once how do I upcast to IUnknown?

Question

Suppose my COM object implements two or more COM interfaces:

class CMyClass : public IPersistFile, public IPersistStream {
};

when implementing

Answer 1

Mark Ransom already gave the correct answer - any will do, as long as it's consistent - but picking the first one has one minor advantage. Due to layout rules, the IUnknown* of the first interface will point to the start of the object. Any other IUnknown* will point to subsequent vtable pointers elsewhere in the object. For debugging purposes, it's very useful to know where ano object begins in memory.