Why do we need “this pointer adjustor thunk”?

Question

I read about adjustor thunk from here. Here\'s some quotation:

Now, there is only one QueryInterface method, but there are two entries, one for ea

Answer 1

I think that it is important to point that in C++ there is no such entity as "interface pointer" or anything close to that. It is an idiom at best built on the concept of a restricted abstract class but still remaining a class. As a such all rules applying to class members and handling 'this' still apply unchanged. So principally an interface class must behave like stand-alone classes of given type regardless of their function and eventual inheritance hierarchy.

We can use the virtual method call mechanism to get to the actual (dynamic type) of the object exposed by an (interface) base class. How it is done is implementation specific including such concepts like the Virtual Method Table and "adjustor thunks". Generally compiler may use its initial 'this' pointer to locate the VMT and then the actual implementation of a given function and call it with eventual adjustment of the 'this' pointer. The thunk adjustment generally is needed to perform the final call if the memory layout of the base class is different than a derived one to which reference we hold like in the case of multiple inheritance.