Geek Answers Handbook

Is using implicit conversion for upcast instead of QueryInterface () legal with multiple inheritance?

Suppose I have a class that implements two or more COM interfaces (exactly like here ):

class CMyClass : public IInterface1, public IInterface2 { 
};

QueryInterface() must return the same pointer for each request of the same interface (for the correct adjustment of the pointer, it needs an explicit upcast):

if( iid == __uuidof( IUnknown ) ) { 
    *ppv = static_cast<IInterface1*>( this );
    //call Addref(), return S_OK 
} else if( iid == __uuidof( IInterface1 ) ) {
    *ppv = static_cast<IInterface1*>( this );
    //call Addref(), return S_OK 
} else if( iid == __uuidof( IInterface2 ) ) {
    *ppv = static_cast<IInterface2*>( this );
    //call Addref(), return S_OK 
} else {
    *ppv = 0;
    return E_NOINTERFACE;
}

Now there are two in the object IUnknown- one of them is the base IInterface1, and the other is the base IInterface2. And they are in different subobjects .

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