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2 COM objects and using interface from one of it in another

Aug 10, 2017 See the question and my original answer on StackOverflow

You can use the idl import directive. The Windows SDK .idl files is full of these (like import "unknwn.idl";). So, in processor.idl:

import "face.idl";  

[
    object,
    uuid(6E0537CC-232A-4E73-A625-358591CA9231),
    pointer_default(unique)
]
interface IProcessor
{
    HRESULT ProcessFace(IFace *pFace);
};

Of course, this will need the .idl files (and generated .h and various artifacts, if any, for compilation phase) to be present in accessible directories.