Unit-testing functions with user-defined types

[terminalXXX]

Thank you Paul. It was just an example of the situation I am in right now.

The requirement we have right now is to upgrade the current library (previous team wrote it). Previous authors didn't include unit-tests in each of the methods they wrote to export as those of Dll's. I am very confused about mock, fake, stub etc because I don't know how to mock a CComPtr type argument that is used in many places in functions in the dlls.
Could you provide me an simple example of mock a CComPtr (COM pointer type) and type that is user-defined, the more complex, the better ? Thank you.