Easy one. C++ is much more strict about types and doesn't allow implicit casting to (void *).
As DoMethod() expect "Object *" as object, either define your MUI objects as Object * instead of APTR, or cast them to (Object*) in DoMethod() call.
Having compiled mui classes for OWB-MorphOS in C++, I can safely say casts are the main difference to C. Can't remember any other issue so far.
