If callin doesn't allow that, then iris callin is not a replacement of cache cpp.  So our c++ program will not be able to be upgraded with IRIS.

SIP provides C/C++ wrappers to the python code. Python is the driver but C/C++ is not. In other words, SIP just allows us to make C/C++ call from Python but does not make it work calling Python from C/C++. At least, I have not had any luck doing it.