boost help: C++ object from PyObject pointer

[Mark Higgins]

I'm just getting started with boost & python, so please excuse my 
ignorance... :)

I've built a python extension from a C++ class using boost - it all
seems to work fine.

However, now another C++ function is receiving a pointer to a PyObject
that represents an instance of this "boosted" python class. How do I
pick off a pointer to the underlying C++ object from the PyObject
pointer?