[C++-sig] two question from boost.python

[simon zhang]

The first question:
The class_ class has four parameters,class template class_<T, Bases,
HeldType, NonCopyable>.
I don't understand the parameters of Bases and HeldType.How to use the
parameters.
this doc: http://www.boost.org/doc/libs/1_51_0/libs/python/doc/v2/class.html


The second question:
I have to make a python function as callback of a c++ function.How to
do?Where can I find some examples??