[C++-sig] wrapping a to_python_converter

[Andreas Beyer]

Assume I have a class A and a shared pointer to it A_ptr. BP converts 
A_ptr correctly to python after regestring the pointer:
BOOST_PYTHON_MODULE(ptr_test) {
   ...
   python::register_ptr_to_python< A::A_ptr >();
   ...
}

Now, I want to call a function f() each time a pointer of type A_ptr is 
returned to python. The function f() does not modify A, nor does it 
change the way A's should be converted to python. In fact, f() only 
registers the pointer (see below).
I guess I would somehow have to wrap the existing converter. Is that the 
correct idea?
But "where" is this converter? (Thus, how/where does the 
register_ptr_to_python template create and register the actual 
converter?) Does anybody have an example of such wrapped 
to_python_converter?
The questions may sound a bit stupid, but I never have done anything 
low-level with BP. Specifically I never wrote a to_python_converter.

In the concrete example I want to call this function before returning A_ptr:

template <class T>
void hold_python(shared_ptr<T>& x)
{
      x = python::extract<shared_ptr<T> >( python::object(x) );
}

Here python::object(x) will call the to_python_converter. Thus, if 
hold_python() would be called from within a to_python_converter I would 
have to avoid recursion, right? I guess hold_python() must explicitely 
call the standard to_python_converter. How do I do that?

Thanks!!
    Andreas