[C++-sig] Re: creating an instance of a class_<> object in c++ and exporting it to python

Francois Ostiguy ostiguy at fnal.gov
Mon Aug 16 19:21:38 CEST 2004

Thank You Dave, Max and Chez for your insights.

I am getting closer to my goal, but it is not quite working yet.
In the process I am slowly gaining more understanding of the way
boost.python works. I have to say that I never cease to be amazed
at how carefully crafted boost.python is.
It is truly a remarkable piece of work.

Back to my problem:

Last Friday, following Max and Dave's suggestions I tried (in simplified

class A;

class_<A, A*, boost::noncopyable> PythonTypeA;

A* get_existing() { return aptr; }


A* aptr = new A();

object py_get_existing =
 make_function( &get_existing,
                return_value_policy<reference_existing_object>()  );

object PythonInstanceOfA  = PythonTypeA(  py_get_existing() );


Another poster pointed out that make_function is a factory for python
objects and therefore the last line was incorrect. Dave added that it was

When I remove this last line, everything compiles properly, but of
course when I print the "py_get_existing" object in python I get

<Boost.Python.function object at 0x834cb28>;

which is not a wrapped class A object.

After thinking about this a bit, it occured to me that what I needed was
more along the lines of

object PythonInstanceOfA = PythonTypeA( aptr ); ///

If I understand correctly, this should instantiate an object using
the automatic converter defined by the extended python type
class_<A, A*, boost::noncopyable> PythonTypeA.
The code compiles without a hitch, but I get a core dump
when the statement is executed. I am still
investigating what might be going on (*** see note below ***) ).

The approach suggested by Chez (chezdique AT yahoo.com) appears more
promising. I have yet to try his code, but a cursory look gives me the
impression that what he is doing is not fundamentally not different from
what I just described. He defines an an empty ObjectWrapper<T> and a
correcponding get_ptr() function. Once again, correct me if I am wrong,
but this appers superflous since T& and T* converters should automatically
be generated by boost.python. An ObjectWrapper class would be useful to
support some kind of smart pointers.

I apologize for the lengthy post.  I hope this is not too confusing
and I would greatly appreciate more comments/feedback.


Dr. Jean-Francois OSTIGUY                              voice: (630) 840-2231
Beam Physics Dept MS220                                  FAX: (630) 840-6039
Fermi National Accelerator Laboratory                email: ostiguy at fnal.gov
Batavia IL 60510-0500                           WWW:www-ap.fnal.gov/~ostiguy

*** note: one complication is that I want to keep all the
    extended python type definitions together. This means that somehow the
    PythonTypeA object has to be global. Since there are typically
    a large number of types declared in each modules, the simplest
    thing seemed to use a global
    class_<A, A*, boost::noncopyable>* PythonTypeAPtr
    and set PythonTypeAPtr = &PythonTypeA.

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