[C++-sig] converting objects to python..

[Dominique Devriese]

David Abrahams writes:

  David> Dominique Devriese
  David> <dominique.devriese at student.kuleuven.ac.be> writes:
  >> Hi,
  >>
  >> I have been looking at this for some time, and I can't seem to
  >> find it.  Maybe someone here can help ?
  >>
  >> (I'm using Boost.python to export the api of an app that i want
  >> to embed python in.. ( http://edu.kde.org/kig in case someone is
  >> interested ) )
  >>
  >> I'm trying to call a python function from C++ code with some
  >> arguments.  The arguments are of type "const ObjectImp*", where
  >> ObjectImp is the abstract base class of a whole lot of other
  >> types.  All of them are properly wrapped, and I have tested the
  >> wrapping in different situations, and it works.  The problem is
  >> that I can't seem to find how to convert a "const ObjectImp*" to
  >> a PyObject.  I tried:
  >>
  >> const ObjectImp* the_object_imp_pointer; object o(
  >> the_object_imp_pointer ); use( o.ptr() ); object o2(
  >> *the_object_imp_pointer ); use( o2.ptr() );
  >>
  >> ( and a whole lot of other things that would only make me look
  >> stupid here, so i'll omit those ;) )
  >>
  >> In both cases, the object constructor throws an error_already_set
  >> exception

  David> What's the Python exception behind it?

No idea, any idea where i can find this ?

  >> and from the debugging in boost.python internals I've done, it
  >> appears that this happens because the lib doesn't have a
  >> converter registered for the types.
  >>
  >> I thought about trying to write a converter myself, but then I
  >> thought that this would be stupid, since I wouldn't know how to
  >> do this, unless i found out how boost.python PyObject's work, and
  >> it seemed more intelligent to just ask here first ;)
  >>
  >> At one point, I also thought that the problem would be that i
  >> should first downcast the pointers to their "real" type, and try
  >> to convert that, but then I thought that would not be the case,
  >> since you're using typeid() in the conversion stuff, and that
  >> only takes into account the "real" type, not the formal type, so
  >> downcasting is not necessary..
  >>
  >> Anyway, I'm kinda stuck on this..  Could you give me some
  >> pointers on where to look ?

  David> http://www.boost.org/libs/python/doc/v2/ptr.html

  David> Consider:

  David>    object o( ptr(the_object_imp_pointer) );

  David> Equivalent to:

  David>    object o( ref(*the_object_imp_pointer) );

  David> But beware dangling reference in Python.

  David>    object o( *the_object_imp_pointer );

  David> is safer, but incurs a copy (and slicing) of your base class.

  David> Using boost::shared_ptr instead of raw pointers works best of
  David> all if it's an option.

great, thanks for the information..
cheers
domi

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