[C++-sig] [pygccxml-development] Parameter passing, ownership semantics

[Gustavo Carneiro]

On 2/20/07, Gustavo Carneiro <gjcarneiro at gmail.com> wrote:
>
> On 2/19/07, Roman Yakovenko <roman.yakovenko at gmail.com> wrote:
> >
> > Hi. Unfortunately I reproduced the error and was not able to  fix it
> > :-(((((.
> > There is something I don't understand  and I think  this is
> > Boost.Python bug.
> >
> > I do made some progress.
> >
> > 1. When you derive Python class from a C++ one you have to define
> > __init__
> >     method, otherwise your code will not work:
> >
> > class MyEvent(EventImpl):
> >     def __init__( self ):
> >         EventImpl.__init__( self )
> >
> >     def Notify(self):
> >         print "Notify!"
> >
> > 2. I did small research for you. Read this post:
> >    http://aspn.activestate.com/ASPN/Mail/Message/cpp-sig/1331901
> >    and take a look on Boost.Python unit tests:
> >    http://boost.cvs.sourceforge.net/boost/boost/libs/python/test/auto_ptr.cpp?view=markup
> >
> >    http://boost.cvs.sourceforge.net/boost/boost/libs/python/test/auto_ptr.py?view=markup
> >
> >
> > What I found is:
> > The ownership is really transfered. You can check this using simple
> > technique:
> >         Add new function to EventImpl:
> >           virtual std::string class_name() const { return "EventImpl"; }
> >         Add free function:
> >            std::string get_class_name( const std::auto_ptr<EventImpl>& e
> > ){
> >                if( e.get() ){
> >                    return e->class_name();
> >                }
> >                else{
> >                   return "no object";
> >                }
> >            }
> >         You will get the answer "no object", after "Schedule" call.
> > So, may be you don't have to delete the Python object.
> >
> > I will submit the bug to the Boost.Python mailing list, hope somebody
> > will be able to help.
>
>
> I found this wiki with a possible solution for the problem:
>
> http://wiki.python.org/moin/boost.python/HowTo#head-927c9493ac51c81b3f2484e486d85567ff316c8a
>
>

  Good news.

  That wiki entry didn't apply anymore probably due to changing
boost::python interfaces, but it provided very useful clues.  I managed to
get it working 100% correctly using another hack.  Here's the diff of the
manual changes I made to the wrapper:

--- ns3.cpp     2007-02-21 23:46:44.000000000 +0000
+++ ns3-working.cpp     2007-02-21 23:44:34.000000000 +0000
@@ -9,19 +9,32 @@
 namespace bp = boost::python;

 struct EventImpl_wrapper : ns3::EventImpl, bp::wrapper< ns3::EventImpl > {
+    ~EventImpl_wrapper() {
+        if (this->pyobj) {
+            Py_DECREF(this->pyobj);
+            this->pyobj = 0;
+        }
+
+    }

     EventImpl_wrapper()
     : ns3::EventImpl()
-      , bp::wrapper< ns3::EventImpl >(){
-        // null constructor
+      , bp::wrapper< ns3::EventImpl > (), pyobj(0) {

     }

     virtual void Notify(  ){
+        if (!this->pyobj) {
+            this->pyobj = bp::detail::wrapper_base_::get_owner(*this);
+            Py_INCREF(this->pyobj);
+        }
         bp::override func_Notify = this->get_override( "Notify" );
         func_Notify(  );
     }

+protected:
+    PyObject *pyobj;
+
 };

 static void Schedule_b8544467c482930a621aca2e7ac87dca( std::auto_ptr<
::ns3::EventImpl > event ){

  Now my question, especially to any boost_python maintainers out there, can
you please do something about this?  I shouldn't have to make these kinds of
hacks to get it working; any chance this INCREF/DECREF of the PyObject can
make it upstream to the standard boost distribution?

-- 
Gustavo J. A. M. Carneiro
"The universe is always one step beyond logic."
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