[C++-sig] Wrap a virtual function which returns a raw pointer

[Birgir Sigurjonsson]

Hi all.
How can I wrap a  virtual function which returns a raw pointer
which will be managed from C++.

I am embedding Python in my C++ application and
I have the follwoing classes that I want to wrap in boost.python:


In the code below the virtual function ALFactory::make() returns a pointer
to an abstract class AL which is also wrapped with boost.python.
I want to be able to override make() in python and in the
implementation of this function create and return a new python
subclass of my C++ AL. The lifetime of the created object is managed
by the ALFactory.


class ALFactory : private boost::noncopyable {
public:

   ALFactory(const std::string& key);
   virtual ~ALFactory();

   virtual  AL* make() = 0;

};

class AL : private boost::noncopyable {
public:
   virtual ~AL();
protected:
   AL();

};

################ Wrap classes
class ALWrap : public AL, public boost::python::wrapper<AL> {
public:
   ALWrap() : AL() {}
};

class ALFactoryWrap : public ALFactory, public boost::python::wrapper<ALFactory> {
public:
   ALFactoryWrap(const std::string& key) : ALFactory(key) {}
   AL* make() {
     return this->get_override("make")();
   }
};


############
BOOST_PYTHON_MODULE(import_framework) {
     class_<ALWrap, boost::noncopyable>("AL")
     ;

     class_<ALFactoryWrap, boost::noncopyable>("ALFactory", init<std::string>())
         .def("make",  pure_virtual(&ALFactory::make), return_value_policy<manage_new_object>())
     ;
}
###############

#### Python module code

from import_framework import AbstractLoader, AbstractLoaderFactory

class Ascii135Loader(AbstractLoader):
     def __init__(self):
         print self.__init__


class Ascii135LoaderFactory(AbstractLoaderFactory):
     def __init__(self, key):
         super(Ascii135LoaderFactory, self).__init__(key)
         print self.__init__

     def make(self):
         return Ascii135Loader()

register_this = Ascii135LoaderFactory("Ascii135")


#######################
				
I get the following error.

pure virtual method called

#########################

I have browsed the documentation and not found a clear answer.
I found the following quote on http://wiki.python.org/moin/boost.python/class
decribes my problem.


"There are several problems with the above arrangement, but the most important
one is that if you keep  the shared_ptr<U> alive longer than its corresponding
Python object, calls to the Python-overridable virtual functions will crash,
because they'll try to call through an invalid pointer."

How can I solve this problem?

Best regards,
Birgir Sigurjonsson.




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