Python object to C++ pointer problem

Question

this is my first post :). I could convert a python extended object into a C++ pointer, but I have a problem. First I will show you my code and then I will explain the problem.

This is my class:

#include <boost/python.hpp>


using namespace boost::python;


class Base
{

public:

    virtual const char* HelloWorld() = 0;

};


class BaseWrapper : public Base, public wrapper<BaseWrapper>
{

public:

    virtual const char* HelloWorld()
    {

        if (override f = this->get_override("HelloWorld"))
            return call<const char*>(f.ptr());

        return "FAILED TO CALL";

    }

};

Boost wrapping:

BOOST_PYTHON_MODULE(hello_ext)
{

     class_<Base, boost::noncopyable>("Base", no_init);

     class_<BaseWrapper, bases<Base> >("BaseWrapper")
         .def("HelloWorld", &BaseWrapper::HelloWorld);

}

The Python code (hello.py):

def NewDerived():
    import hello_ext

    class Derived(hello_ext.BaseWrapper):
        def __init__(self):
                super(Derived, self).__init__()
        def HelloWorld(self):
                return "This is a Hello World!!!"

    return Derived()

and the main file:

int main() 
{

     // Start the interpreter.
     Py_Initialize();

     // Import the module that we need (hello.py)
     object module = import("hello");

     // Get a C++ pointer of the derived python class.
     Base* base = extract< Base* >( module.attr("NewDerived")() );

     // Call the HelloWorld function
     std::cout << base->HelloWorld() << std::endl;

}

When I run my application I can see at the screen "This is a Hello World!!!" as I expected. So, what is the problem??? Suppose I change the python code to:

def NewDerived():
    import hello_ext

    class Derived(hello_ext.BaseWrapper):
        def __init__(self):
                super(Derived, self).__init__()
        def HelloWorld(self):
                return "This is a Hello" # I CHANGED THIS LINE!!!!

    return Derived()

Then, when I run my application again, it crashes, because I got an error in the line:

std::cout << base->HelloWorld() << std::endl;

because base is NULL.

More precisely, the error is "Access violation reading location 0xblablabla". When I debug, the debugguer stops at the function (Boost or Python code, I think)

inline api::object_base::~object_base()
{
    Py_DECREF(m_ptr);
}

What do you think???

score 2 · Answer 1 · answered Jan 19 '11 at 13:21

Finally, another programmer explained me the solution.

I don't know why it worked originally, but the problem is that the object is getting destroyed before I try calling the member function. I need to break the extract call into two parts like so:

object derived = module.attr("NewDerived")();
Base* base = extract< Base* >( derived );

This will keep the object around long enough for me to actually call functions on it.

Python object to C++ pointer problem

1 Answers1