Boost Python: Fails to call a C++ virtual function, which is overridden in a Python derived class

Question

I have a Python class, which is derived from a C++ class and overrides part of the functions. A C++ function receives an object of the Python class and stores it in a vector, so the Python class should not be collected by the GC. I tried two different approaches, but first one doesn't compile and second doesn't see the overrides at run time (the functions of C++ are called instead of the Python class functions). Please tell me what is the right way to do this.

The C++ code:

class MyClass
{
public:
    virtual void PureVirtual(int i) = 0;
    virtual const char* VirtualWithDefaultImpl()
    {
        return "MyClass";
    }
};

void MyFnc(boost::shared_ptr<MyClass> obj)
{
    myVector.push_back(obj);
    std::cout << obj->VirtualWithDefaultImpl() << std::endl;
    obj->PureVirtual(0);
}

The python wrapper:

class MyClassWrap : public MyClass, wrapper<MyClass>
{
public:
    MyClassWrap(PyObject* self) : m_self(self)
    {
        Py_INCREF(self);
    }
    MyClassWrap(PyObject* self, const MyClass& other) : MyClass(other), m_self(self)
    {
        Py_INCREF(self);
    }
    ~MyClassWrap()
    {
        Py_DECREF(m_self);
    }
    virtual void PureVirtual(int i)
    {
        this->get_override("PureVirtual")(i);
    }
    virtual const char* VirtualWithDefaultImpl()
    {
        if (override f = this->get_override("VirtualWithDefaultImpl"))
            return f();
        return MyClass::VirtualWithDefaultImpl();
    }
    const char* DefaultVirtualWithDefaultImpl()
    {
        return this->MyClass::VirtualWithDefaultImpl();
    }
private:
    PyObject* m_self;
};

BOOST_PYTHON_MODULE(MyModule)
{
    // First approach:
    // Fails in compilation with error C2243: 'type cast' : conversion from
    // 'MyClassWrap *' to 'boost::python::wrapper<T> *' exists, but is inaccessible
    //class_<MyClassWrap, boost::shared_ptr<MyClassWrap>, boost::noncopyable>("MyClass")

    // Second approach:
    // Doesn't see the overrides at runtime
    class_<MyClass, boost::shared_ptr<MyClassWrap>, boost::noncopyable>("MyClass")
        .def("PureVirtual", pure_virtual(&MyClass::PureVirtual), args("i"))
        .def("VirtualWithDefaultImpl", &MyClass::VirtualWithDefaultImpl,
            &MyClassWrap::DefaultVirtualWithDefaultImpl);
    def("MyFnc", &MyFnc, args("obj"));
}

The Python code:

class PythonDerived(MyModule.MyClass):
    def PureVirtual(self, i):
        print i

    def VirtualWithDefaultImpl(self):
        return 'PythonDerived'

MyModule.MyFnc(PythonDerived())

Output of the second approach run. As you can see the functions of MyClass are called instead of the PythonDerived functions:

MyClass

File "z:\tmp\tmp.py", line 11, in <module>
  MyModule.MyFnc(PythonDerived())

TypeError: 'NoneType' object is not callable

Answer 1

I have modified your wrapper class and how it is exposed to python and now that is working as expected:

class MyClassWrap : public MyClass, public python::wrapper<MyClass>
{
public:
    MyClassWrap() : MyClass()
    {
    }

    virtual void PureVirtual(int i)
    {
        this->get_override("PureVirtual")(i);
    }
    virtual const char* VirtualWithDefaultImpl()
    {
        if (python::override f = this->get_override("VirtualWithDefaultImpl"))
            return f();
        return MyClass::VirtualWithDefaultImpl();
    }
    const char* DefaultVirtualWithDefaultImpl()
    {
        return this->MyClass::VirtualWithDefaultImpl();
    }

};

BOOST_PYTHON_MODULE(MyModule)
{
python::class_<MyClassWrap, boost::shared_ptr<MyClassWrap>, boost::noncopyable>("MyClass")
    .def("PureVirtual", python::pure_virtual(&MyClassWrap::PureVirtual), python::args("i"))
    .def("VirtualWithDefaultImpl", &MyClassWrap::VirtualWithDefaultImpl,
         &MyClassWrap::DefaultVirtualWithDefaultImpl);
    python::def("MyFnc", &MyFnc, python::args("obj"));
}

Output:

PythonDerived
0