Exposing polymorphism with boost python

Question

i am starting to get really frustrated trying to expose a simple C++ polymorphism to python with boost::python.

I do have the following structure in C++:

struct Base {
    int typeID;
};

struct Derived : public Base {
    int derivedProperty;
}

//and some more from base derived types....    

Base *returnSomethingDerivedFromBase(...) {
    Derived *ret = new Derived;
    ret->derivedProperty = 1234;
    return ret;
}

BOOST_PYTHON_MODULE(foo) 
{
    class_<Base>("Base")
        .add_property("baseProperty", &Base::baseProperty);

    class_<Derived, bases<Base> >("Derived")
        .add_property("derivedProperty", &Derived::derivedProperty);

    def("returnSomethingDerivedFromBase", returnSomethingDerivedFromBase);    
}

And in Python i just want to have the following:

object = returnSomethingFromDerived() #object is of type Base
if object.typeID = 1:
    #here i want to cast to Derived and access "derivedProperty"
    #but this is not working :-( :
    object.__class__ = Derived

Is there a way to accomplish this at all? Or isn´t this possible as it would be in C++ ?

Thanks a lot for your help!!

Answer 1

Ok, i missed the virtual destructor in the Base class. So here is how it works:

struct Base {
    virtual ~Base() {}
    int typeID;
};

struct Derived : public Base {
    int derivedProperty;
}

//and some more from base derived types....    

Base *returnSomethingDerivedFromBase(...) {
    Derived *ret = new Derived;
    ret->derivedProperty = 1234;
    return ret;
}

BOOST_PYTHON_MODULE(foo) 
{
    class_<Base>("Base")
        .add_property("baseProperty", &Base::baseProperty);

    class_<Derived, bases<Base> >("Derived")
        .add_property("derivedProperty", &Derived::derivedProperty);

    def("returnSomethingDerivedFromBase", returnSomethingDerivedFromBase, return_value_policy<manage_new_object>());    
}

But now i have a different problem. When i try to return this type in a tuple i again lose the type info:

tuple returnSomethingDerivedFromBase(...) {
    Derived *ret = new Derived;
    ret->derivedProperty = 1234;
    return make_tuple(ret);
}