retrieve C++object member pointer using Python

Question

I'm using box2d in my application. It has a class b2Body. It has a property void *userData and methods for it's accessing. In other words: we can store there some additional info about physic body. I want to store there some class object:

MyClass *obj = new MyClass();
b2Body body;
body.SetUserData(obj);

Now I have to get user data inside python! I wrote a function-wrapper which returns the extracted pointer:

MyClass *GetBodyUserData(b2Body &body)
{
    void *userData = body.GetUserData();
    if (userData) return (MyClass*)userData;

    std::cout << "Fail.";
    return nullptr;
}

Now, when I call GetBodyUserData in python I have an empty or 'dead' object :(.

I tried to store smart pointer insted of pure one but It's not allowed by SetUserData interface.

Any ideas?

upd Exporting to python:

bp::def("GetBodyUserData", &GetBodyUserData, boost::python::return_value_policy <boost::python::reference_existing_object>())

What you're doing is perfectly fine, C++-wise. How are you passing the pointer to Python? — Adam Rosenfield
– Adam Rosenfield, Commented Jun 29, 2011 at 20:20

Glen Fletcher · Accepted Answer · 2011-07-04 02:49:37Z

You just have to do it C-style, i.e. pass the pointer to a function with returns the specific item you want. C++ objects aren't the same as python objects.

Also Python doesn't even have the definition of your c++ object, so even if they looked the same in memory, python could use it as it doesn't know what its internal values and functions are.

Remember pointers are just a number that represents the memory address of the object.

So say you myclass has a member value you want to be able to edit from python.

You need to define a set and get function

the set function should take a python value and then set the ((MyClass *)obj)->value to this value while the get function will return ((MyClass *)obj)->value

You could also construct a MyClass in python that has the appropriate design, and simply calls the C-style functions.

P.S.

I believe it's also possible to define a python class type from C++, similar to how you define functions, however I've never tried this myself.

I hope this helps you.

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