[capi-sig] CPython and C++ object GC

Csaba Balazs csaba.balazs at researchstudio.at
Mon Mar 30 16:59:36 CEST 2009

Hello Everybody,

I would like to use a C++ gui library with the following (simplified) interface
in Python.

#include <stdio.h>

class Gui;

class GuiObject {
        GuiObject(Gui *Gui) {printf("creating GuiObject(gui: %X)\n", Gui);}
        ~GuiObject() {printf("deleting GuiObject\n");}
        void Move(int x, int y) {printf("GuiObject move(%d, %d)\n", x, y);};

class Gui {
        Gui()  {printf("creating Gui\n");}
        ~Gui() {printf("deleting Gui\n");}
        GuiObject* AddImage() {
                GuiObject* ob = new GuiObject(this);
                return ob;
        void Print() {printf("Gui: %X\n", this);}

int main() {
        Gui *gui = new Gui();
        GuiObject *obj = gui->AddImage();
        obj->Move(50, 50);
        /*GuiObject *obj2 = new GuiObject(gui); // not allowed
        delete obj2;*/
        delete obj;
        delete gui;
        return 0;

I created the Python Gui and GuiObject classes (PyTypeObject), and added it to
main module (PyModule_AddObject).
It works, but there is a problem at the Gui::AddImage(), with constructs a new
GuiObject, which is available in Python layer but finally it is not collected
and freed by GC:

obj = _PyObject_New(&GuiObjectType);
PyObject_Init(obj, &GuiObjectType);

I cannot invoke the GuiObject object constructor directly from Python, because
of the implementation of the C++ gui library (in this case it would be
I use the embedded CPython as an interpreter, so I cannot add additional
external .py file for it.

So the following Python code would be the target:

gui = GUI();

background = gui.AddImage();
#background = GuiObject(gui); <-- Collected but not allowed
background.Move(0, 0);

How could I implement the AddImage function in order to be freed the constructed
object at the end?

Thanks in advance!



More information about the capi-sig mailing list