##
10.8.1 Example Cycle Collector Support

This example shows only enough of the implementation of an extension type to show how the garbage collector support needs to be added. It shows the definition of the object structure, the `tp_traverse`, `tp_clear` and `tp_dealloc` implementations, the type structure, and a constructor -- the module initialization needed to export the constructor to Python is not shown as there are no special considerations there for the collector. To make this interesting, assume that the module exposes ways for the `container` field of the object to be modified. Note that since no checks are made on the type of the object used to initialize `container`, we have to assume that it may be a container.

      #include "Python.h"

      typedef struct {
          PyObject_HEAD
          PyObject *container;
      } MyObject;

      static int
      my_traverse(MyObject *self, visitproc visit, void *arg)
      {
          if (self->container != NULL)
              return visit(self->container, arg);
          else
              return 0;
      }

      static int
      my_clear(MyObject *self)
      {
          Py_XDECREF(self->container);
          self->container = NULL;

          return 0;
      }

      static void
      my_dealloc(MyObject *self)
      {
          PyObject_GC_UnTrack((PyObject *) self);
          Py_XDECREF(self->container);
          PyObject_GC_Del(self);
      }

      statichere PyTypeObject
      MyObject_Type = {
          PyObject_HEAD_INIT(NULL)
          0,
          "MyObject",
          sizeof(MyObject),
          0,
          (destructor)my_dealloc,     /* tp_dealloc */
          0,                          /* tp_print */
          0,                          /* tp_getattr */
          0,                          /* tp_setattr */
          0,                          /* tp_compare */
          0,                          /* tp_repr */
          0,                          /* tp_as_number */
          0,                          /* tp_as_sequence */
          0,                          /* tp_as_mapping */
          0,                          /* tp_hash */
          0,                          /* tp_call */
          0,                          /* tp_str */
          0,                          /* tp_getattro */
          0,                          /* tp_setattro */
          0,                          /* tp_as_buffer */
          Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
          0,                          /* tp_doc */
          (traverseproc)my_traverse,  /* tp_traverse */
          (inquiry)my_clear,          /* tp_clear */
          0,                          /* tp_richcompare */
          0,                          /* tp_weaklistoffset */
      };

      /* This constructor should be made accessible from Python. */
      static PyObject *
      new_object(PyObject *unused, PyObject *args)
      {
          PyObject *container = NULL;
          MyObject *result = NULL;

          if (PyArg_ParseTuple(args, "|O:new_object", &container)) {
              result = PyObject_GC_New(MyObject, &MyObject_Type);
              if (result != NULL) {
                  result->container = container;
                  PyObject_GC_Track(result);
              }
          }
          return (PyObject *) result;
      }