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<H2><A NAME="SECTION005330000000000000000"></A><A NAME="weakref-extension"></A>
<BR>
3.3.3 Weak References in Extension Types

</H2>

<P>
One of the goals of the implementation is to allow any type to
participate in the weak reference mechanism without incurring the
overhead on those objects which do not benefit by weak referencing
(such as numbers).

<P>
For an object to be weakly referencable, the extension must include a
<tt class="ctype">PyObject*</tt> field in the instance structure for the use of the
weak reference mechanism; it must be initialized to <tt class="constant">NULL</tt> by the
object's constructor.  It must also set the <tt class="member">tp_weaklistoffset</tt>
field of the corresponding type object to the offset of the field.
Also, it needs to add <tt class="constant">Py_TPFLAGS_HAVE_WEAKREFS</tt> to the
tp_flags slot.  For example, the instance type is defined with the
following structure:

<P>
<div class="verbatim"><pre>
typedef struct {
    PyObject_HEAD
    PyClassObject *in_class;       /* The class object */
    PyObject      *in_dict;        /* A dictionary */
    PyObject      *in_weakreflist; /* List of weak references */
} PyInstanceObject;
</pre></div>

<P>
The statically-declared type object for instances is defined this way:

<P>
<div class="verbatim"><pre>
PyTypeObject PyInstance_Type = {
    PyObject_HEAD_INIT(&amp;PyType_Type)
    0,
    "module.instance",

    /* Lots of stuff omitted for brevity... */

    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_WEAKREFS   /* tp_flags */
    0,                                          /* tp_doc */
    0,                                          /* tp_traverse */
    0,                                          /* tp_clear */
    0,                                          /* tp_richcompare */
    offsetof(PyInstanceObject, in_weakreflist), /* tp_weaklistoffset */
};
</pre></div>

<P>
The type constructor is responsible for initializing the weak reference
list to <tt class="constant">NULL</tt>:

<P>
<div class="verbatim"><pre>
static PyObject *
instance_new() {
    /* Other initialization stuff omitted for brevity */

    self-&gt;in_weakreflist = NULL;

    return (PyObject *) self;
}
</pre></div>

<P>
The only further addition is that the destructor needs to call the
weak reference manager to clear any weak references.  This should be
done before any other parts of the destruction have occurred, but is
only required if the weak reference list is non-<tt class="constant">NULL</tt>:

<P>
<div class="verbatim"><pre>
static void
instance_dealloc(PyInstanceObject *inst)
{
    /* Allocate temporaries if needed, but do not begin
       destruction just yet.
     */

    if (inst-&gt;in_weakreflist != NULL)
        PyObject_ClearWeakRefs((PyObject *) inst);

    /* Proceed with object destruction normally. */
}
</pre></div>

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<span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span>
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