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54 | <H2><A NAME="SECTION005330000000000000000"></A><A NAME="weakref-extension"></A> | |
55 | <BR> | |
56 | 3.3.3 Weak References in Extension Types | |
57 | ||
58 | </H2> | |
59 | ||
60 | <P> | |
61 | One of the goals of the implementation is to allow any type to | |
62 | participate in the weak reference mechanism without incurring the | |
63 | overhead on those objects which do not benefit by weak referencing | |
64 | (such as numbers). | |
65 | ||
66 | <P> | |
67 | For an object to be weakly referencable, the extension must include a | |
68 | <tt class="ctype">PyObject*</tt> field in the instance structure for the use of the | |
69 | weak reference mechanism; it must be initialized to <tt class="constant">NULL</tt> by the | |
70 | object's constructor. It must also set the <tt class="member">tp_weaklistoffset</tt> | |
71 | field of the corresponding type object to the offset of the field. | |
72 | Also, it needs to add <tt class="constant">Py_TPFLAGS_HAVE_WEAKREFS</tt> to the | |
73 | tp_flags slot. For example, the instance type is defined with the | |
74 | following structure: | |
75 | ||
76 | <P> | |
77 | <div class="verbatim"><pre> | |
78 | typedef struct { | |
79 | PyObject_HEAD | |
80 | PyClassObject *in_class; /* The class object */ | |
81 | PyObject *in_dict; /* A dictionary */ | |
82 | PyObject *in_weakreflist; /* List of weak references */ | |
83 | } PyInstanceObject; | |
84 | </pre></div> | |
85 | ||
86 | <P> | |
87 | The statically-declared type object for instances is defined this way: | |
88 | ||
89 | <P> | |
90 | <div class="verbatim"><pre> | |
91 | PyTypeObject PyInstance_Type = { | |
92 | PyObject_HEAD_INIT(&PyType_Type) | |
93 | 0, | |
94 | "module.instance", | |
95 | ||
96 | /* Lots of stuff omitted for brevity... */ | |
97 | ||
98 | Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_WEAKREFS /* tp_flags */ | |
99 | 0, /* tp_doc */ | |
100 | 0, /* tp_traverse */ | |
101 | 0, /* tp_clear */ | |
102 | 0, /* tp_richcompare */ | |
103 | offsetof(PyInstanceObject, in_weakreflist), /* tp_weaklistoffset */ | |
104 | }; | |
105 | </pre></div> | |
106 | ||
107 | <P> | |
108 | The type constructor is responsible for initializing the weak reference | |
109 | list to <tt class="constant">NULL</tt>: | |
110 | ||
111 | <P> | |
112 | <div class="verbatim"><pre> | |
113 | static PyObject * | |
114 | instance_new() { | |
115 | /* Other initialization stuff omitted for brevity */ | |
116 | ||
117 | self->in_weakreflist = NULL; | |
118 | ||
119 | return (PyObject *) self; | |
120 | } | |
121 | </pre></div> | |
122 | ||
123 | <P> | |
124 | The only further addition is that the destructor needs to call the | |
125 | weak reference manager to clear any weak references. This should be | |
126 | done before any other parts of the destruction have occurred, but is | |
127 | only required if the weak reference list is non-<tt class="constant">NULL</tt>: | |
128 | ||
129 | <P> | |
130 | <div class="verbatim"><pre> | |
131 | static void | |
132 | instance_dealloc(PyInstanceObject *inst) | |
133 | { | |
134 | /* Allocate temporaries if needed, but do not begin | |
135 | destruction just yet. | |
136 | */ | |
137 | ||
138 | if (inst->in_weakreflist != NULL) | |
139 | PyObject_ClearWeakRefs((PyObject *) inst); | |
140 | ||
141 | /* Proceed with object destruction normally. */ | |
142 | } | |
143 | </pre></div> | |
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