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73 lines
2.9 KiB
ReStructuredText
73 lines
2.9 KiB
ReStructuredText
.. highlight:: c
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.. _countingrefs:
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******************
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Reference Counting
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******************
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The macros in this section are used for managing reference counts of Python
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objects.
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.. c:function:: void Py_INCREF(PyObject *o)
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Increment the reference count for object *o*. The object must not be ``NULL``; if
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you aren't sure that it isn't ``NULL``, use :c:func:`Py_XINCREF`.
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.. c:function:: void Py_XINCREF(PyObject *o)
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Increment the reference count for object *o*. The object may be ``NULL``, in
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which case the macro has no effect.
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.. c:function:: void Py_DECREF(PyObject *o)
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Decrement the reference count for object *o*. The object must not be ``NULL``; if
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you aren't sure that it isn't ``NULL``, use :c:func:`Py_XDECREF`. If the reference
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count reaches zero, the object's type's deallocation function (which must not be
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``NULL``) is invoked.
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.. warning::
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The deallocation function can cause arbitrary Python code to be invoked (e.g.
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when a class instance with a :meth:`__del__` method is deallocated). While
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exceptions in such code are not propagated, the executed code has free access to
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all Python global variables. This means that any object that is reachable from
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a global variable should be in a consistent state before :c:func:`Py_DECREF` is
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invoked. For example, code to delete an object from a list should copy a
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reference to the deleted object in a temporary variable, update the list data
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structure, and then call :c:func:`Py_DECREF` for the temporary variable.
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.. c:function:: void Py_XDECREF(PyObject *o)
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Decrement the reference count for object *o*. The object may be ``NULL``, in
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which case the macro has no effect; otherwise the effect is the same as for
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:c:func:`Py_DECREF`, and the same warning applies.
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.. c:function:: void Py_CLEAR(PyObject *o)
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Decrement the reference count for object *o*. The object may be ``NULL``, in
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which case the macro has no effect; otherwise the effect is the same as for
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:c:func:`Py_DECREF`, except that the argument is also set to ``NULL``. The warning
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for :c:func:`Py_DECREF` does not apply with respect to the object passed because
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the macro carefully uses a temporary variable and sets the argument to ``NULL``
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before decrementing its reference count.
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It is a good idea to use this macro whenever decrementing the reference
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count of an object that might be traversed during garbage collection.
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The following functions are for runtime dynamic embedding of Python:
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``Py_IncRef(PyObject *o)``, ``Py_DecRef(PyObject *o)``. They are
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simply exported function versions of :c:func:`Py_XINCREF` and
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:c:func:`Py_XDECREF`, respectively.
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The following functions or macros are only for use within the interpreter core:
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:c:func:`_Py_Dealloc`, :c:func:`_Py_ForgetReference`, :c:func:`_Py_NewReference`,
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as well as the global variable :c:data:`_Py_RefTotal`.
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