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Diffstat (limited to 'Doc/c-api/allocation.rst')
| -rw-r--r-- | Doc/c-api/allocation.rst | 132 |
1 files changed, 107 insertions, 25 deletions
diff --git a/Doc/c-api/allocation.rst b/Doc/c-api/allocation.rst index 7cbc99ad145..f8d01a3f29b 100644 --- a/Doc/c-api/allocation.rst +++ b/Doc/c-api/allocation.rst @@ -16,7 +16,20 @@ Allocating Objects on the Heap Initialize a newly allocated object *op* with its type and initial reference. Returns the initialized object. Other fields of the object are - not affected. + not initialized. Despite its name, this function is unrelated to the + object's :meth:`~object.__init__` method (:c:member:`~PyTypeObject.tp_init` + slot). Specifically, this function does **not** call the object's + :meth:`!__init__` method. + + In general, consider this function to be a low-level routine. Use + :c:member:`~PyTypeObject.tp_alloc` where possible. + For implementing :c:member:`!tp_alloc` for your type, prefer + :c:func:`PyType_GenericAlloc` or :c:func:`PyObject_New`. + + .. note:: + + This function only initializes the object's memory corresponding to the + initial :c:type:`PyObject` structure. It does not zero the rest. .. c:function:: PyVarObject* PyObject_InitVar(PyVarObject *op, PyTypeObject *type, Py_ssize_t size) @@ -24,38 +37,107 @@ Allocating Objects on the Heap This does everything :c:func:`PyObject_Init` does, and also initializes the length information for a variable-size object. + .. note:: + + This function only initializes some of the object's memory. It does not + zero the rest. + .. c:macro:: PyObject_New(TYPE, typeobj) - Allocate a new Python object using the C structure type *TYPE* - and the Python type object *typeobj* (``PyTypeObject*``). - Fields not defined by the Python object header are not initialized. - The caller will own the only reference to the object - (i.e. its reference count will be one). - The size of the memory allocation is determined from the - :c:member:`~PyTypeObject.tp_basicsize` field of the type object. + Allocates a new Python object using the C structure type *TYPE* and the + Python type object *typeobj* (``PyTypeObject*``) by calling + :c:func:`PyObject_Malloc` to allocate memory and initializing it like + :c:func:`PyObject_Init`. The caller will own the only reference to the + object (i.e. its reference count will be one). + + Avoid calling this directly to allocate memory for an object; call the type's + :c:member:`~PyTypeObject.tp_alloc` slot instead. + + When populating a type's :c:member:`~PyTypeObject.tp_alloc` slot, + :c:func:`PyType_GenericAlloc` is preferred over a custom function that + simply calls this macro. + + This macro does not call :c:member:`~PyTypeObject.tp_alloc`, + :c:member:`~PyTypeObject.tp_new` (:meth:`~object.__new__`), or + :c:member:`~PyTypeObject.tp_init` (:meth:`~object.__init__`). + + This cannot be used for objects with :c:macro:`Py_TPFLAGS_HAVE_GC` set in + :c:member:`~PyTypeObject.tp_flags`; use :c:macro:`PyObject_GC_New` instead. + + Memory allocated by this macro must be freed with :c:func:`PyObject_Free` + (usually called via the object's :c:member:`~PyTypeObject.tp_free` slot). + + .. note:: + + The returned memory is not guaranteed to have been completely zeroed + before it was initialized. + + .. note:: + + This macro does not construct a fully initialized object of the given + type; it merely allocates memory and prepares it for further + initialization by :c:member:`~PyTypeObject.tp_init`. To construct a + fully initialized object, call *typeobj* instead. For example:: + + PyObject *foo = PyObject_CallNoArgs((PyObject *)&PyFoo_Type); - Note that this function is unsuitable if *typeobj* has - :c:macro:`Py_TPFLAGS_HAVE_GC` set. For such objects, - use :c:func:`PyObject_GC_New` instead. + .. seealso:: + + * :c:func:`PyObject_Free` + * :c:macro:`PyObject_GC_New` + * :c:func:`PyType_GenericAlloc` + * :c:member:`~PyTypeObject.tp_alloc` .. c:macro:: PyObject_NewVar(TYPE, typeobj, size) - Allocate a new Python object using the C structure type *TYPE* and the - Python type object *typeobj* (``PyTypeObject*``). - Fields not defined by the Python object header - are not initialized. The allocated memory allows for the *TYPE* structure - plus *size* (``Py_ssize_t``) fields of the size - given by the :c:member:`~PyTypeObject.tp_itemsize` field of - *typeobj*. This is useful for implementing objects like tuples, which are - able to determine their size at construction time. Embedding the array of - fields into the same allocation decreases the number of allocations, - improving the memory management efficiency. - - Note that this function is unsuitable if *typeobj* has - :c:macro:`Py_TPFLAGS_HAVE_GC` set. For such objects, - use :c:func:`PyObject_GC_NewVar` instead. + Like :c:macro:`PyObject_New` except: + + * It allocates enough memory for the *TYPE* structure plus *size* + (``Py_ssize_t``) fields of the size given by the + :c:member:`~PyTypeObject.tp_itemsize` field of *typeobj*. + * The memory is initialized like :c:func:`PyObject_InitVar`. + + This is useful for implementing objects like tuples, which are able to + determine their size at construction time. Embedding the array of fields + into the same allocation decreases the number of allocations, improving the + memory management efficiency. + + Avoid calling this directly to allocate memory for an object; call the type's + :c:member:`~PyTypeObject.tp_alloc` slot instead. + + When populating a type's :c:member:`~PyTypeObject.tp_alloc` slot, + :c:func:`PyType_GenericAlloc` is preferred over a custom function that + simply calls this macro. + + This cannot be used for objects with :c:macro:`Py_TPFLAGS_HAVE_GC` set in + :c:member:`~PyTypeObject.tp_flags`; use :c:macro:`PyObject_GC_NewVar` + instead. + + Memory allocated by this function must be freed with :c:func:`PyObject_Free` + (usually called via the object's :c:member:`~PyTypeObject.tp_free` slot). + + .. note:: + + The returned memory is not guaranteed to have been completely zeroed + before it was initialized. + + .. note:: + + This macro does not construct a fully initialized object of the given + type; it merely allocates memory and prepares it for further + initialization by :c:member:`~PyTypeObject.tp_init`. To construct a + fully initialized object, call *typeobj* instead. For example:: + + PyObject *list_instance = PyObject_CallNoArgs((PyObject *)&PyList_Type); + + .. seealso:: + + * :c:func:`PyObject_Free` + * :c:macro:`PyObject_GC_NewVar` + * :c:func:`PyType_GenericAlloc` + * :c:member:`~PyTypeObject.tp_alloc` .. c:function:: void PyObject_Del(void *op) |