kio wrote:
> Hi,
>
> I'm studying the CPython source code. I don’t quite understand why
> they’re using PyObject_VAR_HEAD to define struct like PyListObject.
To
> define such kind of struct, could I use _PyObject_HEAD_EXTRA as a
> header and add "items" pointer and "allocated" count explicity? Is
> there any difference?
>
> Thanks.
>
> Eric
first, _PyObject_HEAD_EXTRA is only useful for debug and defined as:
#ifdef Py_TRACE_REFS
#define _PyObject_HEAD_EXTRA \
struct _object *_ob_next; \
struct _object *_ob_prev;
#define _PyObject_EXTRA_INIT 0, 0,
#else
#define _PyObject_HEAD_EXTRA
#define _PyObject_EXTRA_INIT
#endif
and PyObject_HEAD is defined as
#define PyObject_HEAD \
_PyObject_HEAD_EXTRA \
int ob_refcnt; \
struct _typeobject *ob_type;
PyObject_VAR_HEAD defined as
#define PyObject_VAR_HEAD \
PyObject_HEAD \
int ob_size;
so you can see the differences between them here.
PyListObject is defined as
typedef struct{
PyObject_VAR_HEAD
PyObject **ob_item;
int allocated;
}PyListObject;
After unfolding these macros we can get
typedef struct{
_PyObject_HEAD_EXTRA
int ob_refcnt;
struct _typeobject *ob_type;
int ob_size;
PyObject **ob_item;
int allocated;
}PyListObject;
So, if we don't use macros, we have to specify not only the "item" and
"allocated", but also the ref count and type information of the object
and the size info for a variable object. These information is not only
used by PyListObject, but also PyStringObject and PyDictObject, so
macros make it convenient and consize.
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