use of #define for type structures?

[giohappy]

Hello.
Many times I find the use of #define preprocessor macro to define type/data strctures, but I haven't unsertood very well its syntax. As an example here is anexcerpt from object.h from Python sources:

#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

/* PyObject_HEAD defines the initial segment of every PyObject. */
#define PyObject_HEAD \
_PyObject_HEAD_EXTRA \
Py_ssize_t ob_refcnt; \
struct _typeobject *ob_type;

#define PyObject_HEAD_INIT(type) \
_PyObject_EXTRA_INIT \
1, type,

#define PyVarObject_HEAD_INIT(type, size) \
PyObject_HEAD_INIT(type) size,

Questions:

1 - how the commas are interpreted?
2 - how the expansion works in these cases?
(3 - what's the gain respect to using typedef?)

thanks a lot!
giovanni

[jsmith]

#define is essentially a search-and-replace. Anywhere _PyObject_HEAD_EXTRA
is found, replace it verbatim with the text that follows.

#define has more uses than what typedef has. #define should not be used
when typedef could be.

[giohappy]

thanks jsmith.
So the define explands to a literal replacement. So the following defines

#define _PyObject_EXTRA_INIT 0, 0,

#define PyObject_HEAD_INIT(type) \
_PyObject_EXTRA_INIT \
1, type,

make the following expand to

static PyTypeObject SimpleObjectType = {
************
PyObject_HEAD_INIT(NULL)
************
/*expands to*/
************
0,
0,
1,
NULL,
************
etc.
};


right?
(I don't know this way of defining a type, using curly braces, but this is another question...)

[giohappy]

Many things to learn: this is a simple structure initialization... now everything is clear.