[Python-Dev] Define metatype and a type that uses it

[Erik Groeneveld]

Amaury,

Thank you very much for your detailed explanation.  It helps to
understand it better, and it mostly works now.  There is one thing
however:

On Wed, Jul 8, 2009 at 17:35, Amaury Forgeot d'Arc<amauryfa at gmail.com> wrote:
> - Don't define a JObjectMeta struct, use JObjectType directly instead.
> An instance of the metatype is the type itself!
> - Don't set JObjectMetaType.tp_basicsize, let it inherit from the base
> type (the correct value would be sizeof(PyHeapTypeObject), this is
> important in order to create derived classes in python)

I'd like to add a C pointer field to the metatype instance (the type).
So, contrary to your advice, I have defined:

typedef struct {
    PyHeapTypeObject x;
    void* p;
} JObjectMeta;

This seems the way to do it for objects, but for types, it doesn't
seem right, as the p member turns out to be overwritten unexpectedly
at runtime.

Reading Python's object.h file it turns out that PyHeapTypeObject
'extends' PyTypeObject, which in turn has a PyObject_VAR_HEAD init
macro.  So mustn't:

PyTypeObject JObjectMetaType = {
    PyObject_HEAD_INIT(NULL)
};

actually be:

PyTypeObject JObjectMetaType = {
    PyVarObject_HEAD_INIT(NULL, 1)
};

with:

JObjectMetaType.tp_basic_size = sizeof(JObjectMeta);
JObjectMetaType.tp_itemsize = sizeof(void*);

I tried this, but it doesn't keep my app from dumping core on an
overwritten 'p'.

My question basically is: how can I define a pointer for each type
created with this metatype, such as is intended by the JObjectMeta
struct?

Best regards
Erik Groeneveld