Implementing Metatypes in C

[Phil Thompson]

I want to create a type (in C) that has some additional, type specific, 
information associated with it.

That type will then have many sub-types created, also in C. Instances of those 
sub-types will then be created in Python.

As I understand it, I have to create a metatype for my new types so that they 
get created with the correct size to contain the additional I want to store 
in the type. To do this I am simply sub-typing from PyType_Type but 
specifying an "appropriate" value for the tp_basicsize field. The sub-types 
are then created on the heap by calling the metatype.

The problem I had was understanding what the "appropriate" value was. At first 
my metatype data structure was...

struct metatype {
	PyTypeObject supertype;
	ExtraTypeData extra;
};

Eventually I worked out that Python allocates additional fields (as well as 
PyTypeObject) when allocating new types on the heap. This is from 
typeobject.c in the Python 2.3a2 source...

/* The *real* layout of a type object when allocated on the heap */
/* XXX Should we publish this in a header file? */

If I add the extra fields in my struct metatype, everything works.

Two questions...

1. Is my understanding correct, or is there a more correct way to do what I 
want that avoids the problem?

2. If my solution is correct, can the structure defined in typeobject.c be 
published in a header file as the comment says? Please?

Thanks,
Phil