GObject constructors

[Jacob Kroon <jacob kroon gmail com>]

I'm having problems with figuring out how to initialize my data properly.
My scenario is that I have:

struct Parent {
   GObject parent;
   float *data;
}

struct ParentClass {
   GObjectClass parent;
   int data_size;
}

struct Child {
   Parent parent;
}

struct ChildClass {
   ParentClass parent;
}

Each subclass of Parent will use different, but fixed data sizes, so I 
want Child to tell Parent how much
data it needs, and then let Parent initialize the data accordingly.

The way I thought would work was this:

1. In child_class_init(*klass):
   PARENT_CLASS(klass)->data_size = specific_size_for_this_subclass;
2. In parent_instance_init(*obj)
   klass = PARENT_GET_CLASS(obj);
   obj->data = g_new(float, klass->data_size);

but this wont work since PARENT_GET_CLASS(obj) in parent_instance_init() 
returns the class for Parent,
not the subclass ChildClass, so data_size won't have the value I intended.

Any help would be gladly appreciated.

Regards Jacob