On Wed, Nov 09, 2005 at 06:43:17PM +0100, Jacob Kroon wrote:
> 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.
I had similar problem and came to conclusion it's impossible
to implement it this way, or at least it's against inheritance.
However, you can keep 1. as it is and define a parent_instance_setup()
method that contains the common code and call that from
child constructor:
2a. parent_instance_setup(obj) {
klass = PARENT_GET_CLASS(obj)
obj->data = g_new(float, klass->data_size);
}
2b. child_instance_init(obj) {
parent_instance_setup(obj);
}
Yeti
--
That's enough.
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