Re: Inheritence without knowing the parent at compile time

["Padraig O'Briain" <Padraig Obriain Sun COM>]

Andrew,

I have had to do someting along these lines whoile implementing Accessible 
objects for widgets.

See, for esxample, gtkhtml2/libgtkhtml/a11y/htmlviewaccessible.c.

I wanted the accessible object for HtmlView to inherit behavior from the 
accessible object for GtkLayout, which is what HtmlView derives from.

The GTK module libgail.so contains the accessible objects for GTK widgets bit 
this could not be referred to in gtkhtml2.

You can see my solution.

I have not needed to make the class and instance sizes different.

Padraig


> 
> Hi,
> 
> After playing about with GObject a bit, I'm trying to work out how to
> inherit at runtime.
> 
> Normally, type and class structs have the parent type and class structs
> at their start, in order to make them the right size and give the
> correct offset to the other members of the struct. ie.
> 
> struct _MyObject {
> 	ParentObject parent;
> 
> 	gpointer myfield;
> 	... etc ...
> };
> 
> However, I can't do this because I don't know what the parent is going
> to be. So I'm omitting that from my structs and registering my types
> with this sort of function:
> 
> GType
> tirith_obi_python_new_type (const gchar *class, const gchar *super)
> {
> 	guint type = 0, parent_type = 0;
> 	GTypeQuery query;
> 	
> 	parent_type = g_type_from_name (super);
> 	g_type_query (parent_type, &query);
> 
> 	GTypeInfo obi_info =
> 	{
> 		query.class_size + sizeof (TirithObiPythonClass),
> 		(GBaseInitFunc) NULL,
> 		(GBaseFinalizeFunc) NULL,
> 		(GClassInitFunc) tirith_obi_python_class_init,
> 		(GClassFinalizeFunc) NULL,
> 		NULL,
> 		query.instance_size + sizeof (TirithObiPython),
> 		0,
> 		(GInstanceInitFunc) tirith_obi_python_init,
> 		NULL
> 	};
> 
> 	type = g_type_register_static (g_type_from_name (super), class,
> &obi_info, 0);
> 
> 	return type;
> }
> 
> note the sizeof() addition so the runtime system gets the correct size.
> But now I expect my pointers are pointing to the wrong place when they
> are passed to a function.
> 
> void
> tirith_obi_python_init (TirithObiPython *obi)
> 
> In this function, obi will be pointing to the start the gobject which is
> not the same point as the beginning of the TirithObiPython struct?
> Presumably I have to do something along the lines of:
> 
> newobi = obi + sizeof (parent);
> 
> to get the correct position? But I don't know what the immediate parent
> is. Is there a gobject function where I can find out?
> 
> Basically, I'd like to know if I'm on the right lines here, or if I'm
> doing it totally wrong.
> 
> Thanks a lot for your help.
> 
> -- 
>                  Merry Christmas,
> 
>                                Andrew
> 
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