Re: how are interfaces created? (talking about the Java-like "interface")

[Ben Johnson <ben blarg net>]

On Sat, Jan 29, 2005 at 08:27:55AM +0100, Jean Br?fort wrote:
> Le vendredi 28 janvier 2005 ?? 15:22 -0800, Ben Johnson a ??crit :
> > I want to understand how interfaces are defined and used/implemented.
...
> 
> It's a GInterface object. When creating a new class of object, you
> derive it from an existing class and you can add one or several
> interfaces to enrich your class.
> Here is a sample code I used to add printing support to the gnome canvas
> (this code is used by GChemPaint).
> 
> GType
> gnome_canvas_group_ext_get_type (void)
> {
> 	static GType group_ext_type;
> 
> 	if (!group_ext_type) {
> 		static const GTypeInfo object_info = {
> 			sizeof (GnomeCanvasGroupExtClass),
> 			(GBaseInitFunc) NULL,
> 			(GBaseFinalizeFunc) NULL,
> 			(GClassInitFunc) gnome_canvas_group_ext_class_init,
> 			(GClassFinalizeFunc) NULL,
> 			NULL,			/* class_data */
> 			sizeof (GnomeCanvasGroupExt),
> 			0,			/* n_preallocs */
> 			(GInstanceInitFunc) gnome_canvas_group_ext_init,
> 			NULL			/* value_table */
> 		};
> 
> 		static const GInterfaceInfo print_info = {
> 			(GInterfaceInitFunc) gnome_canvas_group_print_init,
> 			NULL, NULL
> 		};
> 
> 		group_ext_type = g_type_register_static (GNOME_TYPE_CANVAS_GROUP_EXT,
> "GnomeCanvasGroupExt",
> 						    &object_info, 0);
> 
> 		g_type_add_interface_static (group_ext_type, G_TYPE_PRINTABLE,
> &print_info);
> 	}
> 
> 	return group_ext_type;
> }
> 
> Hope this helps,

It does help *very* much.  Thank you.  I hope you don't answering a
couple questions.

Once an object of GnomeCanvasGroupExtClass is instantiated, can it be
cast as something else having to do with the print_info (Printable?)?
How is that done?  And, when the cast is performed, what functionality
is exposed?

Thanks a lot!

- Ben