GTK+ Gnome Application Development by Havoc Pennington

Author:Havoc Pennington
Language: eng
Format: mobi, pdf
Publisher: UNKNOWN
Published: 2018-06-18T23:00:00+00:00

Chaining Up

If an object overrides the shutdown, destroy, or finalize methods, it should chain up to the default implementation, to ensure that each parent class has a chance to clean up. Here is an example of chaining up:

static void

gtk_widget_real_destroy (GtkObject *object)

{

/* ... */ if (parent_class

>destroy)

parent_class>destroy (object);

};

gtk_widget_real_destroy() is installed in the widget’s class struct in the class initialization function, overwriting the GtkObject default. parent_class is a pointer to the parent’s class struct; usually you will want to store this pointer in your class initialization function, as GtkWidget does:

static GtkObjectClass *parent_class = NULL;

/* ... code omitted ... */

static void

gtk_widget_class_init (GtkWidgetClass *klass) {

GtkObjectClass *object_class;

object_class = (GtkObjectClass*) klass;

parent_class = gtk_type_class (gtk_object_get_type ()); /* ... code omitted ... */

object_class>set_arg = gtk_widget_set_arg; object_class>get_arg = gtk_widget_get_arg; object_class>shutdown = gtk_widget_shutdown; object_class>destroy = gtk_widget_real_destroy; object_class>finalize = gtk_widget_finalize;

}

Of course, if parent_class is not a GtkObjectClass*, you will need to cast it with the GTK_OBJECT_CLASS() macro.

An aside: notice that you should not chain up when implementing get_arg and set_arg — GTK+ special-cases these methods ingtk_object_set() andgtk_object_get(). Recall that the GtkObject base class initializer zeroes these two methods, rather than leaving the default implementation. When setting or getting an argument value, GTK+ uses the information provided on argument registration to jump directly to the correct class struct and invoke only the correct get_arg or set_arg method. Chaining up would be a much slower way to implement the same thing (and would require unique argument IDs within the same class ancestry).

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