Virtual self-reference

intro polymorphism idioms patterns events summary, Q/A, literature

A special feature of the hush widget class library is its support for the definition of new composite widgets, which provide to the application programmer the interface of a built-in (or possibly other composite) widget. To realize this flexibility, we introduced a self() function that adds another level of indirection to self-reference. For example, look at the item class below:

  class item {    item  
  
  public item(String x) { _name = x; _self = null; }
  
  String name() { return exists()?self().name():_name; }
  
  public void redirect(item x) { _self = x; } 
  
  boolean exists() { return _self != null; }
  public item self() { return exists()?_self.self():this; }
  
  item _self;
  String _name;
  };

slide: Item with self()

The item class has an instance variable _self, that can be set to an arbitrary instance of item by invoking redirect. Now, when we ask for the name of the item, it is checked whether a redirection exists. If so, the call is redirected to the instance referenced by self(), otherwise the name of the item itself is returned.

  public class go {
  
  public static void main(String[] args) {
  	item a = new item("a");
  	item b = new item("b");
  	a.redirect(b);
  	System.out.println(a.name());  indeed, b
  	}
  
  };

slide: item: go

In combination with the handle/body idiom, we can create composites offering the interface of item, providing access to one or more (inner) items. This will be further illustrated in chapter 4.

Those well-versed in design patterns will recognize the Decorator patterns (as applied in the Interviews MonoGlyph class, [Interviews]).