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Gforth Manual

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5.21.4.2 Basic `oof.fs' Usage

This section uses the same example as for objects (see section 5.21.3.2 Basic `objects.fs' Usage).

You can define a class for graphical objects like this:

 
object class graphical \ "object" is the parent class
  method draw ( x y graphical -- )
class;

This code defines a class graphical with an operation draw. We can perform the operation draw on any graphical object, e.g.:

 
100 100 t-rex draw

where t-rex is an object or object pointer, created with e.g. graphical : t-rex.

How do we create a graphical object? With the present definitions, we cannot create a useful graphical object. The class graphical describes graphical objects in general, but not any concrete graphical object type (C++ users would call it an abstract class); e.g., there is no method for the selector draw in the class graphical.

For concrete graphical objects, we define child classes of the class graphical, e.g.:

 
graphical class circle \ "graphical" is the parent class
  cell var circle-radius
how:
  : draw ( x y -- )
    circle-radius @ draw-circle ;

  : init ( n-radius -- (
    circle-radius ! ;
class;

Here we define a class circle as a child of graphical, with a field circle-radius; it defines new methods for the selectors draw and init (init is defined in object, the parent class of graphical).

Now we can create a circle in the dictionary with:

 
50 circle : my-circle

: invokes init, thus initializing the field circle-radius with 50. We can draw this new circle at (100,100) with:

 
100 100 my-circle draw

Note: You can only invoke a selector if the receiving object belongs to the class where the selector was defined or one of its descendents; e.g., you can invoke draw only for objects belonging to graphical or its descendents (e.g., circle). The scoping mechanism will check if you try to invoke a selector that is not defined in this class hierarchy, so you'll get an error at compilation time.


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