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Gforth Manual

5.21.3.2 Basic `objects.fs' Usage

You can define a class for graphical objects like this:

object class \ "object" is the parent class selector draw ( x y graphical -- ) end-class graphical

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 a word (say, a constant) that produces a graphical object.

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 \ "graphical" is the parent class cell% field circle-radius :noname ( x y circle -- ) circle-radius @ draw-circle ; overrides draw :noname ( n-radius circle -- ) circle-radius ! ; overrides construct end-class circle

Here we define a class circle as a child of graphical, with field circle-radius (which behaves just like a field (see section 5.20 Structures); it defines (using overrides) new methods for the selectors draw and construct (construct is defined in object, the parent class of graphical).

Now we can create a circle on the heap (i.e., allocated memory) with:

50 circle heap-new constant my-circle

heap-new invokes construct, 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 object on the TOS (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). Immediately before end-class, the search order has to be the same as immediately after class.