This section uses the same example as for objects (see 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 -- ) 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.