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Object-Oriented Programming

SwiftForth includes a powerful object-oriented programming package called SWOOP. It includes all the essential features of object-oriented programming, including:

• Encapsulation: combining data and methods into a single package that responds to messages.
• Information hiding: the ability of an object to possess data and methods not accessible outside its class.
• Inheritance: the ability to define a new class based on a previously defined ("parent") class. The new class automatically possesses all members of the parent; it may add to or replace these members, or define behaviors for deferred members.
• Polymorphism: the ability of different sub-classes of a class to respond to the same message in different ways. For example, all vehicles can steer, but bicycles do it differently from automobiles.

SwiftForth's support of object-oriented programming is extensive and powerful. Details are, of course, presented in the product manual, from which the following brief introductory information is adapted.

POINT (defined below) is a simple class we shall use as a primary building-block example for SWOOP. It demonstrates two of the four basic class member types: data and colon. The word following CLASS is the name of the class; all definitions between CLASS and END-CLASS belong to it. These definitions are referred to as the members of the class. When a class name is executed, it leaves its handle (xt) on the stack. The constructor words are the primary consumers of this handle.

 
    CLASS POINT
	    VARIABLE X
	    VARIABLE Y
	    : SHOW ( -- )   X @ . Y @ . ;
	    : DOT ( -- )   ." Point at " SHOW ;
    END-CLASS

The class definition itself does not allocate any instance storage; it only records how much storage is required for each instance of the class. VARIABLE reserves a cell of space and associates it with a member name.

The colon members SHOW and DOT are like normal Forth colon definitions, but are only valid in the execution context of an object of type POINT. X and Y also behave exactly like normal Forth VARIABLEs.

There are five kinds of members:

• Data members include all data definitions. Available data-member-defining words include CREATE (normally followed by data compiled with , or C,), BUFFER: (an array whose length is specified in address units), VARIABLE, CVARIABLE (single char), or CONSTANTs.
• Colon members are definitions that may act on or use data members.
• Other previously defined objects, available within this object.
• Deferred members are colon-like definitions with default behaviors that can be referenced while defining the class, but which may have substitute behaviors defined by sub-classes defined later. These allow for polymorphism and late binding.
• Messages are un-named responses to arbitrary numeric message values (such as Windows messages).

Objects may be static or dynamic. Both early and late binding are supported fully. Instance data and methods are implemented in a manner that is extremely efficient in the context of a Forth development environment.

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