SWOOP Object-Oriented Programming
Libraries, Functions, Callbacks, and Threads
SwiftForth Programming References
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SWOOP Object-Oriented Programming
SwiftForth includes a powerful object-oriented programming package called SWOOP. SWOOP provides the essential features of object-oriented programming.
- 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, adding a pair of scalars is different from adding a pair of vectors.
SwiftForth's support of object-oriented programming is extensive and powerful. The following brief introductory information is excerpted from the SwiftForth Reference Manual.
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 unnamed responses to arbitrary numeric message values.
Objects may be static or dynamic. Both early and late binding are supported. Instance data and methods are implemented in a manner that is very efficient in the context of a Forth development environment.
