Forced Objects

Psion's C development package and Geoworks GEOS SDK attempt a difficult trick: implementing an object-oriented environment atop the nonobject-oriented C language. Both systems require that a project include class-definition files -- category files on the Psion, GOC (GEOS C) files in GEOS. After preprocessing, these definition files are merged (either before or after linking, depending on the platform) with C source code files that implement the classes' behavior.

GEOS uses prepended keywords to identify classes, messages, objects, and instance data. For instance, you define a class and its messages with the following statements:

@class <classname>, <superclass
>;

@message void <message1>;

@message void <message2>;

@endc <classname>, <classattributes>;

Psion's overloading scheme allows a subclass to override its parent's methods. You can't specify this within the C language, which has no concept of class or overriding. The preprocessor recognizes the method keyword preceding a function. In that case, the programmer must give the function's name the form <classname>_<methodname> , which associates method with class (the class having been defined in the category file) while satisfying C's requirement that each function name be unique.