Classes and Objects

 

    MzScheme's object system uses the class-object model: every object is an instance of some class. Object variables a declared within a class, but are not instantiated until an instance of the class is created, Classes and objects are both first-class Scheme values. However, a MzScheme class is not a MzScheme object. (There are no ``meta-classes''.)

A MzScheme class specifies

• a collection of instance variable names,
• initial value expressions for the instance variables, and
• initialization variables that are bound to input initialization arguments.

An object is a collection of instance variables that are instantiated according to a class description. There is no distinction between ``methods'' and ``instance variables''; a method is simply an instance variable with a procedural value.

    The most important feature of the object system is the ability to define a new class (a derived class ) in terms of existing classes (the superclasses ) using inheritance and overriding:

• Inheritance -- An object of a derived class instantiates variables declared in the superclasses as well as variables from the derived class.
• Overriding -- A variable declared in a superclass can be redefined in the derived class. References to the overridden variable from other expressions in the superclass use the new definition.

• An Example
• Creating Classes
  • Initialization Variables
  • Instance Variables
  • Initial Values
• Creating Objects
• Instance Variable Access
  • Generic Procedures
• Utilities
• Superclass Search Algorithm