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Polymorphism 473

14

Mixins and Capabilities Classes ....................................................................

One way to strike a middle ground between multiple inheritance and single inheritance is

to use what are called mixins. Thus, you might have your Horseclass derive from

Animaland from Displayable. Displayablewould just add a few methods for display-

ing any object onscreen.

A mixin, or capability class, is a class that adds specialized functionality without adding

many additional methods or much data.

Capability classes are mixed into a derived class the same as any other class might be, by

declaring the derived class to inherit publicly from them. The only difference between a

capability class and any other class is that the capability class has little or no data. This is

an arbitrary distinction, of course, and is just a shorthand way of noting that at times all

you want to do is mix in some additional capabilities without complicating the derived

class.

This will, for some debuggers, make it easier to work with mixins than with more com-

plex multiply inherited objects. In addition, less likelihood exists of ambiguity in access-

ing the data in the other principal base class.

For example, if Horsederives from Animaland from Displayable,Displayablewould

have no data. Animalwould be just as it always was, so all the data in Horsewould

derive from Animal, but the functions in Horsewould derive from both.

The term mixincomes from an ice cream store in Sommerville,

Massachusetts, where candies and cakes were mixed into the basic ice cream

flavors. This seemed like a good metaphor to some of the object-oriented

programmers who used to take a summer break there, especially while

working with the object-oriented programming language SCOOPS.

NOTE

Abstract Data Types ............................................................................................

Often, you will create a hierarchy of classes together. For example, you might create a

Shapeclass, and derive from that Rectangleand Circle. From Rectangle, you might

derive Squareas a special case of Rectangle.

Each of the derived classes will override the Draw()method, the GetArea()method, and

so forth. Listing 14.7 illustrates a bare-bones implementation of the Shapeclass and its

derived Circleand Rectangleclasses.