Perl Best Practices

Chapter 16. Class Hierarchies

The ham and cheese omelet class is worth special attention because it must inherit characteristics from the pork, dairy, and poultry classes. Thus, we see that the problem cannot be properly solved without multiple inheritance. At run time, the program must create the proper object and send a message to the object that says, "Cook yourself". The semantics of this message depend, of course, on the kind of object, so they have a different meaning to a piece of toast than to scrambled eggs. Reviewing the process so far, we see that the analysis phase has revealed that the primary requirement is to cook any kind of breakfast food. In the design phase, we have discovered some derived requirements. Specifically, we need an object-oriented language with multiple inheritance. Of course, users don't want the eggs to get cold while the bacon is frying, so concurrent processing is required, too.

— Do-While Jones The Breakfast Food Cooker

The disadvantages of implementing classes via blessed hashes become even more pronounced when those classes are used as the bases of inheritance hierarchies. For example, the lack of encapsulation makes it almost inevitable that base-class attributes will be accessed directly in derived-class methods, thereby strongly coupling the two classes.

This notion that derived classes should have some kind of exemption to the encapsulation of their base class—usually known as "protected access"—certainly seemed like a good idea at the time. ...