Chapter 16. Class Hierarchies
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 classusually known as "protected access"certainly seemed like a good idea at the time. But long and bitter experience now strongly suggests that this practice is just as detrimental to the maintainability of class hierarchies as full "public access" is.
Worse still, in a hash-based object, the attributes live in a single namespace (the keys of the hash), so derived classes have to contend with their base classes, and with each other, for ownership of particular attributes.
Other serious problems can also arise in Perl class hierarchies, regardless of their underlying implementation type. Constructor and destructor methods have no privileged status, and constructors usually intermix the creation and initialization of objects. These two factors make it easy for subclassesespecially those inheriting from multiple base classesto misconstruct, incompletely initialize, or only partially clean up their derived objects.
This chapter describes a set of design and coding practices that avoid all these problems.