Advanced Perl Programming

On Inheritance

I have never been quite comfortable with using inheritance liberally, and I don’t subscribe to the theory that this feature is essential for software reuse. There are three related but distinct flavors of inheritance, and in this section, I’ll list what I like or dislike about these aspects. The three types of inheritance are as follows:

• Attribute inheritance

• Implementation inheritance

• Interface inheritance

Attribute Inheritance

The facility provided by a language for a subclass to inherit attributes from a base class or a structure is called attribute inheritance. While C++ and Java provide this facility, Perl doesn’t. The onus is on the Perl programmer to figure out a way for a superclass and a subclass to agree on a common inheritable representation. For this reason, a hash table is a frequent choice, but not necessarily an economical one, as has been pointed out earlier.

My problem with attribute inheritance is that it introduces a tremendous amount of coupling between an inherited class and a derived class. A change in the way a base class is laid out has drastic consequences for the derived class. This is clearly a violation of encapsulation. C++ treats all attributes as private by default but then provides a keyword called “protected,” whereby it makes them freely available to derived classes, while still hiding them from the general public. Bjarne Stroustrup, the creator of C++, regrets this in his excellent book The Design and Evolution of C++ [2]:

One of my ...