Intermediate Perl, 2nd Edition

Chapter 15. Objects with Data

Using the simple syntax introduced in Chapter 13, we have class methods, (multiple) inheritance, overriding, and extending. We’ve been able to factor out common code and to provide a way to reuse implementations with variations. This is at the core of what objects provide, but objects also provide instance data, which we cover in this chapter.

A Horse Is a Horse, of Course of Course—Or Is It?

We look at the code we used for the Animal classes and Horse classes. The Animal class provides the general speak subroutine:

package Animal;

sub speak {
  my $class = shift;
  print "a $class goes ", $class−>sound, "!\n"
}

The Horse class inherits from Animal but provides its specific sound routine:

package Horse;
use parent qw(Animal);
sub sound { 'neigh' }

This lets us invoke Horse−>speak to ripple upward to Animal::speak, calling back to Horse::sound to get the specific sound, and gives us this output:

a Horse goes neigh!

But all Horse objects would have to be absolutely identical. If we add a method, all horses automatically share it. That’s great for making identical horses, but how do we capture the properties of an individual horse? For example, suppose we want to give our horse a name. There’s got to be a way to keep its name separate from those of other horses.

We can do so by establishing an instance. An instance is generally created by a class, much like a car is created by a car factory. An instance will have associated properties, called instance variables (or ...