Programming Perl, 4th Edition

Class Inheritance

As with the rest of Perl’s object system, inheritance of one class by another requires no special syntax to be added to the language. When you invoke a method for which Perl finds no subroutine in the invocant’s package, that package’s @ISA array^[146] is examined. This is how Perl implements inheritance: each element of a given package’s @ISA array holds the name of another package, which is searched when methods are missing. For example, the following makes the Horse class a subclass of the Critter class. (We declare @ISA with our because it has to be a package variable, not a lexical declared with my.)

package Horse;
our @ISA = "Critter";

You might see this with the parent pragma, which handles @ISA for you and loads the parent class at the same time:

package Horse;
use parent qw(Critter);

The parent pragma replaces the older base pragma, which did the same thing but threw in some fields magic if it thought the superclasses used them. If you don’t know what that is, don’t worry about it (just use parent):

package Horse;
use base qw(Critter);

You should now be able to use a Horse class or object everywhere that a Critter was previously used. If your new class passes this empty subclass test, you know that Critter is a proper base class, fit for inheritance.

Suppose you have a Horse object in $steed and invoke a move method on it:

$steed–>move(10);

Because $steed is a Horse, Perl’s first choice for that method is the Horse::move subroutine. If there isn’t one, instead of ...