Programming Perl, 3rd 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^[5] 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 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 raising a run-time exception, Perl consults the first element of @Horse::ISA, which directs it to look in the Critter package for Critter::move. If this subroutine isn't found either, and Critter has its own @Critter::ISA array, then that too will be consulted for the name of an ancestral package that might supply a move method, and so on back up the inheritance hierarchy until we come to a package without ...