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Programming Perl, 3rd Edition by Jon Orwant, Tom Christiansen, Larry Wall

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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 ...

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