A.8. Answer for Chapter 9A.8.1. Exercise (Section 9.15.1)First, start the Animal package: use strict; { package Animal; use Carp qw(croak); And now for the constructor: ## constructors sub named { ref(my $class = shift) and croak "class name needed"; my $name = shift; my $self = { Name => $name, Color => $class->default_color }; bless $self, $class; } Now, for virtual methods: the methods that should be overridden in a subclass. Perl doesn't require virtual methods to be declared in the base class, but it's nice as a documentation item. ## backstops (should be overridden) sub default_color { "brown" } sub sound { croak "subclass must define a sound" } Next comes the methods that work with either a class or an instance: ## class/instance methods sub speak { my $either = shift; print $either->name, " goes ", $either->sound, "\n"; } sub name { my $either = shift; ref $either ? $either->{Name} : "an unnamed $either"; } sub color { my $either = shift; ref $either ? $either->{Color} : $either->default_color; } Finally, the methods that work only for the particular instance: ## instance-only methods sub set_name { ref(my $self = shift) or croak "instance variable needed"; $self->{Name} = shift; } sub set_color { ref(my $self = shift) or croak "instance variable needed"; $self->{Color} = shift; } } Now that you have your abstract base class, define some concrete classes that can have instances: { package Horse; our @ISA = qw(Animal); sub sound { "neigh" } } { package Sheep; our @ISA = qw(Animal); sub color { "white" } # override the default color sub sound { "baaaah" } # no Silence of the Lambs } Finally, a few lines of code to test your classes: my $tv_horse = Horse->named("Mr. Ed"); $tv_horse->set_name("Mister Ed"); $tv_horse->set_color("grey"); print $tv_horse->name, " is ", $tv_horse->color, "\n"; print Sheep->name, " colored ", Sheep->color, " goes ", Sheep->sound, "\n"; Copyright © 2003 O'Reilly & Associates. All rights reserved. |
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