A.8 Answer for Chapter 9

A.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";