package Wheel;

# class for storing information about a Wheel

use warnings;
use strict;
use Carp;

sub new {
	my $class = shift;
	my $name = shift;
	my $self = {
		NAME => $name,
	};
	my $closure = sub {
		my $field = shift;
		if (@_) { $self->{$field} = shift; }
		return $self->{$field};
	};
	bless ($closure,$class);
	return $closure;
}

# public accessors
sub name   { &{ $_[0] }("NAME", @_[1 .. $#_]) }

sub accept {
	my $self = shift;
	my $visitor = shift;
	$visitor->visitWheel($self);
}

1;

##</code><code>##

package Body;

# class for storing information about a Body

use warnings;
use strict;
use Carp;

sub new {
	my $class = shift;
	my $self = {};
	bless $self, $class;
	return $self;
}

sub accept {
	my $self = shift;
	my $visitor = shift;
	$visitor->visitBody($self);
}

1;

##</code><code>##

package Engine;

# class for storing information about an Engine

use warnings;
use strict;
use Carp;

sub new {
	my $class = shift;
	my $self = {};
	bless $self,$class;
	return $self;
}

sub accept {
	my $self = shift;
	my $visitor = shift;
	$visitor->visitEngine($self);
}

1;

##</code><code>##

package Car;

# class for storing information about a Car 

use warnings;
use strict;
use Carp;

sub new {
	my $class = shift;
	my $self = {
		ENGINE => undef, 
		BODY => undef,
		WHEELS => undef, # a list of wheels
	};
	my $closure = sub {
		my $field = shift;
		if (@_) { $self->{$field} = shift; }
		return $self->{$field};
	};
	bless ($closure,$class);
	return $closure;
}

# public accessors
sub engine { &{ $_[0] }("ENGINE", @_[1 .. $#_]) }
sub body   { &{ $_[0] }("BODY", @_[1 .. $#_]) }
sub wheels { &{ $_[0] }("WHEELS", @_[1 .. $#_]) }

sub accept {
	my $self = shift;
	my $visitor = shift;
	$visitor->visitCar($self);
	$self->engine->accept($visitor);
	$self->body->accept($visitor);
 	map { $_->accept($visitor) } @{$self->wheels};
}

1;

##</code><code>##

package PrintVisitor;

# class for grouping together all the different print messages for car parts 

use warnings;
use strict;
use Carp;

sub new {
	my $class = shift;
	my $self = {};
	bless $self,$class;
	return $self;
}

sub visitWheel {
	my $self = shift;
	my $wheel = shift;
	print "Visiting " . $wheel->name . " wheel\n";
}

sub visitEngine {
	my $self = shift;
	my $engine = shift;
	print "Visiting engine\n";
}

sub visitBody {
	my $self = shift;
	my $body = shift;
	print "Visiting body\n";
}

sub visitCar {
	my $self = shift;
	my $car = shift;
	print "Visiting car\n";
}

1;

##</code><code>##

#!/usr/bin/perl

# Using the visitor class

use strict;
use warnings;

use Car;
use PrintVisitor;
use Wheel;
use Body;
use Engine;

my $car = Car->new();
$car->engine( Engine->new() );
$car->body( Body->new() );
$car->wheels( [Wheel->new("front left"), 
               Wheel->new("front right"), 
               Wheel->new("back left"), 
               Wheel->new("back right")] );

my $visitor = PrintVisitor->new();
$car->accept($visitor);