Learning Perl Objects, References amp;amp; Modules [Electronic resources] نسخه متنی

10.5 Using Class Variables

What if you want to iterate over all
the animals we've made so far? Animals may exist all
over the program namespace and are lost once they're
handed back from the named constructor method.

However, you can record the created animal in a hash and iterate over
that hash. The key to the hash can be the stringified form of the
animal reference,[9] while the value can be the actual
reference, allowing you to access its name or class.

[9] Or any other convenient and unique
string.

For example,
let's extend named as
follows:

## in Animal
our %REGISTRY;
sub named {
my $class = shift;
my $name = shift;
my $self = { Name => $name, Color => $class->default_color };
bless $self, $class;
$REGISTRY{$self} = $self;  # also returns $self
}

The
uppercase name for %REGISTRY is a reminder that
this variable is more global than most variables. In this case,
it's a metavariable that contains information about
many instances.

Note that when used as a key, $self
stringifies, which means it turns into a
string unique to the object.

We also need to add a new method:

sub registered {
return map { "a ".ref($_)." named ".$_->name } values %REGISTRY;
}

Now you can see all the animals we've made:

my @cows = map Cow->named($_), qw(Bessie Gwen);
my @horses = map Horse->named($_), ("Trigger", "Mr. Ed");
my @racehorses = RaceHorse->named("Billy Boy");
print "We've seen:\n", map("  $_\n", Animal->registered);
print "End of program.\n";

This prints:

We've seen:
a RaceHorse named Billy Boy
a Horse named Mr. Ed
a Horse named Trigger
a Cow named Gwen
a Cow named Bessie
End of program.
[Billy Boy has died.]
[Billy Boy has gone off to the glue factory.]
[Bessie has died.]
[Gwen has died.]
[Trigger has died.]
[Trigger has gone off to the glue factory.]
[Mr. Ed has died.]
[Mr. Ed has gone off to the glue factory.]

Note that the animals die at their proper time because the variables
holding the animals are all being destroyed at the final step. Or are
they?