Accessing Class Data

It turns out that this is not really a good way to go about handling class data. A good scalable rule is that you must never reference class data directly from an object method. Otherwise you aren't building a scalable, inheritable class. The object must be the rendezvous point for all operations, especially from an object method. The globals (class data) would in some sense be in the ``wrong'' package in your derived classes. In Perl, methods execute in the context of the class they were defined in, not that of the object that triggered them. Therefore, namespace visibility of package globals in methods is unrelated to inheritance.

Got that? Maybe not. Ok, let's say that some other class ``borrowed'' (well, inherited) the DESTROY method as it was defined above. When those objects are destroyed, the original $Census variable will be altered, not the one in the new class's package namespace. Perhaps this is what you want, but probably it isn't.

Here's how to fix this. We'll store a reference to the data in the value accessed by the hash key ``_CENSUS''. Why the underscore? Well, mostly because an initial underscore already conveys strong feelings of magicalness to a C programmer. It's really just a mnemonic device to remind ourselves that this field is special and not to be used as a public data member in the same way that NAME, AGE, and PEERS are. (Because we've been developing this code under the strict pragma, prior to perl version 5.004 we'll have to quote the field name.)

    sub new {
        my $proto = shift;
        my $class = ref($proto) || $proto;
        my $self  = {};
        $self->{NAME}     = undef;
        $self->{AGE}      = undef;
        $self->{PEERS}    = [];
        # "private" data
        $self->{"_CENSUS"} = \$Census;
        bless ($self, $class);
        ++ ${ $self->{"_CENSUS"} };
        return $self;

    sub population {
        my $self = shift;
        if (ref $self) {
            return ${ $self->{"_CENSUS"} };
        } else {
            return $Census;

    sub DESTROY {
        my $self = shift;
        -- ${ $self->{"_CENSUS"} };