Autoloaded Data Methods

You probably began to get a little suspicious about the duplicated code way back earlier when we first showed you the Person class, and then later the Employee class. Each method used to access the hash fields looked virtually identical. This should have tickled that great programming virtue, Impatience, but for the time, we let Laziness win out, and so did nothing. Proxy methods can cure this.

Instead of writing a new function every time we want a new data field, we'll use the autoload mechanism to generate (actually, mimic) methods on the fly. To verify that we're accessing a valid member, we will check against an _permitted (pronounced ``under-permitted'') field, which is a reference to a file-scoped lexical (like a C file static) hash of permitted fields in this record called %fields. Why the underscore? For the same reason as the _CENSUS field we once used: as a marker that means ``for internal use only''.

Here's what the module initialization code and class constructor will look like when taking this approach: 

    package Person;
    use Carp;
    use vars qw($AUTOLOAD);  # it's a package global


    my %fields = (
	name        => undef,
	age         => undef,
	peers       => undef,
    );


    sub new {
	my $that  = shift;
	my $class = ref($that) || $that;
	my $self  = {
	    _permitted => \%fields,
	    %fields,
	};
	bless $self, $class;
	return $self;
    }

If we wanted our record to have default values, we could fill those in where current we have undef in the %fields hash.

Notice how we saved a reference to our class data on the object itself? Remember that it's important to access class data through the object itself instead of having any method reference %fields directly, or else you won't have a decent inheritance.

The real magic, though, is going to reside in our proxy method, which will handle all calls to undefined methods for objects of class Person (or subclasses of Person). It has to be called AUTOLOAD. Again, it's all caps because it's called for us implicitly by Perl itself, not by a user directly. 

    sub AUTOLOAD {
	my $self = shift;
	my $type = ref($self)
		    or croak "$self is not an object";


	my $name = $AUTOLOAD;
	$name =~ s/.*://;   # strip fully-qualified portion


	unless (exists $self->{_permitted}->{$name} ) {
	    croak "Can't access `$name' field in class $type";
	}


	if (@_) {
	    return $self->{$name} = shift;
	} else {
	    return $self->{$name};
	}
    }

Pretty nifty, eh? All we have to do to add new data fields is modify %fields. No new functions need be written.

I could have avoided the _permitted field entirely, but I wanted to demonstrate how to store a reference to class data on the object so you wouldn't have to access that class data directly from an object method.