package Class::Data::Inheritable; use strict qw(vars subs); use vars qw($VERSION); $VERSION = '0.02'; =pod =head1 NAME Class::Data::Inheritable - Inheritable, overridable class data =head1 SYNOPSIS package Stuff; use base qw(Class::Data::Inheritable); # Set up DataFile as inheritable class data. Stuff->mk_classdata('DataFile'); # Declare the location of the data file for this class. Stuff->DataFile('/etc/stuff/data'); =head1 DESCRIPTION Class::Data::Inheritable is for creating accessor/mutators to class data. That is, if you want to store something about your class as a whole (instead of about a single object). This data is then inherited by your subclasses and can be overriden. For example: Pere::Ubu->mk_classdata('Suitcase'); will generate the method Suitcase() in the class Pere::Ubu. This new method can be used to get and set a piece of class data. Pere::Ubu->Suitcase('Red'); $suitcase = Pere::Ubu->Suitcase; The interesting part happens when a class inherits from Pere::Ubu: package Raygun; use base qw(Pere::Ubu); # Raygun's suitcase is Red. $suitcase = Raygun->Suitcase; Raygun inherits its Suitcase class data from Pere::Ubu. Inheritance of class data works analgous to method inheritance. As long as Raygun does not "override" its inherited class data (by using Suitcase() to set a new value) it will continue to use whatever is set in Pere::Ubu and inherit further changes: # Both Raygun's and Pere::Ubu's suitcases are now Blue Pere::Ubu->Suitcase('Blue'); However, should Raygun decide to set its own Suitcase() it has now "overridden" Pere::Ubu and is on its own, just like if it had overriden a method: # Raygun has an orange suitcase, Pere::Ubu's is still Blue. Raygun->Suitcase('Orange'); Now that Raygun has overridden Pere::Ubu futher changes by Pere::Ubu no longer effect Raygun. # Raygun still has an orange suitcase, but Pere::Ubu is using Samsonite. Pere::Ubu->Suitcase('Samsonite'); =head1 Methods =over 4 =item B<mk_classdata> Class->mk_classdata($data_accessor_name); This is a class method used to declare new class data accessors. A new accessor will be created in the Class using the name from $data_accessor_name. To facilitate overriding, mk_classdata creates an alias to the accessor, _field_accessor(). So Suitcase() would have an alias _Suitcase_accessor() that does the exact same thing as Suitcase(). This is useful if you want to alter the behavior of a single accessor yet still get the benefits of inheritable class data. For example. sub Suitcase { my($self) = shift; warn "Fashion tragedy" if @_ and $_[0] eq 'Plaid'; $self->_Suitcase_accessor(@_); } =cut sub mk_classdata { my ($declaredclass, $attribute, $data) = @_; my $accessor = sub { my $wantclass = ref($_[0]) || $_[0]; return $wantclass->mk_classdata($attribute)->(@_) if @_>1 && $wantclass ne $declaredclass; $data = $_[1] if @_>1; return $data; }; my $alias = "_${attribute}_accessor"; *{$declaredclass.'::'.$attribute} = $accessor; *{$declaredclass.'::'.$alias} = $accessor; } =pod =head1 COPYRIGHT Copyright (c) 2000, Damian Conway and Michael G Schwern. All Rights Reserved. This module is free software. It may be used, redistributed and/or modified under the terms of the Perl Artistic License (see http://www.perl.com/perl/misc/Artistic.html) =head1 AUTHOR Original code by Damian Conway. Maintained by Michael G Schwern <schwern@pobox.com> =head1 SEE ALSO L<perltootc> has a very elaborate discussion of class data in Perl. =cut 1;