package Class::Data::Inheritable;

use strict qw(vars subs);
use vars qw($VERSION);
$VERSION = '0.09';

sub mk_classdata {
    my ($declaredclass, $attribute, $data) = @_;

    if( ref $declaredclass ) {
        require Carp;
        Carp::croak("mk_classdata() is a class method, not an object method");

    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;



=head1 NAME

Class::Data::Inheritable - Inheritable, overridable class data


  package Stuff;
  use base qw(Class::Data::Inheritable);

  # Set up DataFile as inheritable class data.

  # Declare the location of the data file for this class.

  # Or, all in one shot:
  Stuff->mk_classdata(DataFile => '/etc/stuff/data');


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 overridden.

For example:


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.

  $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 analogous 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

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
overridden a method:

  # Raygun has an orange suitcase, Pere::Ubu's is still Blue.

Now that Raygun has overridden Pere::Ubu further changes by Pere::Ubu
no longer effect Raygun.

  # Raygun still has an orange suitcase, but Pere::Ubu is using Samsonite.

=head1 Methods

=head2 mk_classdata

  Class->mk_classdata($data_accessor_name => $value);

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, and optionally initially setting it to the given

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';


=head1 AUTHOR

Original code by Damian Conway.

Maintained by Michael G Schwern until September 2005.

Now maintained by Tony Bowden.

=head1 BUGS and QUERIES

Please direct all correspondence regarding this module to:


Copyright (c) 2000-2005, Damian Conway and Michael G Schwern. 
All Rights Reserved.  

This module is free software. It may be used, redistributed and/or
modified under the same terms as Perl itself.

=head1 SEE ALSO

L<perltooc> has a very elaborate discussion of class data in Perl.