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Author image Dave Rolsky
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Moose::Cookbook::Extending::Recipe3 - Providing an alternate base object class


  package MyApp::Base;
  use Moose;

  extends 'Moose::Object';

  before 'new' => sub { warn "Making a new " . $_[0] };

  no Moose;

  package MyApp::UseMyBase;
  use Moose ();
  use Moose::Exporter;

  Moose::Exporter->setup_import_methods( also => 'Moose' );

  sub init_meta {
      return Moose->init_meta( @_, base_class => 'MyApp::Base' );


A common extension is to provide an alternate base class. One way to do that is to make a MyApp::base and add extends 'MyApp::Base' to every class in your application. That's pretty tedious. Instead, you can create a Moose-alike module that sets the base object class to MyApp::Base for you.

Then, instead of writing use Moose you can write use MyApp::UseMyBase.

In this particular example, our base class issues some debugging output every time a new object is created, but you can think of some more interesting things to do with your own base class.

This uses the magic of Moose::Exporter. When we call Moose::Exporter->setup_import_methods( also => 'Moose' ) it builds import and unimport methods for you. The also => 'Moose' bit says that we want to export everything that Moose does.

The import method that gets created will call our init_meta method, passing it for_caller => $caller as its arguments. The $caller is set to the class that actually imported us in the first place.

See the Moose::Exporter docs for more details on its API.

USING MyApp::UseMyBase

To actually use our new base class, we simply use MyApp::UseMyBase instead of Moose. We get all the Moose sugar plus our new base class.

  package Foo;

  use MyApp::UseMyBase;

  has 'size' => ( is => 'rw' );

  no MyApp::UseMyBase;


This is an awful lot of magic for a simple base class. You will often want to combine a metaclass trait with a base class extension, and that's when this technique is useful.


Dave Rolsky <autarch@urth.org>


Copyright 2006-2010 by Infinity Interactive, Inc.


This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.