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Florian Ragwitz > Moose-2.0204 > Moose::Cookbook::Extending::Recipe2


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Moose::Cookbook::Extending::Recipe2 - Providing a role for the base object class


version 2.0204


  package MooseX::Debugging;

  use Moose::Exporter;

      base_class_roles => ['MooseX::Debugging::Role::Object'],

  package MooseX::Debugging::Role::Object;

  use Moose::Role;

  sub BUILD {}
  after BUILD => sub {
      my $self = shift;

      warn "Made a new " . ( ref $self ) . " object\n";


In this example, we provide a role for the base object class that adds some simple debugging output. Every time an object is created, it spits out a warning saying what type of object it was.

Obviously, a real debugging role would do something more interesting, but this recipe is all about how we apply that role.

In this case, with the combination of Moose::Exporter and Moose::Util::MetaRole, we ensure that when a module does use MooseX::Debugging, it automatically gets the debugging role applied to its base object class.

There are a few pieces of code worth looking at more closely.

      base_class_roles => ['MooseX::Debugging::Role::Object'],

This creates an import method in the MooseX::Debugging package. Since we are not actually exporting anything, we do not pass setup_import_methods any parameters related to exports, but we need to have an import method to ensure that our init_meta method is called. The init_meta is created by setup_import_methods for us, since we passed the base_class_roles parameter. The generated init_meta will in turn call Moose::Util::MetaRole::apply_base_class_roles.

  sub BUILD {}
  after BUILD => sub {

Due to the way role composition currently works, if the class that a role is composed into contains a BUILD method, then that will override the BUILD method in any roles it composes, which is typically not what you want. Using a method modifier on BUILD avoids this issue, since method modifiers compose together rather than being overridden. Method modifiers require that a method exists in order to wrap, however, so we also provide a stub method to wrap if no BUILD method exists in the class.


Stevan Little <>


This software is copyright (c) 2011 by Infinity Interactive, Inc..

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.

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