Scroll to navigation

Moose::Cookbook::Extending::Debugging_BaseClassRole(3pm) User Contributed Perl Documentation Moose::Cookbook::Extending::Debugging_BaseClassRole(3pm)


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


version 2.2013


  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 <>
  • Dave Rolsky <>
  • Jesse Luehrs <>
  • Shawn M Moore <>
  • יובל קוג'מן (Yuval Kogman) <>
  • Karen Etheridge <>
  • Florian Ragwitz <>
  • Hans Dieter Pearcey <>
  • Chris Prather <>
  • Matt S Trout <>


This software is copyright (c) 2006 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.

2020-07-23 perl v5.30.3