Moose::Cookbook::Meta::Recipe7(3)

NAME
Moose::Cookbook::Meta::Recipe7 - Creating a glob reference meta-instance class

VERSION
version 2.0205

SYNOPSIS
  package My::Meta::Instance;

  use Scalar::Util qw( weaken );
  use Symbol qw( gensym );

  use Moose;
  extends 'Moose::Meta::Instance';

  sub create_instance {
      my $self = shift;
      my $sym = gensym();
      bless $sym, $self->_class_name;
  }

  sub clone_instance {
      my ( $self, $instance ) = @_;

      my $new_sym = gensym();
      %{*$new_sym} = %{*$instance};

      bless $new_sym, $self->_class_name;
  }

  sub get_slot_value {
      my ( $self, $instance, $slot_name ) = @_;
      return *$instance->{$slot_name};
  }

  sub set_slot_value {
      my ( $self, $instance, $slot_name, $value ) = @_;
      *$instance->{$slot_name} = $value;
  }

  sub deinitialize_slot {
      my ( $self, $instance, $slot_name ) = @_;
      delete *$instance->{$slot_name};;
  }

  sub is_slot_initialized {
      my ( $self, $instance, $slot_name, $value ) = @_;
      exists *$instance->{$slot_name};;
  }

  sub weaken_slot_value {
      my ( $self, $instance, $slot_name ) = @_;
      weaken *$instance->{$slot_name};;
  }

  sub inline_create_instance {
      my ( $self, $class_variable ) = @_;
      return 'do { my $sym = Symbol::gensym(); bless $sym, ' . $class_variable . ' }';
  }

  sub inline_slot_access {
      my ( $self, $instance, $slot_name ) = @_;
      return '*{' . $instance . '}->{' . $slot_name . '}';
  }

  package MyApp::User;

  use metaclass 'Moose::Meta::Class' =>
      ( instance_metaclass => 'My::Meta::Instance' );

  use Moose;

  has 'name' => (
      is  => 'rw',
      isa => 'Str',
  );

  has 'email' => (
      is  => 'rw',
      isa => 'Str',
  );

DESCRIPTION
This recipe shows how to build your own meta-instance. The meta instance is the metaclass that creates object instances and helps manages access to attribute slots.

In this example, we're creating a meta-instance that is based on a glob reference rather than a hash reference. This example is largely based on the Piotr Roszatycki's MooseX::GlobRef module.

Our class is a subclass of Moose::Meta::Instance, which creates hash reference based objects. We need to override all the methods which make assumptions about the object's data structure.

The first method we override is "create_instance":

  sub create_instance {
      my $self = shift;
      my $sym = gensym();
      bless $sym, $self->_class_name;
  }

This returns an glob reference which has been blessed into our meta-instance's associated class.

We also override "clone_instance" to create a new array reference:

  sub clone_instance {
      my ( $self, $instance ) = @_;

      my $new_sym = gensym();
      %{*$new_sym} = %{*$instance};

      bless $new_sym, $self->_class_name;
  }

After that, we have a series of methods which mediate access to the object's slots (attributes are stored in "slots"). In the default instance class, these expect the object to be a hash reference, but we need to change this to expect a glob reference instead.

  sub get_slot_value {
      my ( $self, $instance, $slot_name ) = @_;
      *$instance->{$slot_name};;
  }

This level of indirection probably makes our instance class slower than the default. However, when attribute access is inlined, this lookup will be cached:

  sub inline_create_instance {
      my ( $self, $class_variable ) = @_;
      return 'do { my $sym = Symbol::gensym(); bless $sym, ' . $class_variable . ' }';
  }

The code snippet that the "inline_slot_access" method returns will get "eval"'d once per attribute.

Finally, we use this meta-instance in our "MyApp::User" class:

  use metaclass 'Moose::Meta::Class' =>
      ( instance_metaclass => 'My::Meta::Instance' );

We actually don't recommend the use of metaclass in most cases. However, the other ways of using alternate metaclasses are more complex, and would complicate our example code unnecessarily.

CONCLUSION
This recipe shows how to create your own meta-instance class. It's unlikely that you'll need to do this yourself, but it's interesting to take a peek at how Moose works under the hood.

SEE ALSO
There are a few meta-instance class extensions on CPAN:

•   MooseX::Singleton

    This module extends the instance class in order to ensure that the object is a singleton. The instance it uses is still a blessed hash reference.

•   MooseX::GlobRef

    This module makes the instance a blessed glob reference. This lets you use a handle as an object instance.

AUTHOR
Stevan Little <stevan@iinteractive.com>

COPYRIGHT AND LICENSE
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.