Linux cli command Moose_Cookbook_Meta_GlobRef_InstanceMetaclasspm

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NAME 🖥️ Moose_Cookbook_Meta_GlobRef_InstanceMetaclasspm 🖥️

Creating a glob reference meta-instance class

VERSION

version 2.2207

SYNOPSIS

package My::Meta::Instance; use Scalar::Util qw( weaken ); use Symbol qw( gensym ); use Moose::Role; 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, $slot_name ) = @_; exists *$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 Moose; Moose::Util::MetaRole::apply_metaroles( for => _ _PACKAGE_ _, class_metaroles => { instance => [My::Meta::Instance], }, ); 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 extension is a role which will be applied to 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_slot_access { my ( $self, $instance, $slot_name ) = @_; return *{ . $instance . }->{ . $slot_name . }; }

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:

Moose::Util::MetaRole::apply_metaroles( for => _ _PACKAGE_ _, class_metaroles => { instance => [My::Meta::Instance], }, );

We actually don’t recommend the use of Moose::Util::MetaRole directly in your class in most cases. Typically, this would be provided by a Moose::Exporter-based module which handles applying the role for you.

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.

AUTHORS

COPYRIGHT AND LICENSE

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.

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