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.IX Title "Moose::Cookbook::Meta::GlobRef_InstanceMetaclass 3pm"
.TH Moose::Cookbook::Meta::GlobRef_InstanceMetaclass 3pm "2012-06-28" "perl v5.14.2" "User Contributed Perl Documentation"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
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.nh
.SH "NAME"
Moose::Cookbook::Meta::GlobRef_InstanceMetaclass \- Creating a glob reference meta\-instance class
.SH "VERSION"
.IX Header "VERSION"
version 2.0603
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\&  package My::Meta::Instance;
\&
\&  use Scalar::Util qw( weaken );
\&  use Symbol qw( gensym );
\&
\&  use Moose;
\&  extends \*(AqMoose::Meta::Instance\*(Aq;
\&
\&  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 \*(Aqdo { my $sym = Symbol::gensym(); bless $sym, \*(Aq . $class_variable . \*(Aq }\*(Aq;
\&  }
\&
\&  sub inline_slot_access {
\&      my ( $self, $instance, $slot_name ) = @_;
\&      return \*(Aq*{\*(Aq . $instance . \*(Aq}\->{\*(Aq . $slot_name . \*(Aq}\*(Aq;
\&  }
\&
\&  package MyApp::User;
\&
\&  use metaclass \*(AqMoose::Meta::Class\*(Aq =>
\&      ( instance_metaclass => \*(AqMy::Meta::Instance\*(Aq );
\&
\&  use Moose;
\&
\&  has \*(Aqname\*(Aq => (
\&      is  => \*(Aqrw\*(Aq,
\&      isa => \*(AqStr\*(Aq,
\&  );
\&
\&  has \*(Aqemail\*(Aq => (
\&      is  => \*(Aqrw\*(Aq,
\&      isa => \*(AqStr\*(Aq,
\&  );
.Ve
.SH "DESCRIPTION"
.IX Header "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.
.PP
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.
.PP
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.
.PP
The first method we override is \f(CW\*(C`create_instance\*(C'\fR:
.PP
.Vb 5
\&  sub create_instance {
\&      my $self = shift;
\&      my $sym = gensym();
\&      bless $sym, $self\->_class_name;
\&  }
.Ve
.PP
This returns an glob reference which has been blessed into our
meta-instance's associated class.
.PP
We also override \f(CW\*(C`clone_instance\*(C'\fR to create a new array reference:
.PP
.Vb 2
\&  sub clone_instance {
\&      my ( $self, $instance ) = @_;
\&
\&      my $new_sym = gensym();
\&      %{*$new_sym} = %{*$instance};
\&
\&      bless $new_sym, $self\->_class_name;
\&  }
.Ve
.PP
After that, we have a series of methods which mediate access to the
object's slots (attributes are stored in \*(L"slots\*(R"). 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.
.PP
.Vb 4
\&  sub get_slot_value {
\&      my ( $self, $instance, $slot_name ) = @_;
\&      *$instance\->{$slot_name};
\&  }
.Ve
.PP
This level of indirection probably makes our instance class \fIslower\fR
than the default. However, when attribute access is inlined, this
lookup will be cached:
.PP
.Vb 4
\&  sub inline_slot_access {
\&      my ( $self, $instance, $slot_name ) = @_;
\&      return \*(Aq*{\*(Aq . $instance . \*(Aq}\->{\*(Aq . $slot_name . \*(Aq}\*(Aq;
\&  }
.Ve
.PP
The code snippet that the \f(CW\*(C`inline_slot_access\*(C'\fR method returns will
get \f(CW\*(C`eval\*(C'\fR'd once per attribute.
.PP
Finally, we use this meta-instance in our \f(CW\*(C`MyApp::User\*(C'\fR class:
.PP
.Vb 2
\&  use metaclass \*(AqMoose::Meta::Class\*(Aq =>
\&      ( instance_metaclass => \*(AqMy::Meta::Instance\*(Aq );
.Ve
.PP
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.
.SH "CONCLUSION"
.IX Header "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.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
There are a few meta-instance class extensions on \s-1CPAN:\s0
.IP "\(bu" 4
MooseX::Singleton
.Sp
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.
.IP "\(bu" 4
MooseX::GlobRef
.Sp
This module makes the instance a blessed glob reference. This lets you
use a handle as an object instance.
.SH "AUTHOR"
.IX Header "AUTHOR"
Moose is maintained by the Moose Cabal, along with the help of many contributors. See \*(L"\s-1CABAL\s0\*(R" in Moose and \*(L"\s-1CONTRIBUTORS\s0\*(R" in Moose for details.
.SH "COPYRIGHT AND LICENSE"
.IX Header "COPYRIGHT AND LICENSE"
This software is copyright (c) 2012 by Infinity Interactive, Inc..
.PP
This is free software; you can redistribute it and/or modify it under
the same terms as the Perl 5 programming language system itself.