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and 142 contributors-
Stevan Little
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Dave Rolsky
-
Jesse Luehrs
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Shawn M Moore
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יובל קוג'מן (Yuval Kogman)
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Florian Ragwitz
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Hans Dieter Pearcey
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NAME
Moose::Cookbook::Basics::Person_BUILDARGSAndBUILD - Using BUILDARGS and BUILD to hook into object construction
VERSION
version 2.2201
SYNOPSIS
package Person; has 'ssn' => ( is => 'ro', isa => 'Str', predicate => 'has_ssn', ); has 'country_of_residence' => ( is => 'ro', isa => 'Str', default => 'usa' ); has 'first_name' => ( is => 'ro', isa => 'Str', ); has 'last_name' => ( is => 'ro', isa => 'Str', ); around BUILDARGS => sub { my $orig = shift; my $class = shift; if ( @_ == 1 && ! ref $_[0] ) { return $class->$orig(ssn => $_[0]); } else { return $class->$orig(@_); } }; sub BUILD { my $self = shift; if ( $self->country_of_residence eq 'usa' ) { die 'Cannot create a Person who lives in the USA without an ssn.' unless $self->has_ssn; } }
DESCRIPTION
This recipe demonstrates the use of
BUILDARGS
andBUILD
. By defining these methods, we can hook into the object construction process without overridingnew
.The
BUILDARGS
method is called before an object has been created. It is called as a class method, and receives all of the parameters passed to thenew
method. It is expected to do something with these arguments and return a hash reference. The keys of the hash must be attributeinit_arg
s.The primary purpose of
BUILDARGS
is to allow a class to accept something other than named arguments. In the case of ourPerson
class, we are allowing it to be called with a single argument, a social security number:my $person = Person->new('123-45-6789');
The key part of our
BUILDARGS
is this conditional:if ( @_ == 1 && ! ref $_[0] ) { return $class->$orig(ssn => $_[0]); }
By default, Moose constructors accept a list of key-value pairs, or a hash reference. We need to make sure that
$_[0]
is not a reference before assuming it is a social security number.We call the original
BUILDARGS
method to handle all the other cases. You should always do this in your ownBUILDARGS
methods, since Moose::Object provides its ownBUILDARGS
method that handles hash references and a list of key-value pairs.The
BUILD
method is called after the object is constructed, but before it is returned to the caller. TheBUILD
method provides an opportunity to check the object state as a whole. This is a good place to put logic that cannot be expressed as a type constraint on a single attribute.In the
Person
class, we need to check the relationship between two attributes,ssn
andcountry_of_residence
. We throw an exception if the object is not logically consistent.MORE CONSIDERATIONS
This recipe is made significantly simpler because all of the attributes are read-only. If the
country_of_residence
attribute were settable, we would need to check that a Person had anssn
if the new country wasusa
. This could be done with abefore
modifier.CONCLUSION
We have repeatedly discouraged overriding
new
in Moose classes. This recipe shows how you can useBUILDARGS
andBUILD
to hook into object construction without overridingnew
.The
BUILDARGS
method lets us expand on Moose's built-in parameter handling for constructors. TheBUILD
method lets us implement logical constraints across the whole object after it is created.AUTHORS
Stevan Little <stevan@cpan.org>
Dave Rolsky <autarch@urth.org>
Jesse Luehrs <doy@cpan.org>
Shawn M Moore <sartak@cpan.org>
יובל קוג'מן (Yuval Kogman) <nothingmuch@woobling.org>
Karen Etheridge <ether@cpan.org>
Florian Ragwitz <rafl@debian.org>
Hans Dieter Pearcey <hdp@cpan.org>
Chris Prather <chris@prather.org>
Matt S Trout <mstrout@cpan.org>
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.
Module Install Instructions
To install Moose, copy and paste the appropriate command in to your terminal.
cpanm Moose
perl -MCPAN -e shell install Moose
For more information on module installation, please visit the detailed CPAN module installation guide.