1Moose::Cookbook::BasicsU:s:ePreMrCosooonsnte_r:Bi:UbCIuoLtoDekAdbRoGPoSekAr:nl:dBBDaUosIciLucDms(e:3n:)tPaetrisoonn_BUILDARGSAndBUILD(3)
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NAME

6       Moose::Cookbook::Basics::Person_BUILDARGSAndBUILD - Using BUILDARGS and
7       BUILD to hook into object construction
8

VERSION

10       version 2.2015
11

SYNOPSIS

13         package Person;
14
15         has 'ssn' => (
16             is        => 'ro',
17             isa       => 'Str',
18             predicate => 'has_ssn',
19         );
20
21         has 'country_of_residence' => (
22             is      => 'ro',
23             isa     => 'Str',
24             default => 'usa'
25         );
26
27         has 'first_name' => (
28             is  => 'ro',
29             isa => 'Str',
30         );
31
32         has 'last_name' => (
33             is  => 'ro',
34             isa => 'Str',
35         );
36
37         around BUILDARGS => sub {
38             my $orig = shift;
39             my $class = shift;
40
41             if ( @_ == 1 && ! ref $_[0] ) {
42                 return $class->$orig(ssn => $_[0]);
43             }
44             else {
45                 return $class->$orig(@_);
46             }
47         };
48
49         sub BUILD {
50             my $self = shift;
51
52             if ( $self->country_of_residence eq 'usa' ) {
53                 die 'Cannot create a Person who lives in the USA without an ssn.'
54                     unless $self->has_ssn;
55             }
56         }
57

DESCRIPTION

59       This recipe demonstrates the use of "BUILDARGS" and "BUILD". By
60       defining these methods, we can hook into the object construction
61       process without overriding "new".
62
63       The "BUILDARGS" method is called before an object has been created. It
64       is called as a class method, and receives all of the parameters passed
65       to the "new" method. It is expected to do something with these
66       arguments and return a hash reference. The keys of the hash must be
67       attribute "init_arg"s.
68
69       The primary purpose of "BUILDARGS" is to allow a class to accept
70       something other than named arguments. In the case of our "Person"
71       class, we are allowing it to be called with a single argument, a social
72       security number:
73
74         my $person = Person->new('123-45-6789');
75
76       The key part of our "BUILDARGS" is this conditional:
77
78             if ( @_ == 1 && ! ref $_[0] ) {
79                 return $class->$orig(ssn => $_[0]);
80             }
81
82       By default, Moose constructors accept a list of key-value pairs, or a
83       hash reference. We need to make sure that $_[0] is not a reference
84       before assuming it is a social security number.
85
86       We call the original "BUILDARGS" method to handle all the other cases.
87       You should always do this in your own "BUILDARGS" methods, since
88       Moose::Object provides its own "BUILDARGS" method that handles hash
89       references and a list of key-value pairs.
90
91       The "BUILD" method is called after the object is constructed, but
92       before it is returned to the caller. The "BUILD" method provides an
93       opportunity to check the object state as a whole. This is a good place
94       to put logic that cannot be expressed as a type constraint on a single
95       attribute.
96
97       In the "Person" class, we need to check the relationship between two
98       attributes, "ssn" and "country_of_residence". We throw an exception if
99       the object is not logically consistent.
100

MORE CONSIDERATIONS

102       This recipe is made significantly simpler because all of the attributes
103       are read-only. If the "country_of_residence" attribute were settable,
104       we would need to check that a Person had an "ssn" if the new country
105       was "usa". This could be done with a "before" modifier.
106

CONCLUSION

108       We have repeatedly discouraged overriding "new" in Moose classes. This
109       recipe shows how you can use "BUILDARGS" and "BUILD" to hook into
110       object construction without overriding "new".
111
112       The "BUILDARGS" method lets us expand on Moose's built-in parameter
113       handling for constructors. The "BUILD" method lets us implement logical
114       constraints across the whole object after it is created.
115

AUTHORS

117       •   Stevan Little <stevan@cpan.org>
118
119       •   Dave Rolsky <autarch@urth.org>
120
121       •   Jesse Luehrs <doy@cpan.org>
122
123       •   Shawn M Moore <sartak@cpan.org>
124
125       •   יובל קוג'מן (Yuval Kogman) <nothingmuch@woobling.org>
126
127       •   Karen Etheridge <ether@cpan.org>
128
129       •   Florian Ragwitz <rafl@debian.org>
130
131       •   Hans Dieter Pearcey <hdp@cpan.org>
132
133       •   Chris Prather <chris@prather.org>
134
135       •   Matt S Trout <mstrout@cpan.org>
136

138       This software is copyright (c) 2006 by Infinity Interactive, Inc.
139
140       This is free software; you can redistribute it and/or modify it under
141       the same terms as the Perl 5 programming language system itself.
142
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145perl v5.34.0              Moose::C2o0o2k1b-o0o7k-:2:2Basics::Person_BUILDARGSAndBUILD(3)
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