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.IX Title "Moose::Cookbook::Basics::Person_BUILDARGSAndBUILD 3pm"
.TH Moose::Cookbook::Basics::Person_BUILDARGSAndBUILD 3pm 2024-01-21 "perl v5.38.2" "User Contributed Perl Documentation"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
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.SH NAME
Moose::Cookbook::Basics::Person_BUILDARGSAndBUILD \- Using BUILDARGS and BUILD to hook into object construction
.SH VERSION
.IX Header "VERSION"
version 2.2207
.SH SYNOPSIS
.IX Header "SYNOPSIS"
.Vb 1
\&  package Person;
\&
\&  has \*(Aqssn\*(Aq => (
\&      is        => \*(Aqro\*(Aq,
\&      isa       => \*(AqStr\*(Aq,
\&      predicate => \*(Aqhas_ssn\*(Aq,
\&  );
\&
\&  has \*(Aqcountry_of_residence\*(Aq => (
\&      is      => \*(Aqro\*(Aq,
\&      isa     => \*(AqStr\*(Aq,
\&      default => \*(Aqusa\*(Aq
\&  );
\&
\&  has \*(Aqfirst_name\*(Aq => (
\&      is  => \*(Aqro\*(Aq,
\&      isa => \*(AqStr\*(Aq,
\&  );
\&
\&  has \*(Aqlast_name\*(Aq => (
\&      is  => \*(Aqro\*(Aq,
\&      isa => \*(AqStr\*(Aq,
\&  );
\&
\&  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 \*(Aqusa\*(Aq ) {
\&          die \*(AqCannot create a Person who lives in the USA without an ssn.\*(Aq
\&              unless $self\->has_ssn;
\&      }
\&  }
.Ve
.SH DESCRIPTION
.IX Header "DESCRIPTION"
This recipe demonstrates the use of \f(CW\*(C`BUILDARGS\*(C'\fR and \f(CW\*(C`BUILD\*(C'\fR. By
defining these methods, we can hook into the object construction
process without overriding \f(CW\*(C`new\*(C'\fR.
.PP
The \f(CW\*(C`BUILDARGS\*(C'\fR method is called \fIbefore\fR an object has been
created. It is called as a class method, and receives all of the
parameters passed to the \f(CW\*(C`new\*(C'\fR method. It is expected to do something
with these arguments and return a hash reference. The keys of the hash
must be attribute \f(CW\*(C`init_arg\*(C'\fRs.
.PP
The primary purpose of \f(CW\*(C`BUILDARGS\*(C'\fR is to allow a class to accept
something other than named arguments. In the case of our \f(CW\*(C`Person\*(C'\fR
class, we are allowing it to be called with a single argument, a
social security number:
.PP
.Vb 1
\&  my $person = Person\->new(\*(Aq123\-45\-6789\*(Aq);
.Ve
.PP
The key part of our \f(CW\*(C`BUILDARGS\*(C'\fR is this conditional:
.PP
.Vb 3
\&      if ( @_ == 1 && ! ref $_[0] ) {
\&          return $class\->$orig(ssn => $_[0]);
\&      }
.Ve
.PP
By default, Moose constructors accept a list of key-value pairs, or a
hash reference. We need to make sure that \f(CW$_[0]\fR is not a reference
before assuming it is a social security number.
.PP
We call the original \f(CW\*(C`BUILDARGS\*(C'\fR method to handle all the other
cases. You should always do this in your own \f(CW\*(C`BUILDARGS\*(C'\fR methods,
since Moose::Object provides its own \f(CW\*(C`BUILDARGS\*(C'\fR method that
handles hash references and a list of key-value pairs.
.PP
The \f(CW\*(C`BUILD\*(C'\fR method is called \fIafter\fR the object is constructed, but
before it is returned to the caller. The \f(CW\*(C`BUILD\*(C'\fR 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.
.PP
In the \f(CW\*(C`Person\*(C'\fR class, we need to check the relationship between two
attributes, \f(CW\*(C`ssn\*(C'\fR and \f(CW\*(C`country_of_residence\*(C'\fR. We throw an exception
if the object is not logically consistent.
.SH "MORE CONSIDERATIONS"
.IX Header "MORE CONSIDERATIONS"
This recipe is made significantly simpler because all of the
attributes are read-only. If the \f(CW\*(C`country_of_residence\*(C'\fR attribute
were settable, we would need to check that a Person had an \f(CW\*(C`ssn\*(C'\fR if
the new country was \f(CW\*(C`usa\*(C'\fR. This could be done with a \f(CW\*(C`before\*(C'\fR
modifier.
.SH CONCLUSION
.IX Header "CONCLUSION"
We have repeatedly discouraged overriding \f(CW\*(C`new\*(C'\fR in Moose
classes. This recipe shows how you can use \f(CW\*(C`BUILDARGS\*(C'\fR and \f(CW\*(C`BUILD\*(C'\fR
to hook into object construction without overriding \f(CW\*(C`new\*(C'\fR.
.PP
The \f(CW\*(C`BUILDARGS\*(C'\fR method lets us expand on Moose's built-in parameter
handling for constructors. The \f(CW\*(C`BUILD\*(C'\fR method lets us implement
logical constraints across the whole object after it is created.
.SH AUTHORS
.IX Header "AUTHORS"
.IP \(bu 4
Stevan Little <stevan@cpan.org>
.IP \(bu 4
Dave Rolsky <autarch@urth.org>
.IP \(bu 4
Jesse Luehrs <doy@cpan.org>
.IP \(bu 4
Shawn M Moore <sartak@cpan.org>
.IP \(bu 4
יובל קוג'מן (Yuval Kogman) <nothingmuch@woobling.org>
.IP \(bu 4
Karen Etheridge <ether@cpan.org>
.IP \(bu 4
Florian Ragwitz <rafl@debian.org>
.IP \(bu 4
Hans Dieter Pearcey <hdp@cpan.org>
.IP \(bu 4
Chris Prather <chris@prather.org>
.IP \(bu 4
Matt S Trout <mstrout@cpan.org>
.SH "COPYRIGHT AND LICENSE"
.IX Header "COPYRIGHT AND LICENSE"
This software is copyright (c) 2006 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.