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.IX Title "Moose::Cookbook::Basics::Person_BUILDARGSAndBUILD 3pm"
.TH Moose::Cookbook::Basics::Person_BUILDARGSAndBUILD 3pm "2012-06-28" "perl v5.14.2" "User Contributed Perl Documentation"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
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.nh
.SH "NAME"
Moose::Cookbook::Basics::Person_BUILDARGSAndBUILD \- Using BUILDARGS and BUILD to hook into object construction
.SH "VERSION"
.IX Header "VERSION"
version 2.0603
.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 "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.