link to the docs that say "avoid auto_deref".

[gitmo/Moose.git] / lib / Moose / Manual / Construction.pod

package Moose::Manual::Construction;

# ABSTRACT: Object construction (and destruction) with Moose

__END__

=pod

=head1 WHERE'S THE CONSTRUCTOR?

B<Do not define a C<new()> method for your classes!>

When you C<use Moose> in your class, your class becomes a subclass of
L<Moose::Object>. The L<Moose::Object> provides a C<new()> method for your
class. If you follow our recommendations in L<Moose::Manual::BestPractices>
and make your class immutable, then you actually get a class-specific C<new()>
method "inlined" in your class.

=head1 OBJECT CONSTRUCTION AND ATTRIBUTES

The Moose-provided constructor accepts a hash or hash reference of
named parameters matching your attributes (actually, matching their
C<init_arg>s). This is just another way in which Moose keeps you from
worrying I<how> classes are implemented. Simply define a class and
you're ready to start creating objects!

=head1 OBJECT CONSTRUCTION HOOKS

Moose lets you hook into object construction. You can validate an
object's state, do logging, customize construction from parameters which
do not match your attributes, or maybe allow non-hash(ref) constructor
arguments. You can do this by creating C<BUILD> and/or C<BUILDARGS>
methods.

If these methods exist in your class, Moose will arrange for them to
be called as part of the object construction process.

=head2 BUILDARGS

The C<BUILDARGS> method is called as a class method I<before> an
object is created. It will receive all of the arguments that were
passed to C<new()> I<as-is>, and is expected to return a hash
reference. This hash reference will be used to construct the object,
so it should contain keys matching your attributes' names (well,
C<init_arg>s).

One common use for C<BUILDARGS> is to accommodate a non-hash(ref)
calling style. For example, we might want to allow our Person class to
be called with a single argument of a social security number, C<<
Person->new($ssn) >>.

Without a C<BUILDARGS> method, Moose will complain, because it expects
a hash or hash reference. We can use the C<BUILDARGS> method to
accommodate this calling style:

  around BUILDARGS => sub {
      my $orig  = shift;
      my $class = shift;

      if ( @_ == 1 && !ref $_[0] ) {
          return $class->$orig( ssn => $_[0] );
      }
      else {
          return $class->$orig(@_);
      }
  };

Note the call to C<< $class->$orig >>. This will call the default C<BUILDARGS>
in L<Moose::Object>. This method takes care of distinguishing between a hash
reference and a plain hash for you.

=head2 BUILD

The C<BUILD> method is called I<after> an object is created. There are
several reasons to use a C<BUILD> method. One of the most common is to
check that the object state is valid. While we can validate individual
attributes through the use of types, we can't validate the state of a
whole object that way.

  sub BUILD {
      my $self = shift;

      if ( $self->country_of_residence eq 'USA' ) {
          die 'All US residents must have an SSN'
              unless $self->has_ssn;
      }
  }

Another use of a C<BUILD> method could be for logging or tracking
object creation.

  sub BUILD {
      my $self = shift;

      debug( 'Made a new person - SSN = ', $self->ssn, );
  }


The C<BUILD> method is called with the hash reference of the parameters passed
to the constructor (after munging by C<BUILDARGS>). This gives you a chance to
do something with parameters that do not represent object attributes.

  sub BUILD {
      my $self = shift;
      my $args = shift;

      $self->add_friend(
          My::User->new(
              user_id => $args->{user_id},
          )
      );
  }

=head3 BUILD and parent classes

The interaction between multiple C<BUILD> methods in an inheritance hierarchy
is different from normal Perl methods. B<You should never call C<<
$self->SUPER::BUILD >>>, nor should you ever apply a method modifier to
C<BUILD>.

Moose arranges to have all of the C<BUILD> methods in a hierarchy
called when an object is constructed, I<from parents to
children>. This might be surprising at first, because it reverses the
normal order of method inheritance.

The theory behind this is that C<BUILD> methods can only be used for
increasing specialization of a class's constraints, so it makes sense
to call the least specific C<BUILD> method first. Also, this is how
Perl 6 does it.

=head1 OBJECT DESTRUCTION

Moose provides a hook for object destruction with the C<DEMOLISH>
method. As with C<BUILD>, you should never explicitly call C<<
$self->SUPER::DEMOLISH >>. Moose will arrange for all of the
C<DEMOLISH> methods in your hierarchy to be called, from most to least
specific.

Each C<DEMOLISH> method is called with a single argument.

In most cases, Perl's built-in garbage collection is sufficient, and
you won't need to provide a C<DEMOLISH> method.

=head2 Error Handling During Destruction

The interaction of object destruction and Perl's global C<$@> and C<$?>
variables can be very confusing.

Moose always localizes C<$?> when an object is being destroyed. This means
that if you explicitly call C<exit>, that exit code will be preserved even if
an object's destructor makes a system call.

Moose also preserves C<$@> against any C<eval> calls that may happen during
object destruction. However, if an object's C<DEMOLISH> method actually dies,
Moose explicitly rethrows that error.

If you do not like this behavior, you will have to provide your own C<DESTROY>
method and use that instead of the one provided by L<Moose::Object>. You can
do this to preserve C<$@> I<and> capture any errors from object destruction by
creating an error stack.

=cut