phpman > perldoc > Moose::Manual::Construction(3pm)

NAME
    Moose::Manual::Construction - Object construction (and destruction) with Moose

VERSION
    version 2.2200

WHERE'S THE CONSTRUCTOR?
    Do not define a "new()" method for your classes!

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

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

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 "BUILD" and/or "BUILDARGS"
    methods.

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

  BUILDARGS
    The "BUILDARGS" method is called as a class method *before* an object is created. It will
    receive all of the arguments that were passed to "new()" *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, "init_arg"s).

    One common use for "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, "Person->new($ssn)".

    Without a "BUILDARGS" method, Moose will complain, because it expects a hash or hash reference.
    We can use the "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 "$class->$orig". This will call the default "BUILDARGS" in Moose::Object. This
    method takes care of distinguishing between a hash reference and a plain hash for you.

  BUILD
    The "BUILD" method is called *after* an object is created. There are several reasons to use a
    "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 "BUILD" method could be for logging or tracking object creation.

      sub BUILD {
          my $self = shift;

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

    The "BUILD" method is called with the hash reference of the parameters passed to the constructor
    (after munging by "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},
              )
          );
      }

   BUILD and parent classes
    The interaction between multiple "BUILD" methods in an inheritance hierarchy is different from
    normal Perl methods. You should never call "$self->SUPER::BUILD", nor should you ever apply a
    method modifier to "BUILD". Roles are an exception to this rule, though: it's completely
    acceptable to apply a method modifier to "BUILD" in a role; you can even provide an empty
    "BUILD" subroutine in a role so the role is applicable even to classes without their own
    "BUILD".

    Moose arranges to have all of the "BUILD" methods in a hierarchy called when an object is
    constructed, *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 "BUILD" methods can only be used for increasing specialization of
    a class's constraints, so it makes sense to call the least specific "BUILD" method first. Also,
    this is how Perl 6 does it.

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

    Each "DEMOLISH" method is called with a single argument. This is a boolean value indicating
    whether or not this method was called as part of the global destruction process (when the Perl
    interpreter exits).

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

  Error Handling During Destruction
    The interaction of object destruction and Perl's global $@ and $? variables can be very
    confusing.

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

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

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

AUTHORS
    *   Stevan Little <stevan AT cpan.org>

    *   Dave Rolsky <autarch AT urth.org>

    *   Jesse Luehrs <doy AT cpan.org>

    *   Shawn M Moore <sartak AT cpan.org>

    *   יובל קוג'מן (Yuval Kogman) <nothingmuch AT woobling.org>

    *   Karen Etheridge <ether AT cpan.org>

    *   Florian Ragwitz <rafl AT debian.org>

    *   Hans Dieter Pearcey <hdp AT cpan.org>

    *   Chris Prather <chris AT prather.org>

    *   Matt S Trout <mstrout AT 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.