Moose::Cookbook::Basics::Point_AttributesAndSubclassing - phpMan

NAME
    Moose::Cookbook::Basics::Point_AttributesAndSubclassing - Point and
    Point3D classes, showing basic attributes and subclassing.

VERSION
    version 2.2200

SYNOPSIS
      package Point;
      use Moose;

      has 'x' => (isa => 'Int', is => 'rw', required => 1);
      has 'y' => (isa => 'Int', is => 'rw', required => 1);

      sub clear {
          my $self = shift;
          $self->x(0);
          $self->y(0);
      }

      package Point3D;
      use Moose;

      extends 'Point';

      has 'z' => (isa => 'Int', is => 'rw', required => 1);

      after 'clear' => sub {
          my $self = shift;
          $self->z(0);
      };

      package main;

      # hash or hashrefs are ok for the constructor
      my $point1 = Point->new(x => 5, y => 7);
      my $point2 = Point->new({x => 5, y => 7});

      my $point3d = Point3D->new(x => 5, y => 42, z => -5);

DESCRIPTION
    This is the classic Point example. It is taken directly from the Perl 6
    Apocalypse 12 document, and is similar to the example found in the
    classic K&R C book as well.

    As with all Perl 5 classes, a Moose class is defined in a package. Moose
    handles turning on "strict" and "warnings" for us, so all we need to do
    is say "use Moose", and no kittens will die.

    When Moose is loaded, it exports a set of sugar functions into our
    package. This means that we import some functions which serve as Moose
    "keywords". These aren't real language keywords, they're just Perl
    functions exported into our package.

    Moose automatically makes our package a subclass of Moose::Object. The
    Moose::Object class provides us with a constructor that respects our
    attributes, as well other features. See Moose::Object for details.

    Now, onto the keywords. The first one we see here is "has", which
    defines an instance attribute in our class:

      has 'x' => (isa => 'Int', is => 'rw', required => 1);

    This will create an attribute named "x". The "isa" parameter says that
    we expect the value stored in this attribute to pass the type constraint
    for "Int" (1). The accessor generated for this attribute will be
    read-write.

    The "required => 1" parameter means that this attribute must be provided
    when a new object is created. A point object without coordinates doesn't
    make much sense, so we don't allow it.

    We have defined our attributes; next we define our methods. In Moose, as
    with regular Perl 5 OO, a method is just a subroutine defined within the
    package:

      sub clear {
          my $self = shift;
          $self->x(0);
          $self->y(0);
      }

    That concludes the Point class.

    Next we have a subclass of Point, Point3D. To declare our superclass, we
    use the Moose keyword "extends":

      extends 'Point';

    The "extends" keyword works much like "use base"/"use parent". First, it
    will attempt to load your class if needed. However, unlike "base", the
    "extends" keyword will *overwrite* any previous values in your package's
    @ISA, where "use base" will "push" values onto the package's @ISA.

    It is my opinion that the behavior of "extends" is more intuitive. (2).

    Next we create a new attribute for Point3D called "z".

      has 'z' => (isa => 'Int', is => 'rw', required => 1);

    This attribute is just like Point's "x" and "y" attributes.

    The "after" keyword demonstrates a Moose feature called "method
    modifiers" (or "advice" for the AOP inclined):

      after 'clear' => sub {
          my $self = shift;
          $self->z(0);
      };

    When "clear" is called on a Point3D object, our modifier method gets
    called as well. Unsurprisingly, the modifier is called *after* the real
    method.

    In this case, the real "clear" method is inherited from Point. Our
    modifier method receives the same arguments as those passed to the
    modified method (just $self here).

    Of course, using the "after" modifier is not the only way to accomplish
    this. This is Perl, right? You can get the same results with this code:

      sub clear {
          my $self = shift;
          $self->SUPER::clear();
          $self->z(0);
      }

    You could also use another Moose method modifier, "override":

      override 'clear' => sub {
          my $self = shift;
          super();
          $self->z(0);
      };

    The "override" modifier allows you to use the "super" keyword to
    dispatch to the superclass's method in a very Ruby-ish style.

    The choice of whether to use a method modifier, and which one to use, is
    often a question of style as much as functionality.

    Since Point inherits from Moose::Object, it will also inherit the
    default Moose::Object constructor:

      my $point1 = Point->new(x => 5, y => 7);
      my $point2 = Point->new({x => 5, y => 7});

      my $point3d = Point3D->new(x => 5, y => 42, z => -5);

    The "new" constructor accepts a named argument pair for each attribute
    defined by the class, which you can provide as a hash or hash reference.
    In this particular example, the attributes are required, and calling
    "new" without them will throw an error.

      my $point = Point->new( x => 5 ); # no y, kaboom!

    From here on, we can use $point and $point3d just as you would any other
    Perl 5 object. For a more detailed example of what can be done, you can
    refer to the t/recipes/basics_point_attributesandsubclassing.t test
    file.

  Moose Objects are Just Hashrefs
    While this all may appear rather magical, it's important to realize that
    Moose objects are just hash references under the hood (3). For example,
    you could pass $self to "Data::Dumper" and you'd get exactly what you'd
    expect.

    You could even poke around inside the object's data structure, but that
    is strongly discouraged.

    The fact that Moose objects are hashrefs means it is easy to use Moose
    to extend non-Moose classes, as long as they too are hash references. If
    you want to extend a non-hashref class, check out "MooseX::InsideOut".

CONCLUSION
    This recipe demonstrates some basic Moose concepts, attributes,
    subclassing, and a simple method modifier.

FOOTNOTES
    (1) Moose provides a number of builtin type constraints, of which "Int"
        is one. For more information on the type constraint system, see
        Moose::Util::TypeConstraints.

    (2) The "extends" keyword supports multiple inheritance. Simply pass all
        of your superclasses to "extends" as a list:

          extends 'Foo', 'Bar', 'Baz';

    (3) Moose supports using instance structures other than blessed hash
        references (such as glob references - see MooseX::GlobRef).

SEE ALSO
    Method Modifiers
        The concept of method modifiers is directly ripped off from CLOS. A
        great explanation of them can be found by following this link.

        <http://www.gigamonkeys.com/book/object-reorientation-generic-functi
        ons.html>

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.