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Code Editor : Point_AttributesAndSubclassing.pod

# PODNAME: Moose::Cookbook::Basics::Point_AttributesAndSubclassing
# ABSTRACT: Point and Point3D classes, showing basic attributes and subclassing.

__END__

=pod

=encoding UTF-8

=head1 NAME

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

=head1 VERSION

version 2.2013

=head1 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);

=head1 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 C<strict> and C<warnings> for us, so all we
need to do is say C<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 L<Moose::Object>.
The L<Moose::Object> class provides us with a constructor that
respects our attributes, as well other features. See L<Moose::Object>
for details.

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

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

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

The C<< 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 B<Point> class.

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

extends 'Point';

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

It is my opinion that the behavior of C<extends> is more intuitive.
(2).

Next we create a new attribute for B<Point3D> called C<z>.

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

This attribute is just like B<Point>'s C<x> and C<y> attributes.

The C<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 C<clear> is called on a B<Point3D> object, our modifier method
gets called as well. Unsurprisingly, the modifier is called I<after>
the real method.

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

Of course, using the C<after> modifier is not the only way to
accomplish this. This B<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, C<override>:

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

The C<override> modifier allows you to use the C<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 B<Point> inherits from L<Moose::Object>, it will also inherit
the default L<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 C<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 C<new> without them will throw an error.

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

From here on, we can use C<$point> and C<$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
F<t/recipes/basics_point_attributesandsubclassing.t> test file.

=head2 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 C<$self> to C<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
C<MooseX::InsideOut>.

=head1 CONCLUSION

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

=head1 FOOTNOTES

=over 4

=item (1)

Moose provides a number of builtin type constraints, of which C<Int>
is one. For more information on the type constraint system, see
L<Moose::Util::TypeConstraints>.

=item (2)

The C<extends> keyword supports multiple inheritance. Simply pass all
of your superclasses to C<extends> as a list:

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

=item (3)

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

=back

=head1 SEE ALSO

=over 4

=item 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.

L<http://www.gigamonkeys.com/book/object-reorientation-generic-functions.html>

=back

=begin testing

my $point = Point->new( x => 1, y => 2 );
isa_ok( $point, 'Point' );
isa_ok( $point, 'Moose::Object' );

is( $point->x, 1, '... got the right value for x' );
is( $point->y, 2, '... got the right value for y' );

$point->y(10);
is( $point->y, 10, '... got the right (changed) value for y' );

isnt(
    exception {
        $point->y('Foo');
    },
    undef,
    '... cannot assign a non-Int to y'
);

isnt(
    exception {
        Point->new();
    },
    undef,
    '... must provide required attributes to new'
);

$point->clear();

is( $point->x, 0, '... got the right (cleared) value for x' );
is( $point->y, 0, '... got the right (cleared) value for y' );

# check the type constraints on the constructor

is(
    exception {
        Point->new( x => 0, y => 0 );
    },
    undef,
    '... can assign a 0 to x and y'
);

isnt(
    exception {
        Point->new( x => 10, y => 'Foo' );
    },
    undef,
    '... cannot assign a non-Int to y'
);

isnt(
    exception {
        Point->new( x => 'Foo', y => 10 );
    },
    undef,
    '... cannot assign a non-Int to x'
);

# Point3D

my $point3d = Point3D->new( { x => 10, y => 15, z => 3 } );
isa_ok( $point3d, 'Point3D' );
isa_ok( $point3d, 'Point' );
isa_ok( $point3d, 'Moose::Object' );

is( $point3d->x,     10, '... got the right value for x' );
is( $point3d->y,     15, '... got the right value for y' );
is( $point3d->{'z'}, 3,  '... got the right value for z' );

$point3d->clear();

is( $point3d->x, 0, '... got the right (cleared) value for x' );
is( $point3d->y, 0, '... got the right (cleared) value for y' );
is( $point3d->z, 0, '... got the right (cleared) value for z' );

isnt(
    exception {
        Point3D->new( x => 10, y => 'Foo', z => 3 );
    },
    undef,
    '... cannot assign a non-Int to y'
);

isnt(
    exception {
        Point3D->new( x => 'Foo', y => 10, z => 3 );
    },
    undef,
    '... cannot assign a non-Int to x'
);

isnt(
    exception {
        Point3D->new( x => 0, y => 10, z => 'Bar' );
    },
    undef,
    '... cannot assign a non-Int to z'
);

isnt(
    exception {
        Point3D->new( x => 10, y => 3 );
    },
    undef,
    '... z is a required attribute for Point3D'
);

# test some class introspection

can_ok( 'Point', 'meta' );
isa_ok( Point->meta, 'Moose::Meta::Class' );

can_ok( 'Point3D', 'meta' );
isa_ok( Point3D->meta, 'Moose::Meta::Class' );

isnt(
    Point->meta, Point3D->meta,
    '... they are different metaclasses as well'
);

# poke at Point

is_deeply(
    [ Point->meta->superclasses ],
    ['Moose::Object'],
    '... Point got the automagic base class'
);

my @Point_methods = qw(meta x y clear);
my @Point_attrs = ( 'x', 'y' );

is_deeply(
    [ sort @Point_methods ],
    [ sort Point->meta->get_method_list() ],
    '... we match the method list for Point'
);

is_deeply(
    [ sort @Point_attrs ],
    [ sort Point->meta->get_attribute_list() ],
    '... we match the attribute list for Point'
);

foreach my $method (@Point_methods) {
    ok( Point->meta->has_method($method),
        '... Point has the method "' . $method . '"' );
}

foreach my $attr_name (@Point_attrs) {
    ok( Point->meta->has_attribute($attr_name),
        '... Point has the attribute "' . $attr_name . '"' );
    my $attr = Point->meta->get_attribute($attr_name);
    ok( $attr->has_type_constraint,
        '... Attribute ' . $attr_name . ' has a type constraint' );
    isa_ok( $attr->type_constraint, 'Moose::Meta::TypeConstraint' );
    is( $attr->type_constraint->name, 'Int',
        '... Attribute ' . $attr_name . ' has an Int type constraint' );
}

# poke at Point3D

is_deeply(
    [ Point3D->meta->superclasses ],
    ['Point'],
    '... Point3D gets the parent given to it'
);

my @Point3D_methods = qw( meta z clear );
my @Point3D_attrs   = ('z');

is_deeply(
    [ sort @Point3D_methods ],
    [ sort Point3D->meta->get_method_list() ],
    '... we match the method list for Point3D'
);

is_deeply(
    [ sort @Point3D_attrs ],
    [ sort Point3D->meta->get_attribute_list() ],
    '... we match the attribute list for Point3D'
);

foreach my $method (@Point3D_methods) {
    ok( Point3D->meta->has_method($method),
        '... Point3D has the method "' . $method . '"' );
}

foreach my $attr_name (@Point3D_attrs) {
    ok( Point3D->meta->has_attribute($attr_name),
        '... Point3D has the attribute "' . $attr_name . '"' );
    my $attr = Point3D->meta->get_attribute($attr_name);
    ok( $attr->has_type_constraint,
        '... Attribute ' . $attr_name . ' has a type constraint' );
    isa_ok( $attr->type_constraint, 'Moose::Meta::TypeConstraint' );
    is( $attr->type_constraint->name, 'Int',
        '... Attribute ' . $attr_name . ' has an Int type constraint' );
}

=end testing

=head1 AUTHORS

=over 4

=item *

Stevan Little <stevan.little@iinteractive.com>

=item *

Dave Rolsky <autarch@urth.org>

=item *

Jesse Luehrs <doy@tozt.net>

=item *

Shawn M Moore <code@sartak.org>

=item *

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

=item *

Karen Etheridge <ether@cpan.org>

=item *

Florian Ragwitz <rafl@debian.org>

=item *

Hans Dieter Pearcey <hdp@weftsoar.net>

=item *

Chris Prather <chris@prather.org>

=item *

Matt S Trout <mst@shadowcat.co.uk>

=back

=head1 COPYRIGHT AND LICENSE

This is free software; you can redistribute it and/or modify it under
the same terms as the Perl 5 programming language system itself.

=cut

${this.title}

Code Editor : Point_AttributesAndSubclassing.pod