Provided by: libmoose-perl_2.1005-1_amd64 
NAME
Moose::Cookbook::Basics::Point_AttributesAndSubclassing - Point and Point3D classes,
showing basic attributes and subclassing.
VERSION
version 2.1005
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". 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/moose_cookbook_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-functions.html>
AUTHOR
Moose is maintained by the Moose Cabal, along with the help of many contributors. See
"CABAL" in Moose and "CONTRIBUTORS" in Moose for details.
COPYRIGHT AND LICENSE
This software is copyright (c) 2013 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.
perl v5.18.1 Moose::Cookbook::Basics::Point_AttributesAndSubclassing(3pm)