PERLOOTUT(1) Perl Programmers Reference Guide PERLOOTUT(1)
NAME
perlootut - Object-Oriented Programming in Perl Tutorial
DATE
This document was created in February, 2011.
DESCRIPTION
This document provides an introduction to object-oriented programming
in Perl. It begins with a brief overview of the concepts behind object
oriented design. Then it introduces several different OO systems from
CPAN <http://search.cpan.org>; which build on top of what Perl provides.
By default, Perl's built-in OO system is very minimal, leaving you to
do most of the work. This minimalism made a lot of sense in 1994, but
in the years since Perl 5.0 we've seen a number of common patterns
emerge in Perl OO. Fortunately, Perl's flexibility has allowed a rich
ecosystem of Perl OO systems to flourish.
If you want to know how Perl OO works under the hood, the perlobj
document explains the nitty gritty details.
This document assumes that you already understand the basics of Perl
syntax, variable types, operators, and subroutine calls. If you don't
understand these concepts yet, please read perlintro first. You should
also read the perlsyn, perlop, and perlsub documents.
OBJECT-ORIENTED FUNDAMENTALS
Most object systems share a number of common concepts. You've probably
heard terms like "class", "object, "method", and "attribute" before.
Understanding the concepts will make it much easier to read and write
object-oriented code. If you're already familiar with these terms, you
should still skim this section, since it explains each concept in terms
of Perl's OO implementation.
Perl's OO system is class-based. Class-based OO is fairly common. It's
used by Java, C++, C#, Python, Ruby, and many other languages. There
are other object orientation paradigms as well. JavaScript is the most
popular language to use another paradigm. JavaScript's OO system is
prototype-based.
Object
An object is a data structure that bundles together data and
subroutines which operate on that data. An object's data is called
attributes, and its subroutines are called methods. An object can be
thought of as a noun (a person, a web service, a computer).
An object represents a single discrete thing. For example, an object
might represent a file. The attributes for a file object might include
its path, content, and last modification time. If we created an object
to represent /etc/hostname on a machine named "foo.example.com", that
object's path would be "/etc/hostname", its content would be "foo\n",
and it's last modification time would be 1304974868 seconds since the
beginning of the epoch.
The methods associated with a file might include "rename()" and
"write()".
In Perl most objects are hashes, but the OO systems we recommend keep
you from having to worry about this. In practice, it's best to consider
an object's internal data structure opaque.
Class
A class defines the behavior of a category of objects. A class is a
name for a category (like "File"), and a class also defines the
behavior of objects in that category.
All objects belong to a specific class. For example, our /etc/hostname
object belongs to the "File" class. When we want to create a specific
object, we start with its class, and construct or instantiate an
object. A specific object is often referred to as an instance of a
class.
In Perl, any package can be a class. The difference between a package
which is a class and one which isn't is based on how the package is
used. Here's our "class declaration" for the "File" class:
package File;
In Perl, there is no special keyword for constructing an object.
However, most OO modules on CPAN use a method named "new()" to
construct a new object:
my $hostname = File->new(
path => '/etc/hostname',
content => "foo\n",
last_mod_time => 1304974868,
);
(Don't worry about that "->" operator, it will be explained later.)
Blessing
As we said earlier, most Perl objects are hashes, but an object can be
an instance of any Perl data type (scalar, array, etc.). Turning a
plain data structure into an object is done by blessing that data
structure using Perl's "bless" function.
While we strongly suggest you don't build your objects from scratch,
you should know the term bless. A blessed data structure (aka "a
referent") is an object. We sometimes say that an object has been
"blessed into a class".
Once a referent has been blessed, the "blessed" function from the
Scalar::Util core module can tell us its class name. This subroutine
returns an object's class when passed an object, and false otherwise.
use Scalar::Util 'blessed';
print blessed($hash); # undef
print blessed($hostname); # File
Constructor
A constructor creates a new object. In Perl, a class's constructor is
just another method, unlike some other languages, which provide syntax
for constructors. Most Perl classes use "new" as the name for their
constructor:
my $file = File->new(...);
Methods
You already learned that a method is a subroutine that operates on an
object. You can think of a method as the things that an object can do.
If an object is a noun, then methods are its verbs (save, print, open).
In Perl, methods are simply subroutines that live in a class's package.
Methods are always written to receive the object as their first
argument:
sub print_info {
my $self = shift;
print "This file is at ", $self->path, "\n";
}
$file->print_info;
# The file is at /etc/hostname
What makes a method special is how it's called. The arrow operator
("->") tells Perl that we are calling a method.
When we make a method call, Perl arranges for the method's invocant to
be passed as the first argument. Invocant is a fancy name for the thing
on the left side of the arrow. The invocant can either be a class name
or an object. We can also pass additional arguments to the method:
sub print_info {
my $self = shift;
my $prefix = shift // "This file is at ";
print $prefix, ", ", $self->path, "\n";
}
$file->print_info("The file is located at ");
# The file is located at /etc/hostname
Attributes
Each class can define its attributes. When we instantiate an object, we
assign values to those attributes. For example, every "File" object has
a path. Attributes are sometimes called properties.
Perl has no special syntax for attributes. Under the hood, attributes
are often stored as keys in the object's underlying hash, but don't
worry about this.
We recommend that you only access attributes via accessor methods.
These are methods that can get or set the value of each attribute. We
saw this earlier in the "print_info()" example, which calls
"$self->path".
You might also see the terms getter and setter. These are two types of
accessors. A getter gets the attribute's value, while a setter sets it.
Another term for a setter is mutator
Attributes are typically defined as read-only or read-write. Read-only
attributes can only be set when the object is first created, while
read-write attributes can be altered at any time.
The value of an attribute may itself be another object. For example,
instead of returning its last mod time as a number, the "File" class
could return a DateTime object representing that value.
It's possible to have a class that does not expose any publicly
settable attributes. Not every class has attributes and methods.
Polymorphism
Polymorphism is a fancy way of saying that objects from two different
classes share an API. For example, we could have "File" and "WebPage"
classes which both have a "print_content()" method. This method might
produce different output for each class, but they share a common
interface.
While the two classes may differ in many ways, when it comes to the
"print_content()" method, they are the same. This means that we can try
to call the "print_content()" method on an object of either class, and
we don't have to know what class the object belongs to!
Polymorphism is one of the key concepts of object-oriented design.
Inheritance
Inheritance lets you create a specialized version of an existing class.
Inheritance lets the new class to reuse the methods and attributes of
another class.
For example, we could create an "File::MP3" class which inherits from
"File". An "File::MP3" is-a more specific type of "File". All mp3
files are files, but not all files are mp3 files.
We often refer to inheritance relationships as parent-child or
"superclass/subclass" relationships. Sometimes we say that the child
has an is-a relationship with its parent class.
"File" is a superclass of "File::MP3", and "File::MP3" is a subclass of
"File".
package File::MP3;
use parent 'File';
The parent module is one of several ways that Perl lets you define
inheritance relationships.
Perl allows multiple inheritance, which means that a class can inherit
from multiple parents. While this is possible, we strongly recommend
against it. Generally, you can use roles to do everything you can do
with multiple inheritance, but in a cleaner way.
Note that there's nothing wrong with defining multiple subclasses of a
given class. This is both common and safe. For example, we might define
"File::MP3::FixedBitrate" and "File::MP3::VariableBitrate" classes to
distinguish between different types of mp3 file.
Overriding methods and method resolution
Inheritance allows two classes to share code. By default, every method
in the parent class is also available in the child. The child can
explicitly override a parent's method to provide its own
implementation. For example, if we have an "File::MP3" object, it has
the "print_info()" method from "File":
my $cage = File::MP3->new(
path => 'mp3s/My-Body-Is-a-Cage.mp3',
content => $mp3_data,
last_mod_time => 1304974868,
title => 'My Body Is a Cage',
);
$cage->print_info;
# The file is at mp3s/My-Body-Is-a-Cage.mp3
If we wanted to include the mp3's title in the greeting, we could
override the method:
package File::MP3;
use parent 'File';
sub print_info {
my $self = shift;
print "This file is at ", $self->path, "\n";
print "Its title is ", $self->title, "\n";
}
$cage->print_info;
# The file is at mp3s/My-Body-Is-a-Cage.mp3
# Its title is My Body Is a Cage
The process of determining what method should be used is called method
resolution. What Perl does is look at the object's class first
("File::MP3" in this case). If that class defines the method, then that
class's version of the method is called. If not, Perl looks at each
parent class in turn. For "File::MP3", its only parent is "File". If
"File::MP3" does not define the method, but "File" does, then Perl
calls the method in "File".
If "File" inherited from "DataSource", which inherited from "Thing",
then Perl would keep looking "up the chain" if necessary.
It is possible to explicitly call a parent method from a child:
package File::MP3;
use parent 'File';
sub print_info {
my $self = shift;
$self->SUPER::print_info();
print "Its title is ", $self->title, "\n";
}
The "SUPER::" bit tells Perl to look for the "print_info()" in the
"File::MP3" class's inheritance chain. When it finds the parent class
that implements this method, the method is called.
We mentioned multiple inheritance earlier. The main problem with
multiple inheritance is that it greatly complicates method resolution.
See perlobj for more details.
Encapsulation
Encapsulation is the idea that an object is opaque. When another
developer uses your class, they don't need to know how it is
implemented, they just need to know what it does.
Encapsulation is important for several reasons. First, it allows you to
separate the public API from the private implementation. This means you
can change that implementation without breaking the API.
Second, when classes are well encapsulated, they become easier to
subclass. Ideally, a subclass uses the same APIs to access object data
that its parent class uses. In reality, subclassing sometimes involves
violating encapsulation, but a good API can minimize the need to do
this.
We mentioned earlier that most Perl objects are implemented as hashes
under the hood. The principle of encapsulation tells us that we should
not rely on this. Instead, we should use accessor methods to access the
data in that hash. The object systems that we recommend below all
automate the generation of accessor methods. If you use one of them,
you should never have to access the object as a hash directly.
Composition
In object-oriented code, we often find that one object references
another object. This is called composition, or a has-a relationship.
Earlier, we mentioned that the "File" class's "last_mod_time" accessor
could return a DateTime object. This is a perfect example of
composition. We could go even further, and make the "path" and
"content" accessors return objects as well. The "File" class would then
be composed of several other objects.
Roles
Roles are something that a class does, rather than something that it
is. Roles are relatively new to Perl, but have become rather popular.
Roles are applied to classes. Sometimes we say that classes consume
roles.
Roles are an alternative to inheritance for providing polymorphism.
Let's assume we have two classes, "Radio" and "Computer". Both of these
things have on/off switches. We want to model that in our class
definitions.
We could have both classes inherit from a common parent, like
"Machine", but not all machines have on/off switches. We could create a
parent class called "HasOnOffSwitch", but that is very artificial.
Radios and computers are not specializations of this parent. This
parent is really a rather ridiculous creation.
This is where roles come in. It makes a lot of sense to create a
"HasOnOffSwitch" role and apply it to both classes. This role would
define a known API like providing "turn_on()" and "turn_off()" methods.
Perl does not have any built-in way to express roles. In the past,
people just bit the bullet and used multiple inheritance. Nowadays,
there are several good choices on CPAN for using roles.
When to Use OO
Object Orientation is not the best solution to every problem. In Perl
Best Practices (copyright 2004, Published by O'Reilly Media, Inc.),
Damian Conway provides a list of criteria to use when deciding if OO is
the right fit for your problem:
o The system being designed is large, or is likely to become large.
o The data can be aggregated into obvious structures, especially if
there's a large amount of data in each aggregate.
o The various types of data aggregate form a natural hierarchy that
facilitates the use of inheritance and polymorphism.
o You have a piece of data on which many different operations are
applied.
o You need to perform the same general operations on related types of
data, but with slight variations depending on the specific type of
data the operations are applied to.
o It's likely you'll have to add new data types later.
o The typical interactions between pieces of data are best
represented by operators.
o The implementation of individual components of the system is likely
to change over time.
o The system design is already object-oriented.
o Large numbers of other programmers will be using your code modules.
PERL OO SYSTEMS
As we mentioned before, Perl's built-in OO system is very minimal, but
also quite flexible. Over the years, many people have developed systems
which build on top of Perl's built-in system to provide more features
and convenience.
We strongly recommend that you use one of these systems. Even the most
minimal of them eliminates a lot of repetitive boilerplate. There's
really no good reason to write your classes from scratch in Perl.
If you are interested in the guts underlying these systems, check out
perlobj.
Moose
Moose bills itself as a "postmodern object system for Perl 5". Don't be
scared, the "postmodern" label is a callback to Larry's description of
Perl as "the first postmodern computer language".
"Moose" provides a complete, modern OO system. Its biggest influence is
the Common Lisp Object System, but it also borrows ideas from Smalltalk
and several other languages. "Moose" was created by Stevan Little, and
draws heavily from his work on the Perl 6 OO design.
Here is our "File" class using "Moose":
package File;
use Moose;
has path => ( is => 'ro' );
has content => ( is => 'ro' );
has last_mod_time => ( is => 'ro' );
sub print_info {
my $self = shift;
print "This file is at ", $self->path, "\n";
}
"Moose" provides a number of features:
o Declarative sugar
"Moose" provides a layer of declarative "sugar" for defining
classes. That sugar is just a set of exported functions that make
declaring how your class works simpler and more palatable. This
lets you describe what your class is, rather than having to tell
Perl how to implement your class.
The "has()" subroutine declares an attribute, and "Moose"
automatically creates accessors for these attributes. It also takes
care of creating a "new()" method for you. This constructor knows
about the attributes you declared, so you can set them when
creating a new "File".
o Roles built-in
"Moose" lets you define roles the same way you define classes:
package HasOnOfSwitch;
use Moose::Role;
has is_on => (
is => 'rw',
isa => 'Bool',
);
sub turn_on {
my $self = shift;
$self->is_on(1);
}
sub turn_off {
my $self = shift;
$self->is_on(0);
}
o A miniature type system
In the example above, you can see that we passed "isa => 'Bool'" to
"has()" when creating our "is_on" attribute. This tells "Moose"
that this attribute must be a boolean value. If we try to set it to
an invalid value, our code will throw an error.
o Full introspection and manipulation
Perl's built-in introspection features are fairly minimal. "Moose"
builds on top of them and creates a full introspection layer for
your classes. This lets you ask questions like "what methods does
the File class implement?" It also lets you modify your classes
programmatically.
o Self-hosted and extensible
"Moose" describes itself using its own introspection API. Besides
being a cool trick, this means that you can extend "Moose" using
"Moose" itself.
o Rich ecosystem
There is a rich ecosystem of "Moose" extensions on CPAN under the
MooseX <http://search.cpan.org/search?query=MooseX&mode=dist>;
namespace. In addition, many modules on CPAN already use "Moose",
providing you with lots of examples to learn from.
o Many more features
"Moose" is a very powerful tool, and we can't cover all of its
features here. We encourage you to learn more by reading the
"Moose" documentation, starting with Moose::Manual
<http://search.cpan.org/perldoc?Moose::Manual>.
Of course, "Moose" isn't perfect.
"Moose" can make your code slower to load. "Moose" itself is not small,
and it does a lot of code generation when you define your class. This
code generation means that your runtime code is as fast as it can be,
but you pay for this when your modules are first loaded.
This load time hit can be a problem when startup speed is important,
such as with a command-line script or a "plain vanilla" CGI script that
must be loaded each time it is executed.
Before you panic, know that many people do use "Moose" for command-line
tools and other startup-sensitive code. We encourage you to try "Moose"
out first before worrying about startup speed.
"Moose" also has several dependencies on other modules. Most of these
are small stand-alone modules, a number of which have been spun off
from "Moose". "Moose" itself, and some of its dependencies, require a
compiler. If you need to install your software on a system without a
compiler, or if having any dependencies is a problem, then "Moose" may
not be right for you.
Mouse
If you try "Moose" and find that one of these issues is preventing you
from using "Moose", we encourage you to consider Mouse next. "Mouse"
implements a subset of "Moose"'s functionality in a simpler package.
For all features that it does implement, the end-user API is identical
to "Moose", meaning you can switch from "Mouse" to "Moose" quite
easily.
"Mouse" does not implement most of "Moose"'s introspection API, so it's
often faster when loading your modules. Additionally, all of its
required dependencies ship with the Perl core, and it can run without a
compiler. If you do have a compiler, "Mouse" will use it to compile
some of its code for a speed boost.
Finally, it ships with a "Mouse::Tiny" module that takes most of
"Mouse"'s features and bundles them up in a single module file. You can
copy this module file into your application's library directory for
easy bundling.
The "Moose" authors hope that one day "Mouse" can be made obsolete by
improving "Moose" enough, but for now it provides a worthwhile
alternative to "Moose".
Class::Accessor
Class::Accessor is the polar opposite of "Moose". It provides very few
features, nor is it self-hosting.
It is, however, very simple, pure Perl, and it has no non-core
dependencies. It also provides a "Moose-like" API on demand for the
features it supports.
Even though it doesn't do much, it is still preferable to writing your
own classes from scratch.
Here's our "File" class with "Class::Accessor":
package File;
use Class::Accessor 'antlers';
has path => ( is => 'ro' );
has content => ( is => 'ro' );
has last_mod_time => ( is => 'ro' );
sub print_info {
my $self = shift;
print "This file is at ", $self->path, "\n";
}
The "antlers" import flag tells "Class::Accessor" that you want to
define your attributes using "Moose"-like syntax. The only parameter
that you can pass to "has" is "is". We recommend that you use this
Moose-like syntax if you choose "Class::Accessor" since it means you
will have a smoother upgrade path if you later decide to move to
"Moose".
Like "Moose", "Class::Accessor" generates accessor methods and a
constructor for your class.
Object::Tiny
Finally, we have Object::Tiny. This module truly lives up to its name.
It has an incredibly minimal API and absolutely no dependencies (core
or not). Still, we think it's a lot easier to use than writing your own
OO code from scratch.
Here's our "File" class once more:
package File;
use Object::Tiny qw( path content last_mod_time );
sub print_info {
my $self = shift;
print "This file is at ", $self->path, "\n";
}
That's it!
With "Object::Tiny", all accessors are read-only. It generates a
constructor for you, as well as the accessors you define.
Role::Tiny
As we mentioned before, roles provide an alternative to inheritance,
but Perl does not have any built-in role support. If you choose to use
Moose, it comes with a full-fledged role implementation. However, if
you use one of our other recommended OO modules, you can still use
roles with Role::Tiny
"Role::Tiny" provides some of the same features as Moose's role system,
but in a much smaller package. Most notably, it doesn't support any
sort of attribute declaration, so you have to do that by hand. Still,
it's useful, and works well with "Class::Accessor" and "Object::Tiny"
OO System Summary
Here's a brief recap of the options we covered:
o Moose
"Moose" is the maximal option. It has a lot of features, a big
ecosystem, and a thriving user base. We also covered Mouse briefly.
"Mouse" is "Moose" lite, and a reasonable alternative when Moose
doesn't work for your application.
o Class::Accessor
"Class::Accessor" does a lot less than "Moose", and is a nice
alternative if you find "Moose" overwhelming. It's been around a
long time and is well battle-tested. It also has a minimal "Moose"
compatibility mode which makes moving from "Class::Accessor" to
"Moose" easy.
o Object::Tiny
"Object::Tiny" is the absolute minimal option. It has no
dependencies, and almost no syntax to learn. It's a good option for
a super minimal environment and for throwing something together
quickly without having to worry about details.
o Role::Tiny
Use "Role::Tiny" with "Class::Accessor" or "Object::Tiny" if you
find yourself considering multiple inheritance. If you go with
"Moose", it comes with its own role implementation.
Other OO Systems
There are literally dozens of other OO-related modules on CPAN besides
those covered here, and you're likely to run across one or more of them
if you work with other people's code.
In addition, plenty of code in the wild does all of its OO "by hand",
using just the Perl built-in OO features. If you need to maintain such
code, you should read perlobj to understand exactly how Perl's built-in
OO works.
CONCLUSION
As we said before, Perl's minimal OO system has led to a profusion of
OO systems on CPAN. While you can still drop down to the bare metal and
write your classes by hand, there's really no reason to do that with
modern Perl.
For small systems, Object::Tiny and Class::Accessor both provide
minimal object systems that take care of basic boilerplate for you.
For bigger projects, Moose provides a rich set of features that will
let you focus on implementing your business logic.
We encourage you to play with and evaluate Moose, Class::Accessor, and
Object::Tiny to see which OO system is right for you.
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