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PERLOOTUT(1)	       Perl Programmers	Reference Guide		  PERLOOTUT(1)

NAME
       perlootut - Object-Oriented Programming in Perl Tutorial

DATE
       This document was created in February, 2011, and	the last major
       revision	was in February, 2013.

       If you are reading this in the future then it's possible	that the state
       of the art has changed. We recommend you	start by reading the perlootut
       document	in the latest stable release of	Perl, rather than this
       version.

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 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 HasOnOffSwitch;
	     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.

       Moo

       If you try "Moose" and find that	one of these issues is preventing you
       from using "Moose", we encourage	you to consider	Moo next. "Moo"
       implements a subset of "Moose"'s	functionality in a simpler package.
       For most	features that it does implement, the end-user API is identical
       to "Moose", meaning you can switch from "Moo" to	"Moose"	quite easily.

       "Moo" does not implement	most of	"Moose"'s introspection	API, so	it's
       often faster when loading your modules. Additionally, none of its
       dependencies require XS,	so it can be installed on machines without a
       compiler.

       One of "Moo"'s most compelling features is its interoperability with
       "Moose".	When someone tries to use "Moose"'s introspection API on a
       "Moo" class or role, it is transparently	inflated into a	"Moose"	class
       or role.	This makes it easier to	incorporate "Moo"-using	code into a
       "Moose" code base and vice versa.

       For example, a "Moose" class can	subclass a "Moo" class using "extends"
       or consume a "Moo" role using "with".

       The "Moose" authors hope	that one day "Moo" 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.

   Class::Tiny
       Finally,	we have	Class::Tiny. This module truly lives up	to its name.
       It has an incredibly minimal API	and absolutely no dependencies on any
       recent Perl. 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 Class::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 "Class::Tiny", all accessors are read-write. It generates a
       constructor for you, as well as the accessors you define.

       You can also use	Class::Tiny::Antlers for "Moose"-like syntax.

   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 "Class::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	Moo briefly.
	   "Moo" 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   Class::Tiny

	   "Class::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 "Class::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, Class::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. Moo provides a nice
       alternative to Moose when you want a lot	of features but	need faster
       compile time or to avoid	XS.

       We encourage you	to play	with and evaluate Moose, Moo, Class::Accessor,
       and Class::Tiny to see which OO system is right for you.

perl v5.28.3			  2020-05-14			  PERLOOTUT(1)

NAME | DATE | DESCRIPTION | OBJECT-ORIENTED FUNDAMENTALS | PERL OO SYSTEMS | CONCLUSION

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