PERLOOTUT(1) Perl Programmers Reference Guide PERLOOTUT(1) #
PERLOOTUT(1) Perl Programmers Reference Guide PERLOOTUT(1)
NNAAMMEE #
perlootut - Object-Oriented Programming in Perl Tutorial
DDAATTEE #
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.
DDEESSCCRRIIPPTTIIOONN #
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 <https://www.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.
OOBBJJEECCTT--OORRIIEENNTTEEDD FFUUNNDDAAMMEENNTTAALLSS #
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.
OObbjjeecctt An oobbjjeecctt is a data structure that bundles together data and subroutines which operate on that data. An object’s data is called aattttrriibbuutteess, and its subroutines are called mmeetthhooddss. 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 _/_e_t_c_/_h_o_s_t_n_a_m_e 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.
CCllaassss A ccllaassss 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 _/_e_t_c_/_h_o_s_t_n_a_m_e
object belongs to the "File" class. When we want to create a specific
object, we start with its class, and ccoonnssttrruucctt or iinnssttaannttiiaattee an object.
A specific object is often referred to as an iinnssttaannccee 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 $hostname = File->new(
path => '/etc/hostname',
content => "foo\n",
last_mod_time => 1304974868,
);
(Don't worry about that "->" operator, it will be explained later.)
_B_l_e_s_s_i_n_g
As we said earlier, most Perl objects are hashes, but an object can be an
instance of any Perl data type (scalar, array, etc.). Turning a plain
data structure into an object is done by bblleessssiinngg that data structure
using Perl's "bless" function.
While we strongly suggest you don't build your objects from scratch, you
should know the term bblleessss. A bblleesssseedd data structure (aka "a referent")
is an object. We sometimes say that an object has been "blessed into a
class".
Once a referent has been blessed, the "blessed" function from the
Scalar::Util core module can tell us its class name. This subroutine
returns an object's class when passed an object, and false otherwise.
use Scalar::Util 'blessed';
print blessed($hash); # undef
print blessed($hostname); # File
_C_o_n_s_t_r_u_c_t_o_r
A ccoonnssttrruuccttoorr 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(...);
MMeetthhooddss You already learned that a mmeetthhoodd is a subroutine that operates on an object. You can think of a method as the things that an object can _d_o. 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 _h_o_w _i_t_'_s _c_a_l_l_e_d. The arrow operator ("->")
tells Perl that we are calling a method.
When we make a method call, Perl arranges for the method's iinnvvooccaanntt to be
passed as the first argument. IInnvvooccaanntt 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";
}
$file->print_info("The file is located at ");
# The file is located at /etc/hostname
AAttttrriibbuutteess Each class can define its aattttrriibbuutteess. When we instantiate an object, we assign values to those attributes. For example, every “File” object has a path. Attributes are sometimes called pprrooppeerrttiieess.
Perl has no special syntax for attributes. Under the hood, attributes are
often stored as keys in the object's underlying hash, but don't worry
about this.
We recommend that you only access attributes via aacccceessssoorr methods. These
are methods that can get or set the value of each attribute. We saw this
earlier in the "print_info()" example, which calls "$self->path".
You might also see the terms ggeetttteerr and sseetttteerr. These are two types of
accessors. A getter gets the attribute's value, while a setter sets it.
Another term for a setter is mmuuttaattoorr
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.
PPoollyymmoorrpphhiissmm PPoollyymmoorrpphhiissmm 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 wwee
ddoonn''tt hhaavvee ttoo kknnooww wwhhaatt ccllaassss tthhee oobbjjeecctt bbeelloonnggss ttoo!!
Polymorphism is one of the key concepts of object-oriented design.
IInnhheerriittaannccee IInnhheerriittaannccee 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 iinnhheerriittss from
"File". An "File::MP3" iiss--aa _m_o_r_e _s_p_e_c_i_f_i_c type of "File". All mp3 files
are files, but not all files are mp3 files.
We often refer to inheritance relationships as ppaarreenntt--cchhiilldd or
"superclass"/"subclass" relationships. Sometimes we say that the child
has an iiss--aa relationship with its parent class.
"File" is a ssuuppeerrccllaassss of "File::MP3", and "File::MP3" is a ssuubbccllaassss 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 rroolleess 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.
_O_v_e_r_r_i_d_i_n_g _m_e_t_h_o_d_s _a_n_d _m_e_t_h_o_d _r_e_s_o_l_u_t_i_o_n
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
oovveerrrriiddee 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 mmeetthhoodd
rreessoolluuttiioonn. 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.
EEnnccaappssuullaattiioonn EEnnccaappssuullaattiioonn is the idea that an object is opaque. When another developer uses your class, they don’t need to know _h_o_w it is implemented, they just need to know _w_h_a_t 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.
CCoommppoossiittiioonn In object-oriented code, we often find that one object references another object. This is called ccoommppoossiittiioonn, or a hhaass--aa 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 ccoommppoosseedd of
several other objects.
RRoolleess RRoolleess are something that a class _d_o_e_s, rather than something that it _i_s. Roles are relatively new to Perl, but have become rather popular. Roles are aapppplliieedd to classes. Sometimes we say that classes ccoonnssuummee 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.
WWhheenn ttoo UUssee OOOO Object Orientation is not the best solution to every problem. In _P_e_r_l _B_e_s_t _P_r_a_c_t_i_c_e_s (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:
• The system being designed is large, or is likely to become large.
• The data can be aggregated into obvious structures, especially if
there's a large amount of data in each aggregate.
• The various types of data aggregate form a natural hierarchy that
facilitates the use of inheritance and polymorphism.
• You have a piece of data on which many different operations are
applied.
• 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.
• It's likely you'll have to add new data types later.
• The typical interactions between pieces of data are best represented
by operators.
• The implementation of individual components of the system is likely
to change over time.
• The system design is already object-oriented.
• Large numbers of other programmers will be using your code modules.
PPEERRLL OOOO SSYYSSTTEEMMSS #
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.
MMoooossee 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 Raku 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:
• 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 _w_h_a_t your class is, rather than having to tell Perl _h_o_w 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".
• 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);
}
• 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.
• 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.
• 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.
• Rich ecosystem
There is a rich ecosystem of "Moose" extensions on CPAN under the
MooseX <https://metacpan.org/search?q=MooseX> namespace. In addition,
many modules on CPAN already use "Moose", providing you with lots of
examples to learn from.
• 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
<https://metacpan.org/pod/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 _l_o_t 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 _a_n_y dependencies is a problem, then "Moose" may
not be right for you.
_M_o_o
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 _i_d_e_n_t_i_c_a_l 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".
CCllaassss::::AAcccceessssoorr 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.
CCllaassss::::TTiinnyy 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.
RRoollee::::TTiinnyy 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"
OOOO SSyysstteemm SSuummmmaarryy Here’s a brief recap of the options we covered:
• 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.
• 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.
• 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.
• 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.
OOtthheerr OOOO SSyysstteemmss 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.
CCOONNCCLLUUSSIIOONN #
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 v5.36.3 2021-03-02 PERLOOTUT(1)