1PERLOOTUT(1) Perl Programmers Reference Guide PERLOOTUT(1)
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6 perlootut - Object-Oriented Programming in Perl Tutorial
7
9 This document was created in February, 2011, and the last major
10 revision was in February, 2013.
11 If you are reading this in the future then it's possible that the state
13 of the art has changed. We recommend you start by reading the perlootut
14 document in the latest stable release of Perl, rather than this
15 version.
16
18 This document provides an introduction to object-oriented programming
19 in Perl. It begins with a brief overview of the concepts behind object
20 oriented design. Then it introduces several different OO systems from
21 CPAN <https://www.cpan.org> which build on top of what Perl provides.
22 By default, Perl's built-in OO system is very minimal, leaving you to
24 do most of the work. This minimalism made a lot of sense in 1994, but
25 in the years since Perl 5.0 we've seen a number of common patterns
26 emerge in Perl OO. Fortunately, Perl's flexibility has allowed a rich
27 ecosystem of Perl OO systems to flourish.
28 If you want to know how Perl OO works under the hood, the perlobj
30 document explains the nitty gritty details.
31 This document assumes that you already understand the basics of Perl
33 syntax, variable types, operators, and subroutine calls. If you don't
34 understand these concepts yet, please read perlintro first. You should
35 also read the perlsyn, perlop, and perlsub documents.
36
38 Most object systems share a number of common concepts. You've probably
39 heard terms like "class", "object, "method", and "attribute" before.
40 Understanding the concepts will make it much easier to read and write
41 object-oriented code. If you're already familiar with these terms, you
42 should still skim this section, since it explains each concept in terms
43 of Perl's OO implementation.
44 Perl's OO system is class-based. Class-based OO is fairly common. It's
46 used by Java, C++, C#, Python, Ruby, and many other languages. There
47 are other object orientation paradigms as well. JavaScript is the most
48 popular language to use another paradigm. JavaScript's OO system is
49 prototype-based.
50 Object
52 An object is a data structure that bundles together data and
53 subroutines which operate on that data. An object's data is called
54 attributes, and its subroutines are called methods. An object can be
55 thought of as a noun (a person, a web service, a computer).
56 An object represents a single discrete thing. For example, an object
58 might represent a file. The attributes for a file object might include
59 its path, content, and last modification time. If we created an object
60 to represent /etc/hostname on a machine named "foo.example.com", that
61 object's path would be "/etc/hostname", its content would be "foo\n",
62 and it's last modification time would be 1304974868 seconds since the
63 beginning of the epoch.
64 The methods associated with a file might include rename() and write().
66 In Perl most objects are hashes, but the OO systems we recommend keep
68 you from having to worry about this. In practice, it's best to consider
69 an object's internal data structure opaque.
70 Class
72 A class defines the behavior of a category of objects. A class is a
73 name for a category (like "File"), and a class also defines the
74 behavior of objects in that category.
75 All objects belong to a specific class. For example, our /etc/hostname
77 object belongs to the "File" class. When we want to create a specific
78 object, we start with its class, and construct or instantiate an
79 object. A specific object is often referred to as an instance of a
80 class.
81 In Perl, any package can be a class. The difference between a package
83 which is a class and one which isn't is based on how the package is
84 used. Here's our "class declaration" for the "File" class:
85 package File;
87 In Perl, there is no special keyword for constructing an object.
89 However, most OO modules on CPAN use a method named new() to construct
90 a new object:
91 my $hostname = File->new(
93 path => '/etc/hostname',
94 content => "foo\n",
95 last_mod_time => 1304974868,
96 );
97 (Don't worry about that "->" operator, it will be explained later.)
99 Blessing
101 As we said earlier, most Perl objects are hashes, but an object can be
103 an instance of any Perl data type (scalar, array, etc.). Turning a
104 plain data structure into an object is done by blessing that data
105 structure using Perl's "bless" function.
106 While we strongly suggest you don't build your objects from scratch,
108 you should know the term bless. A blessed data structure (aka "a
109 referent") is an object. We sometimes say that an object has been
110 "blessed into a class".
111 Once a referent has been blessed, the "blessed" function from the
113 Scalar::Util core module can tell us its class name. This subroutine
114 returns an object's class when passed an object, and false otherwise.
115 use Scalar::Util 'blessed';
117 print blessed($hash); # undef
119 print blessed($hostname); # File
120 Constructor
122 A constructor creates a new object. In Perl, a class's constructor is
124 just another method, unlike some other languages, which provide syntax
125 for constructors. Most Perl classes use "new" as the name for their
126 constructor:
127 my $file = File->new(...);
129 Methods
131 You already learned that a method is a subroutine that operates on an
132 object. You can think of a method as the things that an object can do.
133 If an object is a noun, then methods are its verbs (save, print, open).
134 In Perl, methods are simply subroutines that live in a class's package.
136 Methods are always written to receive the object as their first
137 argument:
138 sub print_info {
140 my $self = shift;
141 print "This file is at ", $self->path, "\n";
143 }
144 $file->print_info;
146 # The file is at /etc/hostname
147 What makes a method special is how it's called. The arrow operator
149 ("->") tells Perl that we are calling a method.
150 When we make a method call, Perl arranges for the method's invocant to
152 be passed as the first argument. Invocant is a fancy name for the thing
153 on the left side of the arrow. The invocant can either be a class name
154 or an object. We can also pass additional arguments to the method:
155 sub print_info {
157 my $self = shift;
158 my $prefix = shift // "This file is at ";
159 print $prefix, ", ", $self->path, "\n";
161 }
162 $file->print_info("The file is located at ");
164 # The file is located at /etc/hostname
165 Attributes
167 Each class can define its attributes. When we instantiate an object, we
168 assign values to those attributes. For example, every "File" object has
169 a path. Attributes are sometimes called properties.
170 Perl has no special syntax for attributes. Under the hood, attributes
172 are often stored as keys in the object's underlying hash, but don't
173 worry about this.
174 We recommend that you only access attributes via accessor methods.
176 These are methods that can get or set the value of each attribute. We
177 saw this earlier in the print_info() example, which calls
178 "$self->path".
179 You might also see the terms getter and setter. These are two types of
181 accessors. A getter gets the attribute's value, while a setter sets it.
182 Another term for a setter is mutator
183 Attributes are typically defined as read-only or read-write. Read-only
185 attributes can only be set when the object is first created, while
186 read-write attributes can be altered at any time.
187 The value of an attribute may itself be another object. For example,
189 instead of returning its last mod time as a number, the "File" class
190 could return a DateTime object representing that value.
191 It's possible to have a class that does not expose any publicly
193 settable attributes. Not every class has attributes and methods.
194 Polymorphism
196 Polymorphism is a fancy way of saying that objects from two different
197 classes share an API. For example, we could have "File" and "WebPage"
198 classes which both have a print_content() method. This method might
199 produce different output for each class, but they share a common
200 interface.
201 While the two classes may differ in many ways, when it comes to the
203 print_content() method, they are the same. This means that we can try
204 to call the print_content() method on an object of either class, and we
205 don't have to know what class the object belongs to!
206 Polymorphism is one of the key concepts of object-oriented design.
208 Inheritance
210 Inheritance lets you create a specialized version of an existing class.
211 Inheritance lets the new class reuse the methods and attributes of
212 another class.
213 For example, we could create an "File::MP3" class which inherits from
215 "File". An "File::MP3" is-a more specific type of "File". All mp3
216 files are files, but not all files are mp3 files.
217 We often refer to inheritance relationships as parent-child or
219 "superclass"/"subclass" relationships. Sometimes we say that the child
220 has an is-a relationship with its parent class.
221 "File" is a superclass of "File::MP3", and "File::MP3" is a subclass of
223 "File".
224 package File::MP3;
226 use parent 'File';
228 The parent module is one of several ways that Perl lets you define
230 inheritance relationships.
231 Perl allows multiple inheritance, which means that a class can inherit
233 from multiple parents. While this is possible, we strongly recommend
234 against it. Generally, you can use roles to do everything you can do
235 with multiple inheritance, but in a cleaner way.
236 Note that there's nothing wrong with defining multiple subclasses of a
238 given class. This is both common and safe. For example, we might define
239 "File::MP3::FixedBitrate" and "File::MP3::VariableBitrate" classes to
240 distinguish between different types of mp3 file.
241 Overriding methods and method resolution
243 Inheritance allows two classes to share code. By default, every method
245 in the parent class is also available in the child. The child can
246 explicitly override a parent's method to provide its own
247 implementation. For example, if we have an "File::MP3" object, it has
248 the print_info() method from "File":
249 my $cage = File::MP3->new(
251 path => 'mp3s/My-Body-Is-a-Cage.mp3',
252 content => $mp3_data,
253 last_mod_time => 1304974868,
254 title => 'My Body Is a Cage',
255 );
256 $cage->print_info;
258 # The file is at mp3s/My-Body-Is-a-Cage.mp3
259 If we wanted to include the mp3's title in the greeting, we could
261 override the method:
262 package File::MP3;
264 use parent 'File';
266 sub print_info {
268 my $self = shift;
269 print "This file is at ", $self->path, "\n";
271 print "Its title is ", $self->title, "\n";
272 }
273 $cage->print_info;
275 # The file is at mp3s/My-Body-Is-a-Cage.mp3
276 # Its title is My Body Is a Cage
277 The process of determining what method should be used is called method
279 resolution. What Perl does is look at the object's class first
280 ("File::MP3" in this case). If that class defines the method, then that
281 class's version of the method is called. If not, Perl looks at each
282 parent class in turn. For "File::MP3", its only parent is "File". If
283 "File::MP3" does not define the method, but "File" does, then Perl
284 calls the method in "File".
285 If "File" inherited from "DataSource", which inherited from "Thing",
287 then Perl would keep looking "up the chain" if necessary.
288 It is possible to explicitly call a parent method from a child:
290 package File::MP3;
292 use parent 'File';
294 sub print_info {
296 my $self = shift;
297 $self->SUPER::print_info();
299 print "Its title is ", $self->title, "\n";
300 }
301 The "SUPER::" bit tells Perl to look for the print_info() in the
303 "File::MP3" class's inheritance chain. When it finds the parent class
304 that implements this method, the method is called.
305 We mentioned multiple inheritance earlier. The main problem with
307 multiple inheritance is that it greatly complicates method resolution.
308 See perlobj for more details.
309 Encapsulation
311 Encapsulation is the idea that an object is opaque. When another
312 developer uses your class, they don't need to know how it is
313 implemented, they just need to know what it does.
314 Encapsulation is important for several reasons. First, it allows you to
316 separate the public API from the private implementation. This means you
317 can change that implementation without breaking the API.
318 Second, when classes are well encapsulated, they become easier to
320 subclass. Ideally, a subclass uses the same APIs to access object data
321 that its parent class uses. In reality, subclassing sometimes involves
322 violating encapsulation, but a good API can minimize the need to do
323 this.
324 We mentioned earlier that most Perl objects are implemented as hashes
326 under the hood. The principle of encapsulation tells us that we should
327 not rely on this. Instead, we should use accessor methods to access the
328 data in that hash. The object systems that we recommend below all
329 automate the generation of accessor methods. If you use one of them,
330 you should never have to access the object as a hash directly.
331 Composition
333 In object-oriented code, we often find that one object references
334 another object. This is called composition, or a has-a relationship.
335 Earlier, we mentioned that the "File" class's "last_mod_time" accessor
337 could return a DateTime object. This is a perfect example of
338 composition. We could go even further, and make the "path" and
339 "content" accessors return objects as well. The "File" class would then
340 be composed of several other objects.
341 Roles
343 Roles are something that a class does, rather than something that it
344 is. Roles are relatively new to Perl, but have become rather popular.
345 Roles are applied to classes. Sometimes we say that classes consume
346 roles.
347 Roles are an alternative to inheritance for providing polymorphism.
349 Let's assume we have two classes, "Radio" and "Computer". Both of these
350 things have on/off switches. We want to model that in our class
351 definitions.
352 We could have both classes inherit from a common parent, like
354 "Machine", but not all machines have on/off switches. We could create a
355 parent class called "HasOnOffSwitch", but that is very artificial.
356 Radios and computers are not specializations of this parent. This
357 parent is really a rather ridiculous creation.
358 This is where roles come in. It makes a lot of sense to create a
360 "HasOnOffSwitch" role and apply it to both classes. This role would
361 define a known API like providing turn_on() and turn_off() methods.
362 Perl does not have any built-in way to express roles. In the past,
364 people just bit the bullet and used multiple inheritance. Nowadays,
365 there are several good choices on CPAN for using roles.
366 When to Use OO
368 Object Orientation is not the best solution to every problem. In Perl
369 Best Practices (copyright 2004, Published by O'Reilly Media, Inc.),
370 Damian Conway provides a list of criteria to use when deciding if OO is
371 the right fit for your problem:
372 • The system being designed is large, or is likely to become large.
374 • The data can be aggregated into obvious structures, especially if
376 there's a large amount of data in each aggregate.
377 • The various types of data aggregate form a natural hierarchy that
379 facilitates the use of inheritance and polymorphism.
380 • You have a piece of data on which many different operations are
382 applied.
383 • You need to perform the same general operations on related types of
385 data, but with slight variations depending on the specific type of
386 data the operations are applied to.
387 • It's likely you'll have to add new data types later.
389 • The typical interactions between pieces of data are best
391 represented by operators.
392 • The implementation of individual components of the system is likely
394 to change over time.
395 • The system design is already object-oriented.
397 • Large numbers of other programmers will be using your code modules.
399
401 As we mentioned before, Perl's built-in OO system is very minimal, but
402 also quite flexible. Over the years, many people have developed systems
403 which build on top of Perl's built-in system to provide more features
404 and convenience.
405 We strongly recommend that you use one of these systems. Even the most
407 minimal of them eliminates a lot of repetitive boilerplate. There's
408 really no good reason to write your classes from scratch in Perl.
409 If you are interested in the guts underlying these systems, check out
411 perlobj.
412 Moose
414 Moose bills itself as a "postmodern object system for Perl 5". Don't be
415 scared, the "postmodern" label is a callback to Larry's description of
416 Perl as "the first postmodern computer language".
417 "Moose" provides a complete, modern OO system. Its biggest influence is
419 the Common Lisp Object System, but it also borrows ideas from Smalltalk
420 and several other languages. "Moose" was created by Stevan Little, and
421 draws heavily from his work on the Raku OO design.
422 Here is our "File" class using "Moose":
424 package File;
426 use Moose;
427 has path => ( is => 'ro' );
429 has content => ( is => 'ro' );
430 has last_mod_time => ( is => 'ro' );
431 sub print_info {
433 my $self = shift;
434 print "This file is at ", $self->path, "\n";
436 }
437 "Moose" provides a number of features:
439 • Declarative sugar
441 "Moose" provides a layer of declarative "sugar" for defining
443 classes. That sugar is just a set of exported functions that make
444 declaring how your class works simpler and more palatable. This
445 lets you describe what your class is, rather than having to tell
446 Perl how to implement your class.
447 The has() subroutine declares an attribute, and "Moose"
449 automatically creates accessors for these attributes. It also takes
450 care of creating a new() method for you. This constructor knows
451 about the attributes you declared, so you can set them when
452 creating a new "File".
453 • Roles built-in
455 "Moose" lets you define roles the same way you define classes:
457 package HasOnOffSwitch;
459 use Moose::Role;
460 has is_on => (
462 is => 'rw',
463 isa => 'Bool',
464 );
465 sub turn_on {
467 my $self = shift;
468 $self->is_on(1);
469 }
470 sub turn_off {
472 my $self = shift;
473 $self->is_on(0);
474 }
475 • A miniature type system
477 In the example above, you can see that we passed "isa => 'Bool'" to
479 has() when creating our "is_on" attribute. This tells "Moose" that
480 this attribute must be a boolean value. If we try to set it to an
481 invalid value, our code will throw an error.
482 • Full introspection and manipulation
484 Perl's built-in introspection features are fairly minimal. "Moose"
486 builds on top of them and creates a full introspection layer for
487 your classes. This lets you ask questions like "what methods does
488 the File class implement?" It also lets you modify your classes
489 programmatically.
490 • Self-hosted and extensible
492 "Moose" describes itself using its own introspection API. Besides
494 being a cool trick, this means that you can extend "Moose" using
495 "Moose" itself.
496 • Rich ecosystem
498 There is a rich ecosystem of "Moose" extensions on CPAN under the
500 MooseX <https://metacpan.org/search?q=MooseX> namespace. In
501 addition, many modules on CPAN already use "Moose", providing you
502 with lots of examples to learn from.
503 • Many more features
505 "Moose" is a very powerful tool, and we can't cover all of its
507 features here. We encourage you to learn more by reading the
508 "Moose" documentation, starting with Moose::Manual
509 <https://metacpan.org/pod/Moose::Manual>.
510 Of course, "Moose" isn't perfect.
512 "Moose" can make your code slower to load. "Moose" itself is not small,
514 and it does a lot of code generation when you define your class. This
515 code generation means that your runtime code is as fast as it can be,
516 but you pay for this when your modules are first loaded.
517 This load time hit can be a problem when startup speed is important,
519 such as with a command-line script or a "plain vanilla" CGI script that
520 must be loaded each time it is executed.
521 Before you panic, know that many people do use "Moose" for command-line
523 tools and other startup-sensitive code. We encourage you to try "Moose"
524 out first before worrying about startup speed.
525 "Moose" also has several dependencies on other modules. Most of these
527 are small stand-alone modules, a number of which have been spun off
528 from "Moose". "Moose" itself, and some of its dependencies, require a
529 compiler. If you need to install your software on a system without a
530 compiler, or if having any dependencies is a problem, then "Moose" may
531 not be right for you.
532 Moo
534 If you try "Moose" and find that one of these issues is preventing you
536 from using "Moose", we encourage you to consider Moo next. "Moo"
537 implements a subset of "Moose"'s functionality in a simpler package.
538 For most features that it does implement, the end-user API is identical
539 to "Moose", meaning you can switch from "Moo" to "Moose" quite easily.
540 "Moo" does not implement most of "Moose"'s introspection API, so it's
542 often faster when loading your modules. Additionally, none of its
543 dependencies require XS, so it can be installed on machines without a
544 compiler.
545 One of "Moo"'s most compelling features is its interoperability with
547 "Moose". When someone tries to use "Moose"'s introspection API on a
548 "Moo" class or role, it is transparently inflated into a "Moose" class
549 or role. This makes it easier to incorporate "Moo"-using code into a
550 "Moose" code base and vice versa.
551 For example, a "Moose" class can subclass a "Moo" class using "extends"
553 or consume a "Moo" role using "with".
554 The "Moose" authors hope that one day "Moo" can be made obsolete by
556 improving "Moose" enough, but for now it provides a worthwhile
557 alternative to "Moose".
558 Class::Accessor
560 Class::Accessor is the polar opposite of "Moose". It provides very few
561 features, nor is it self-hosting.
562 It is, however, very simple, pure Perl, and it has no non-core
564 dependencies. It also provides a "Moose-like" API on demand for the
565 features it supports.
566 Even though it doesn't do much, it is still preferable to writing your
568 own classes from scratch.
569 Here's our "File" class with "Class::Accessor":
571 package File;
573 use Class::Accessor 'antlers';
574 has path => ( is => 'ro' );
576 has content => ( is => 'ro' );
577 has last_mod_time => ( is => 'ro' );
578 sub print_info {
580 my $self = shift;
581 print "This file is at ", $self->path, "\n";
583 }
584 The "antlers" import flag tells "Class::Accessor" that you want to
586 define your attributes using "Moose"-like syntax. The only parameter
587 that you can pass to "has" is "is". We recommend that you use this
588 Moose-like syntax if you choose "Class::Accessor" since it means you
589 will have a smoother upgrade path if you later decide to move to
590 "Moose".
591 Like "Moose", "Class::Accessor" generates accessor methods and a
593 constructor for your class.
594 Class::Tiny
596 Finally, we have Class::Tiny. This module truly lives up to its name.
597 It has an incredibly minimal API and absolutely no dependencies on any
598 recent Perl. Still, we think it's a lot easier to use than writing your
599 own OO code from scratch.
600 Here's our "File" class once more:
602 package File;
604 use Class::Tiny qw( path content last_mod_time );
605 sub print_info {
607 my $self = shift;
608 print "This file is at ", $self->path, "\n";
610 }
611 That's it!
613 With "Class::Tiny", all accessors are read-write. It generates a
615 constructor for you, as well as the accessors you define.
616 You can also use Class::Tiny::Antlers for "Moose"-like syntax.
618 Role::Tiny
620 As we mentioned before, roles provide an alternative to inheritance,
621 but Perl does not have any built-in role support. If you choose to use
622 Moose, it comes with a full-fledged role implementation. However, if
623 you use one of our other recommended OO modules, you can still use
624 roles with Role::Tiny
625 "Role::Tiny" provides some of the same features as Moose's role system,
627 but in a much smaller package. Most notably, it doesn't support any
628 sort of attribute declaration, so you have to do that by hand. Still,
629 it's useful, and works well with "Class::Accessor" and "Class::Tiny"
630 OO System Summary
632 Here's a brief recap of the options we covered:
633 • Moose
635 "Moose" is the maximal option. It has a lot of features, a big
637 ecosystem, and a thriving user base. We also covered Moo briefly.
638 "Moo" is "Moose" lite, and a reasonable alternative when Moose
639 doesn't work for your application.
640 • Class::Accessor
642 "Class::Accessor" does a lot less than "Moose", and is a nice
644 alternative if you find "Moose" overwhelming. It's been around a
645 long time and is well battle-tested. It also has a minimal "Moose"
646 compatibility mode which makes moving from "Class::Accessor" to
647 "Moose" easy.
648 • Class::Tiny
650 "Class::Tiny" is the absolute minimal option. It has no
652 dependencies, and almost no syntax to learn. It's a good option for
653 a super minimal environment and for throwing something together
654 quickly without having to worry about details.
655 • Role::Tiny
657 Use "Role::Tiny" with "Class::Accessor" or "Class::Tiny" if you
659 find yourself considering multiple inheritance. If you go with
660 "Moose", it comes with its own role implementation.
661 Other OO Systems
663 There are literally dozens of other OO-related modules on CPAN besides
664 those covered here, and you're likely to run across one or more of them
665 if you work with other people's code.
666 In addition, plenty of code in the wild does all of its OO "by hand",
668 using just the Perl built-in OO features. If you need to maintain such
669 code, you should read perlobj to understand exactly how Perl's built-in
670 OO works.
671
673 As we said before, Perl's minimal OO system has led to a profusion of
674 OO systems on CPAN. While you can still drop down to the bare metal and
675 write your classes by hand, there's really no reason to do that with
676 modern Perl.
677 For small systems, Class::Tiny and Class::Accessor both provide minimal
679 object systems that take care of basic boilerplate for you.
680 For bigger projects, Moose provides a rich set of features that will
682 let you focus on implementing your business logic. Moo provides a nice
683 alternative to Moose when you want a lot of features but need faster
684 compile time or to avoid XS.
685 We encourage you to play with and evaluate Moose, Moo, Class::Accessor,
687 and Class::Tiny to see which OO system is right for you.
688perl v5.38.2 2023-11-30 PERLOOTUT(1)