3 perlboot - Beginner's Object-Oriented Tutorial
7 If you're not familiar with objects from other languages, some of the
8 other Perl object documentation may be a little daunting, such as
9 L<perlobj>, a basic reference in using objects, and L<perltoot>, which
10 introduces readers to the peculiarities of Perl's object system in a
13 So, let's take a different approach, presuming no prior object
14 experience. It helps if you know about subroutines (L<perlsub>),
15 references (L<perlref> et. seq.), and packages (L<perlmod>), so become
16 familiar with those first if you haven't already.
18 =head2 If we could talk to the animals...
20 Let's let the animals talk for a moment:
23 print "a Cow goes moooo!\n";
26 print "a Horse goes neigh!\n";
29 print "a Sheep goes baaaah!\n";
42 Nothing spectacular here. Simple subroutines, albeit from separate
43 packages, and called using the full package name. So let's create
46 # Cow::speak, Horse::speak, Sheep::speak as before
47 @pasture = qw(Cow Cow Horse Sheep Sheep);
48 foreach $animal (@pasture) {
60 Wow. That symbolic coderef de-referencing there is pretty nasty.
61 We're counting on C<no strict subs> mode, certainly not recommended
62 for larger programs. And why was that necessary? Because the name of
63 the package seems to be inseparable from the name of the subroutine we
64 want to invoke within that package.
68 =head2 Introducing the method invocation arrow
70 For now, let's say that C<< Class->method >> invokes subroutine
71 C<method> in package C<Class>. (Here, "Class" is used in its
72 "category" meaning, not its "scholastic" meaning.) That's not
73 completely accurate, but we'll do this one step at a time. Now let's
76 # Cow::speak, Horse::speak, Sheep::speak as before
81 And once again, this results in:
87 That's not fun yet. Same number of characters, all constant, no
88 variables. But yet, the parts are separable now. Watch:
91 $a->speak; # invokes Cow->speak
93 Ahh! Now that the package name has been parted from the subroutine
94 name, we can use a variable package name. And this time, we've got
95 something that works even when C<use strict refs> is enabled.
97 =head2 Invoking a barnyard
99 Let's take that new arrow invocation and put it back in the barnyard
103 print "a Cow goes moooo!\n";
106 print "a Horse goes neigh!\n";
109 print "a Sheep goes baaaah!\n";
112 @pasture = qw(Cow Cow Horse Sheep Sheep);
113 foreach $animal (@pasture) {
117 There! Now we have the animals all talking, and safely at that,
118 without the use of symbolic coderefs.
120 But look at all that common code. Each of the C<speak> routines has a
121 similar structure: a C<print> operator and a string that contains
122 common text, except for two of the words. It'd be nice if we could
123 factor out the commonality, in case we decide later to change it all
124 to C<says> instead of C<goes>.
126 And we actually have a way of doing that without much fuss, but we
127 have to hear a bit more about what the method invocation arrow is
128 actually doing for us.
130 =head2 The extra parameter of method invocation
136 attempts to invoke subroutine C<Class::method> as:
138 Class::method("Class", @args);
140 (If the subroutine can't be found, "inheritance" kicks in, but we'll
141 get to that later.) This means that we get the class name as the
142 first parameter (the only parameter, if no arguments are given). So
143 we can rewrite the C<Sheep> speaking subroutine as:
147 print "a $class goes baaaah!\n";
150 And the other two animals come out similarly:
154 print "a $class goes moooo!\n";
158 print "a $class goes neigh!\n";
161 In each case, C<$class> will get the value appropriate for that
162 subroutine. But once again, we have a lot of similar structure. Can
163 we factor that out even further? Yes, by calling another method in
166 =head2 Calling a second method to simplify things
168 Let's call out from C<speak> to a helper method called C<sound>.
169 This method provides the constant text for the sound itself.
172 sub sound { "moooo" }
175 print "a $class goes ", $class->sound, "!\n";
179 Now, when we call C<< Cow->speak >>, we get a C<$class> of C<Cow> in
180 C<speak>. This in turn selects the C<< Cow->sound >> method, which
181 returns C<moooo>. But how different would this be for the C<Horse>?
184 sub sound { "neigh" }
187 print "a $class goes ", $class->sound, "!\n";
191 Only the name of the package and the specific sound change. So can we
192 somehow share the definition for C<speak> between the Cow and the
193 Horse? Yes, with inheritance!
195 =head2 Inheriting the windpipes
197 We'll define a common subroutine package called C<Animal>, with the
198 definition for C<speak>:
203 print "a $class goes ", $class->sound, "!\n";
207 Then, for each animal, we say it "inherits" from C<Animal>, along
208 with the animal-specific sound:
212 sub sound { "moooo" }
215 Note the added C<@ISA> array. We'll get to that in a minute.
217 But what happens when we invoke C<< Cow->speak >> now?
219 First, Perl constructs the argument list. In this case, it's just
220 C<Cow>. Then Perl looks for C<Cow::speak>. But that's not there, so
221 Perl checks for the inheritance array C<@Cow::ISA>. It's there,
222 and contains the single name C<Animal>.
224 Perl next checks for C<speak> inside C<Animal> instead, as in
225 C<Animal::speak>. And that's found, so Perl invokes that subroutine
226 with the already frozen argument list.
228 Inside the C<Animal::speak> subroutine, C<$class> becomes C<Cow> (the
229 first argument). So when we get to the step of invoking
230 C<< $class->sound >>, it'll be looking for C<< Cow->sound >>, which
231 gets it on the first try without looking at C<@ISA>. Success!
233 =head2 A few notes about @ISA
235 This magical C<@ISA> variable (pronounced "is a" not "ice-uh"), has
236 declared that C<Cow> "is a" C<Animal>. Note that it's an array,
237 not a simple single value, because on rare occasions, it makes sense
238 to have more than one parent class searched for the missing methods.
240 If C<Animal> also had an C<@ISA>, then we'd check there too. The
241 search is recursive, depth-first, left-to-right in each C<@ISA> by
242 default (see L<mro> for alternatives). Typically, each C<@ISA> has
243 only one element (multiple elements means multiple inheritance and
244 multiple headaches), so we get a nice tree of inheritance.
246 When we turn on C<use strict>, we'll get complaints on C<@ISA>, since
247 it's not a variable containing an explicit package name, nor is it a
248 lexical ("my") variable. We can't make it a lexical variable though
249 (it has to belong to the package to be found by the inheritance mechanism),
250 so there's a couple of straightforward ways to handle that.
252 The easiest is to just spell the package name out:
254 @Cow::ISA = qw(Animal);
256 Or declare it as package global variable:
259 our @ISA = qw(Animal);
261 Or allow it as an implicitly named package variable:
267 If you're bringing in the class from outside, via an object-oriented
280 And that's pretty darn compact.
282 =head2 Overriding the methods
284 Let's add a mouse, which can barely be heard:
286 # Animal package from before
289 sub sound { "squeak" }
292 print "a $class goes ", $class->sound, "!\n";
293 print "[but you can barely hear it!]\n";
302 [but you can barely hear it!]
304 Here, C<Mouse> has its own speaking routine, so C<< Mouse->speak >>
305 doesn't immediately invoke C<< Animal->speak >>. This is known as
306 "overriding". In fact, we didn't even need to say that a C<Mouse> was
307 an C<Animal> at all, since all of the methods needed for C<speak> are
308 completely defined with C<Mouse>.
310 But we've now duplicated some of the code from C<< Animal->speak >>,
311 and this can once again be a maintenance headache. So, can we avoid
312 that? Can we say somehow that a C<Mouse> does everything any other
313 C<Animal> does, but add in the extra comment? Sure!
315 First, we can invoke the C<Animal::speak> method directly:
317 # Animal package from before
320 sub sound { "squeak" }
323 Animal::speak($class);
324 print "[but you can barely hear it!]\n";
328 Note that we have to include the C<$class> parameter (almost surely
329 the value of C<"Mouse">) as the first parameter to C<Animal::speak>,
330 since we've stopped using the method arrow. Why did we stop? Well,
331 if we invoke C<< Animal->speak >> there, the first parameter to the
332 method will be C<"Animal"> not C<"Mouse">, and when time comes for it
333 to call for the C<sound>, it won't have the right class to come back
336 Invoking C<Animal::speak> directly is a mess, however. What if
337 C<Animal::speak> didn't exist before, and was being inherited from a
338 class mentioned in C<@Animal::ISA>? Because we are no longer using
339 the method arrow, we get one and only one chance to hit the right
342 Also note that the C<Animal> classname is now hardwired into the
343 subroutine selection. This is a mess if someone maintains the code,
344 changing C<@ISA> for C<Mouse> and didn't notice C<Animal> there in
345 C<speak>. So, this is probably not the right way to go.
347 =head2 Starting the search from a different place
349 A better solution is to tell Perl to search from a higher place
350 in the inheritance chain:
352 # same Animal as before
354 # same @ISA, &sound as before
357 $class->Animal::speak;
358 print "[but you can barely hear it!]\n";
362 Ahh. This works. Using this syntax, we start with C<Animal> to find
363 C<speak>, and use all of C<Animal>'s inheritance chain if not found
364 immediately. And yet the first parameter will be C<$class>, so the
365 found C<speak> method will get C<Mouse> as its first entry, and
366 eventually work its way back to C<Mouse::sound> for the details.
368 But this isn't the best solution. We still have to keep the C<@ISA>
369 and the initial search package coordinated. Worse, if C<Mouse> had
370 multiple entries in C<@ISA>, we wouldn't necessarily know which one
371 had actually defined C<speak>. So, is there an even better way?
373 =head2 The SUPER way of doing things
375 By changing the C<Animal> class to the C<SUPER> class in that
376 invocation, we get a search of all of our super classes (classes
377 listed in C<@ISA>) automatically:
379 # same Animal as before
381 # same @ISA, &sound as before
384 $class->SUPER::speak;
385 print "[but you can barely hear it!]\n";
389 So, C<SUPER::speak> means look in the current package's C<@ISA> for
390 C<speak>, invoking the first one found. Note that it does I<not> look in
391 the C<@ISA> of C<$class>.
393 =head2 Where we're at so far...
395 So far, we've seen the method arrow syntax:
397 Class->method(@args);
404 which constructs an argument list of:
408 and attempts to invoke
410 Class::method("Class", @Args);
412 However, if C<Class::method> is not found, then C<@Class::ISA> is examined
413 (recursively) to locate a package that does indeed contain C<method>,
414 and that subroutine is invoked instead.
416 Using this simple syntax, we have class methods, (multiple)
417 inheritance, overriding, and extending. Using just what we've seen so
418 far, we've been able to factor out common code, and provide a nice way
419 to reuse implementations with variations. This is at the core of what
420 objects provide, but objects also provide instance data, which we
421 haven't even begun to cover.
423 =head2 A horse is a horse, of course of course -- or is it?
425 Let's start with the code for the C<Animal> class
426 and the C<Horse> class:
431 print "a $class goes ", $class->sound, "!\n";
436 sub sound { "neigh" }
439 This lets us invoke C<< Horse->speak >> to ripple upward to
440 C<Animal::speak>, calling back to C<Horse::sound> to get the specific
441 sound, and the output of:
445 But all of our Horse objects would have to be absolutely identical.
446 If I add a subroutine, all horses automatically share it. That's
447 great for making horses the same, but how do we capture the
448 distinctions about an individual horse? For example, suppose I want
449 to give my first horse a name. There's got to be a way to keep its
450 name separate from the other horses.
452 We can do that by drawing a new distinction, called an "instance".
453 An "instance" is generally created by a class. In Perl, any reference
454 can be an instance, so let's start with the simplest reference
455 that can hold a horse's name: a scalar reference.
458 my $talking = \$name;
460 So now C<$talking> is a reference to what will be the instance-specific
461 data (the name). The final step in turning this into a real instance
462 is with a special operator called C<bless>:
464 bless $talking, Horse;
466 This operator stores information about the package named C<Horse> into
467 the thing pointed at by the reference. At this point, we say
468 C<$talking> is an instance of C<Horse>. That is, it's a specific
469 horse. The reference is otherwise unchanged, and can still be used
470 with traditional dereferencing operators.
472 =head2 Invoking an instance method
474 The method arrow can be used on instances, as well as names of
475 packages (classes). So, let's get the sound that C<$talking> makes:
477 my $noise = $talking->sound;
479 To invoke C<sound>, Perl first notes that C<$talking> is a blessed
480 reference (and thus an instance). It then constructs an argument
481 list, in this case from just C<($talking)>. (Later we'll see that
482 arguments will take their place following the instance variable,
483 just like with classes.)
485 Now for the fun part: Perl takes the class in which the instance was
486 blessed, in this case C<Horse>, and uses that to locate the subroutine
487 to invoke the method. In this case, C<Horse::sound> is found directly
488 (without using inheritance), yielding the final subroutine invocation:
490 Horse::sound($talking)
492 Note that the first parameter here is still the instance, not the name
493 of the class as before. We'll get C<neigh> as the return value, and
494 that'll end up as the C<$noise> variable above.
496 If Horse::sound had not been found, we'd be wandering up the
497 C<@Horse::ISA> list to try to find the method in one of the
498 superclasses, just as for a class method. The only difference between
499 a class method and an instance method is whether the first parameter
500 is an instance (a blessed reference) or a class name (a string).
502 =head2 Accessing the instance data
504 Because we get the instance as the first parameter, we can now access
505 the instance-specific data. In this case, let's add a way to get at
510 sub sound { "neigh" }
517 Now we call for the name:
519 print $talking->name, " says ", $talking->sound, "\n";
521 Inside C<Horse::name>, the C<@_> array contains just C<$talking>,
522 which the C<shift> stores into C<$self>. (It's traditional to shift
523 the first parameter off into a variable named C<$self> for instance
524 methods, so stay with that unless you have strong reasons otherwise.)
525 Then, C<$self> gets de-referenced as a scalar ref, yielding C<Mr. Ed>,
526 and we're done with that. The result is:
530 =head2 How to build a horse
532 Of course, if we constructed all of our horses by hand, we'd most
533 likely make mistakes from time to time. We're also violating one of
534 the properties of object-oriented programming, in that the "inside
535 guts" of a Horse are visible. That's good if you're a veterinarian,
536 but not if you just like to own horses. So, let's let the Horse class
541 sub sound { "neigh" }
549 bless \$name, $class;
553 Now with the new C<named> method, we can build a horse:
555 my $talking = Horse->named("Mr. Ed");
557 Notice we're back to a class method, so the two arguments to
558 C<Horse::named> are C<Horse> and C<Mr. Ed>. The C<bless> operator
559 not only blesses C<$name>, it also returns the reference to C<$name>,
560 so that's fine as a return value. And that's how to build a horse.
562 We've called the constructor C<named> here, so that it quickly denotes
563 the constructor's argument as the name for this particular C<Horse>.
564 You can use different constructors with different names for different
565 ways of "giving birth" to the object (like maybe recording its
566 pedigree or date of birth). However, you'll find that most people
567 coming to Perl from more limited languages use a single constructor
568 named C<new>, with various ways of interpreting the arguments to
569 C<new>. Either style is fine, as long as you document your particular
570 way of giving birth to an object. (And you I<were> going to do that,
573 =head2 Inheriting the constructor
575 But was there anything specific to C<Horse> in that method? No. Therefore,
576 it's also the same recipe for building anything else that inherited from
577 C<Animal>, so let's put it there:
582 print "a $class goes ", $class->sound, "!\n";
591 bless \$name, $class;
596 sub sound { "neigh" }
599 Ahh, but what happens if we invoke C<speak> on an instance?
601 my $talking = Horse->named("Mr. Ed");
604 We get a debugging value:
606 a Horse=SCALAR(0xaca42ac) goes neigh!
608 Why? Because the C<Animal::speak> routine is expecting a classname as
609 its first parameter, not an instance. When the instance is passed in,
610 we'll end up using a blessed scalar reference as a string, and that
611 shows up as we saw it just now.
613 =head2 Making a method work with either classes or instances
615 All we need is for a method to detect if it is being called on a class
616 or called on an instance. The most straightforward way is with the
617 C<ref> operator. This returns a string (the classname) when used on a
618 blessed reference, and an empty string when used on a string (like a
619 classname). Let's modify the C<name> method first to notice the change:
624 ? $$either # it's an instance, return name
625 : "an unnamed $either"; # it's a class, return generic
628 Here, the C<?:> operator comes in handy to select either the
629 dereference or a derived string. Now we can use this with either an
630 instance or a class. Note that I've changed the first parameter
631 holder to C<$either> to show that this is intended:
633 my $talking = Horse->named("Mr. Ed");
634 print Horse->name, "\n"; # prints "an unnamed Horse\n"
635 print $talking->name, "\n"; # prints "Mr Ed.\n"
637 and now we'll fix C<speak> to use this:
641 print $either->name, " goes ", $either->sound, "\n";
644 And since C<sound> already worked with either a class or an instance,
647 =head2 Adding parameters to a method
649 Let's train our animals to eat:
655 bless \$name, $class;
660 ? $$either # it's an instance, return name
661 : "an unnamed $either"; # it's a class, return generic
665 print $either->name, " goes ", $either->sound, "\n";
670 print $either->name, " eats $food.\n";
675 sub sound { "neigh" }
679 sub sound { "baaaah" }
684 my $talking = Horse->named("Mr. Ed");
685 $talking->eat("hay");
691 an unnamed Sheep eats grass.
693 An instance method with parameters gets invoked with the instance,
694 and then the list of parameters. So that first invocation is like:
696 Animal::eat($talking, "hay");
698 =head2 More interesting instances
700 What if an instance needs more data? Most interesting instances are
701 made of many items, each of which can in turn be a reference or even
702 another object. The easiest way to store these is often in a hash.
703 The keys of the hash serve as the names of parts of the object (often
704 called "instance variables" or "member variables"), and the
705 corresponding values are, well, the values.
707 But how do we turn the horse into a hash? Recall that an object was
708 any blessed reference. We can just as easily make it a blessed hash
709 reference as a blessed scalar reference, as long as everything that
710 looks at the reference is changed accordingly.
712 Let's make a sheep that has a name and a color:
714 my $bad = bless { Name => "Evil", Color => "black" }, Sheep;
716 so C<< $bad->{Name} >> has C<Evil>, and C<< $bad->{Color} >> has
717 C<black>. But we want to make C<< $bad->name >> access the name, and
718 that's now messed up because it's expecting a scalar reference. Not
719 to worry, because that's pretty easy to fix up:
726 "an unnamed $either";
729 And of course C<named> still builds a scalar sheep, so let's fix that
736 my $self = { Name => $name, Color => $class->default_color };
740 What's this C<default_color>? Well, if C<named> has only the name,
741 we still need to set a color, so we'll have a class-specific initial color.
742 For a sheep, we might define it as white:
745 sub default_color { "white" }
747 And then to keep from having to define one for each additional class,
748 we'll define a "backstop" method that serves as the "default default",
749 directly in C<Animal>:
752 sub default_color { "brown" }
754 Now, because C<name> and C<named> were the only methods that
755 referenced the "structure" of the object, the rest of the methods can
756 remain the same, so C<speak> still works as before.
758 =head2 A horse of a different color
760 But having all our horses be brown would be boring. So let's add a
761 method or two to get and set the color.
768 $_[0]->{Color} = $_[1];
771 Note the alternate way of accessing the arguments: C<$_[0]> is used
772 in-place, rather than with a C<shift>. (This saves us a bit of time
773 for something that may be invoked frequently.) And now we can fix
774 that color for Mr. Ed:
776 my $talking = Horse->named("Mr. Ed");
777 $talking->set_color("black-and-white");
778 print $talking->name, " is colored ", $talking->color, "\n";
782 Mr. Ed is colored black-and-white
786 So, now we have class methods, constructors, instance methods,
787 instance data, and even accessors. But that's still just the
788 beginning of what Perl has to offer. We haven't even begun to talk
789 about accessors that double as getters and setters, destructors,
790 indirect object notation, subclasses that add instance data, per-class
791 data, overloading, "isa" and "can" tests, C<UNIVERSAL> class, and so
792 on. That's for the rest of the Perl documentation to cover.
793 Hopefully, this gets you started, though.
797 For more information, see L<perlobj> (for all the gritty details about
798 Perl objects, now that you've seen the basics), L<perltoot> (the
799 tutorial for those who already know objects), L<perltooc> (dealing
800 with class data), L<perlbot> (for some more tricks), and books such as
801 Damian Conway's excellent I<Object Oriented Perl>.
803 Some modules which might prove interesting are Class::Accessor,
804 Class::Class, Class::Contract, Class::Data::Inheritable,
805 Class::MethodMaker and Tie::SecureHash
809 Copyright (c) 1999, 2000 by Randal L. Schwartz and Stonehenge
810 Consulting Services, Inc. Permission is hereby granted to distribute
811 this document intact with the Perl distribution, and in accordance
812 with the licenses of the Perl distribution; derived documents must
813 include this copyright notice intact.
815 Portions of this text have been derived from Perl Training materials
816 originally appearing in the I<Packages, References, Objects, and
817 Modules> course taught by instructors for Stonehenge Consulting
818 Services, Inc. and used with permission.
820 Portions of this text have been derived from materials originally
821 appearing in I<Linux Magazine> and used with permission.