extra code in pp_concat, Take 2
[p5sagit/p5-mst-13.2.git] / pod / perlboot.pod
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694468e3 1=head1 NAME
2
3perlboot - Beginner's Object-Oriented Tutorial
4
5=head1 DESCRIPTION
6
7If you're not familiar with objects from other languages, some of the
8other Perl object documentation may be a little daunting, such as
9L<perlobj>, a basic reference in using objects, and L<perltoot>, which
dbe48302 10introduces readers to the peculiarities of Perl's object system in a
694468e3 11tutorial way.
12
13So, let's take a different approach, presuming no prior object
14experience. It helps if you know about subroutines (L<perlsub>),
15references (L<perlref> et. seq.), and packages (L<perlmod>), so become
16familiar with those first if you haven't already.
17
18=head2 If we could talk to the animals...
19
20Let's let the animals talk for a moment:
21
22 sub Cow::speak {
23 print "a Cow goes moooo!\n";
24 }
25 sub Horse::speak {
26 print "a Horse goes neigh!\n";
27 }
28 sub Sheep::speak {
29 print "a Sheep goes baaaah!\n"
30 }
31
32 Cow::speak;
33 Horse::speak;
34 Sheep::speak;
35
36This results in:
37
38 a Cow goes moooo!
39 a Horse goes neigh!
40 a Sheep goes baaaah!
41
42Nothing spectacular here. Simple subroutines, albeit from separate
43packages, and called using the full package name. So let's create
44an entire pasture:
45
46 # Cow::speak, Horse::speak, Sheep::speak as before
84f709e7 47 @pasture = qw(Cow Cow Horse Sheep Sheep);
48 foreach $animal (@pasture) {
694468e3 49 &{$animal."::speak"};
50 }
51
52This results in:
53
54 a Cow goes moooo!
55 a Cow goes moooo!
56 a Horse goes neigh!
57 a Sheep goes baaaah!
58 a Sheep goes baaaah!
59
60Wow. That symbolic coderef de-referencing there is pretty nasty.
84f709e7 61We're counting on C<no strict subs> mode, certainly not recommended
62for larger programs. And why was that necessary? Because the name of
63the package seems to be inseparable from the name of the subroutine we
64want to invoke within that package.
694468e3 65
66Or is it?
67
68=head2 Introducing the method invocation arrow
69
c47ff5f1 70For now, let's say that C<< Class->method >> invokes subroutine
694468e3 71C<method> in package C<Class>. (Here, "Class" is used in its
72"category" meaning, not its "scholastic" meaning.) That's not
73completely accurate, but we'll do this one step at a time. Now let's
74use it like so:
75
76 # Cow::speak, Horse::speak, Sheep::speak as before
77 Cow->speak;
78 Horse->speak;
79 Sheep->speak;
80
81And once again, this results in:
82
83 a Cow goes moooo!
84 a Horse goes neigh!
85 a Sheep goes baaaah!
86
87That's not fun yet. Same number of characters, all constant, no
88variables. But yet, the parts are separable now. Watch:
89
84f709e7 90 $a = "Cow";
694468e3 91 $a->speak; # invokes Cow->speak
92
93Ahh! Now that the package name has been parted from the subroutine
94name, we can use a variable package name. And this time, we've got
84f709e7 95something that works even when C<use strict refs> is enabled.
694468e3 96
97=head2 Invoking a barnyard
98
99Let's take that new arrow invocation and put it back in the barnyard
100example:
101
102 sub Cow::speak {
103 print "a Cow goes moooo!\n";
104 }
105 sub Horse::speak {
106 print "a Horse goes neigh!\n";
107 }
108 sub Sheep::speak {
109 print "a Sheep goes baaaah!\n"
110 }
111
84f709e7 112 @pasture = qw(Cow Cow Horse Sheep Sheep);
113 foreach $animal (@pasture) {
694468e3 114 $animal->speak;
115 }
116
117There! Now we have the animals all talking, and safely at that,
118without the use of symbolic coderefs.
119
120But look at all that common code. Each of the C<speak> routines has a
121similar structure: a C<print> operator and a string that contains
122common text, except for two of the words. It'd be nice if we could
123factor out the commonality, in case we decide later to change it all
124to C<says> instead of C<goes>.
125
126And we actually have a way of doing that without much fuss, but we
127have to hear a bit more about what the method invocation arrow is
128actually doing for us.
129
130=head2 The extra parameter of method invocation
131
132The invocation of:
133
134 Class->method(@args)
135
136attempts to invoke subroutine C<Class::method> as:
137
138 Class::method("Class", @args);
139
140(If the subroutine can't be found, "inheritance" kicks in, but we'll
141get to that later.) This means that we get the class name as the
dbe48302 142first parameter (the only parameter, if no arguments are given). So
143we can rewrite the C<Sheep> speaking subroutine as:
694468e3 144
145 sub Sheep::speak {
146 my $class = shift;
147 print "a $class goes baaaah!\n";
148 }
149
150And the other two animals come out similarly:
151
152 sub Cow::speak {
153 my $class = shift;
154 print "a $class goes moooo!\n";
155 }
156 sub Horse::speak {
157 my $class = shift;
158 print "a $class goes neigh!\n";
159 }
160
161In each case, C<$class> will get the value appropriate for that
162subroutine. But once again, we have a lot of similar structure. Can
163we factor that out even further? Yes, by calling another method in
164the same class.
165
166=head2 Calling a second method to simplify things
167
168Let's call out from C<speak> to a helper method called C<sound>.
169This method provides the constant text for the sound itself.
170
84f709e7 171 { package Cow;
694468e3 172 sub sound { "moooo" }
173 sub speak {
84f709e7 174 my $class = shift;
175 print "a $class goes ", $class->sound, "!\n"
694468e3 176 }
177 }
178
c47ff5f1 179Now, when we call C<< Cow->speak >>, we get a C<$class> of C<Cow> in
180C<speak>. This in turn selects the C<< Cow->sound >> method, which
694468e3 181returns C<moooo>. But how different would this be for the C<Horse>?
182
84f709e7 183 { package Horse;
694468e3 184 sub sound { "neigh" }
185 sub speak {
186 my $class = shift;
187 print "a $class goes ", $class->sound, "!\n"
188 }
189 }
190
191Only the name of the package and the specific sound change. So can we
192somehow share the definition for C<speak> between the Cow and the
193Horse? Yes, with inheritance!
194
195=head2 Inheriting the windpipes
196
197We'll define a common subroutine package called C<Animal>, with the
198definition for C<speak>:
199
84f709e7 200 { package Animal;
694468e3 201 sub speak {
202 my $class = shift;
203 print "a $class goes ", $class->sound, "!\n"
204 }
205 }
206
207Then, for each animal, we say it "inherits" from C<Animal>, along
208with the animal-specific sound:
209
84f709e7 210 { package Cow;
694468e3 211 @ISA = qw(Animal);
212 sub sound { "moooo" }
213 }
214
215Note the added C<@ISA> array. We'll get to that in a minute.
216
c47ff5f1 217But what happens when we invoke C<< Cow->speak >> now?
694468e3 218
219First, Perl constructs the argument list. In this case, it's just
220C<Cow>. Then Perl looks for C<Cow::speak>. But that's not there, so
221Perl checks for the inheritance array C<@Cow::ISA>. It's there,
222and contains the single name C<Animal>.
223
224Perl next checks for C<speak> inside C<Animal> instead, as in
225C<Animal::speak>. And that's found, so Perl invokes that subroutine
226with the already frozen argument list.
227
228Inside the C<Animal::speak> subroutine, C<$class> becomes C<Cow> (the
229first argument). So when we get to the step of invoking
c47ff5f1 230C<< $class->sound >>, it'll be looking for C<< Cow->sound >>, which
694468e3 231gets it on the first try without looking at C<@ISA>. Success!
232
233=head2 A few notes about @ISA
234
235This magical C<@ISA> variable (pronounced "is a" not "ice-uh"), has
236declared that C<Cow> "is a" C<Animal>. Note that it's an array,
237not a simple single value, because on rare occasions, it makes sense
238to have more than one parent class searched for the missing methods.
239
240If C<Animal> also had an C<@ISA>, then we'd check there too. The
241search is recursive, depth-first, left-to-right in each C<@ISA>.
242Typically, each C<@ISA> has only one element (multiple elements means
243multiple inheritance and multiple headaches), so we get a nice tree of
244inheritance.
245
246When we turn on C<use strict>, we'll get complaints on C<@ISA>, since
247it's not a variable containing an explicit package name, nor is it a
dbe48302 248lexical ("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),
694468e3 250so there's a couple of straightforward ways to handle that.
251
252The easiest is to just spell the package name out:
253
254 @Cow::ISA = qw(Animal);
255
dbe48302 256Or allow it as an implicitly named package variable:
694468e3 257
258 package Cow;
84f709e7 259 use vars qw(@ISA);
260 @ISA = qw(Animal);
694468e3 261
262If you're bringing in the class from outside, via an object-oriented
263module, you change:
264
265 package Cow;
266 use Animal;
84f709e7 267 use vars qw(@ISA);
268 @ISA = qw(Animal);
694468e3 269
270into just:
271
272 package Cow;
273 use base qw(Animal);
274
84f709e7 275And that's pretty darn compact.
694468e3 276
277=head2 Overriding the methods
278
279Let's add a mouse, which can barely be heard:
280
84f709e7 281 # Animal package from before
282 { package Mouse;
283 @ISA = qw(Animal);
694468e3 284 sub sound { "squeak" }
285 sub speak {
286 my $class = shift;
287 print "a $class goes ", $class->sound, "!\n";
288 print "[but you can barely hear it!]\n";
289 }
290 }
291
292 Mouse->speak;
293
294which results in:
295
296 a Mouse goes squeak!
297 [but you can barely hear it!]
298
c47ff5f1 299Here, C<Mouse> has its own speaking routine, so C<< Mouse->speak >>
300doesn't immediately invoke C<< Animal->speak >>. This is known as
694468e3 301"overriding". In fact, we didn't even need to say that a C<Mouse> was
302an C<Animal> at all, since all of the methods needed for C<speak> are
303completely defined with C<Mouse>.
304
c47ff5f1 305But we've now duplicated some of the code from C<< Animal->speak >>,
694468e3 306and this can once again be a maintenance headache. So, can we avoid
307that? Can we say somehow that a C<Mouse> does everything any other
308C<Animal> does, but add in the extra comment? Sure!
309
310First, we can invoke the C<Animal::speak> method directly:
311
84f709e7 312 # Animal package from before
313 { package Mouse;
314 @ISA = qw(Animal);
694468e3 315 sub sound { "squeak" }
316 sub speak {
317 my $class = shift;
318 Animal::speak($class);
319 print "[but you can barely hear it!]\n";
320 }
321 }
322
323Note that we have to include the C<$class> parameter (almost surely
324the value of C<"Mouse">) as the first parameter to C<Animal::speak>,
325since we've stopped using the method arrow. Why did we stop? Well,
c47ff5f1 326if we invoke C<< Animal->speak >> there, the first parameter to the
694468e3 327method will be C<"Animal"> not C<"Mouse">, and when time comes for it
328to call for the C<sound>, it won't have the right class to come back
329to this package.
330
331Invoking C<Animal::speak> directly is a mess, however. What if
332C<Animal::speak> didn't exist before, and was being inherited from a
333class mentioned in C<@Animal::ISA>? Because we are no longer using
334the method arrow, we get one and only one chance to hit the right
335subroutine.
336
337Also note that the C<Animal> classname is now hardwired into the
338subroutine selection. This is a mess if someone maintains the code,
339changing C<@ISA> for <Mouse> and didn't notice C<Animal> there in
340C<speak>. So, this is probably not the right way to go.
341
342=head2 Starting the search from a different place
343
344A better solution is to tell Perl to search from a higher place
345in the inheritance chain:
346
347 # same Animal as before
84f709e7 348 { package Mouse;
694468e3 349 # same @ISA, &sound as before
350 sub speak {
351 my $class = shift;
352 $class->Animal::speak;
353 print "[but you can barely hear it!]\n";
354 }
355 }
356
357Ahh. This works. Using this syntax, we start with C<Animal> to find
358C<speak>, and use all of C<Animal>'s inheritance chain if not found
359immediately. And yet the first parameter will be C<$class>, so the
360found C<speak> method will get C<Mouse> as its first entry, and
361eventually work its way back to C<Mouse::sound> for the details.
362
363But this isn't the best solution. We still have to keep the C<@ISA>
364and the initial search package coordinated. Worse, if C<Mouse> had
365multiple entries in C<@ISA>, we wouldn't necessarily know which one
366had actually defined C<speak>. So, is there an even better way?
367
368=head2 The SUPER way of doing things
369
370By changing the C<Animal> class to the C<SUPER> class in that
371invocation, we get a search of all of our super classes (classes
372listed in C<@ISA>) automatically:
373
374 # same Animal as before
84f709e7 375 { package Mouse;
694468e3 376 # same @ISA, &sound as before
377 sub speak {
378 my $class = shift;
379 $class->SUPER::speak;
380 print "[but you can barely hear it!]\n";
381 }
382 }
383
384So, C<SUPER::speak> means look in the current package's C<@ISA> for
029f3b44 385C<speak>, invoking the first one found. Note that it does I<not> look in
386the C<@ISA> of C<$class>.
694468e3 387
388=head2 Where we're at so far...
389
390So far, we've seen the method arrow syntax:
391
392 Class->method(@args);
393
394or the equivalent:
395
84f709e7 396 $a = "Class";
694468e3 397 $a->method(@args);
398
399which constructs an argument list of:
400
401 ("Class", @args)
402
403and attempts to invoke
404
405 Class::method("Class", @Args);
406
407However, if C<Class::method> is not found, then C<@Class::ISA> is examined
408(recursively) to locate a package that does indeed contain C<method>,
409and that subroutine is invoked instead.
410
411Using this simple syntax, we have class methods, (multiple)
412inheritance, overriding, and extending. Using just what we've seen so
413far, we've been able to factor out common code, and provide a nice way
414to reuse implementations with variations. This is at the core of what
415objects provide, but objects also provide instance data, which we
416haven't even begun to cover.
417
418=head2 A horse is a horse, of course of course -- or is it?
419
420Let's start with the code for the C<Animal> class
421and the C<Horse> class:
422
84f709e7 423 { package Animal;
694468e3 424 sub speak {
425 my $class = shift;
426 print "a $class goes ", $class->sound, "!\n"
427 }
428 }
84f709e7 429 { package Horse;
430 @ISA = qw(Animal);
694468e3 431 sub sound { "neigh" }
432 }
433
c47ff5f1 434This lets us invoke C<< Horse->speak >> to ripple upward to
694468e3 435C<Animal::speak>, calling back to C<Horse::sound> to get the specific
436sound, and the output of:
437
438 a Horse goes neigh!
439
440But all of our Horse objects would have to be absolutely identical.
441If I add a subroutine, all horses automatically share it. That's
442great for making horses the same, but how do we capture the
443distinctions about an individual horse? For example, suppose I want
444to give my first horse a name. There's got to be a way to keep its
445name separate from the other horses.
446
447We can do that by drawing a new distinction, called an "instance".
448An "instance" is generally created by a class. In Perl, any reference
449can be an instance, so let's start with the simplest reference
450that can hold a horse's name: a scalar reference.
451
84f709e7 452 my $name = "Mr. Ed";
694468e3 453 my $talking = \$name;
454
455So now C<$talking> is a reference to what will be the instance-specific
456data (the name). The final step in turning this into a real instance
457is with a special operator called C<bless>:
458
459 bless $talking, Horse;
460
461This operator stores information about the package named C<Horse> into
462the thing pointed at by the reference. At this point, we say
463C<$talking> is an instance of C<Horse>. That is, it's a specific
464horse. The reference is otherwise unchanged, and can still be used
465with traditional dereferencing operators.
466
467=head2 Invoking an instance method
468
469The method arrow can be used on instances, as well as names of
470packages (classes). So, let's get the sound that C<$talking> makes:
471
472 my $noise = $talking->sound;
473
474To invoke C<sound>, Perl first notes that C<$talking> is a blessed
475reference (and thus an instance). It then constructs an argument
476list, in this case from just C<($talking)>. (Later we'll see that
477arguments will take their place following the instance variable,
478just like with classes.)
479
480Now for the fun part: Perl takes the class in which the instance was
481blessed, in this case C<Horse>, and uses that to locate the subroutine
482to invoke the method. In this case, C<Horse::sound> is found directly
483(without using inheritance), yielding the final subroutine invocation:
484
485 Horse::sound($talking)
486
487Note that the first parameter here is still the instance, not the name
488of the class as before. We'll get C<neigh> as the return value, and
489that'll end up as the C<$noise> variable above.
490
491If Horse::sound had not been found, we'd be wandering up the
492C<@Horse::ISA> list to try to find the method in one of the
493superclasses, just as for a class method. The only difference between
494a class method and an instance method is whether the first parameter
dbe48302 495is an instance (a blessed reference) or a class name (a string).
694468e3 496
497=head2 Accessing the instance data
498
499Because we get the instance as the first parameter, we can now access
500the instance-specific data. In this case, let's add a way to get at
501the name:
502
84f709e7 503 { package Horse;
504 @ISA = qw(Animal);
694468e3 505 sub sound { "neigh" }
506 sub name {
507 my $self = shift;
508 $$self;
509 }
510 }
c47ff5f1 511
694468e3 512Now we call for the name:
513
514 print $talking->name, " says ", $talking->sound, "\n";
515
516Inside C<Horse::name>, the C<@_> array contains just C<$talking>,
517which the C<shift> stores into C<$self>. (It's traditional to shift
518the first parameter off into a variable named C<$self> for instance
519methods, so stay with that unless you have strong reasons otherwise.)
520Then, C<$self> gets de-referenced as a scalar ref, yielding C<Mr. Ed>,
521and we're done with that. The result is:
522
523 Mr. Ed says neigh.
524
525=head2 How to build a horse
526
527Of course, if we constructed all of our horses by hand, we'd most
528likely make mistakes from time to time. We're also violating one of
529the properties of object-oriented programming, in that the "inside
530guts" of a Horse are visible. That's good if you're a veterinarian,
531but not if you just like to own horses. So, let's let the Horse class
532build a new horse:
533
84f709e7 534 { package Horse;
535 @ISA = qw(Animal);
694468e3 536 sub sound { "neigh" }
537 sub name {
538 my $self = shift;
539 $$self;
540 }
541 sub named {
542 my $class = shift;
84f709e7 543 my $name = shift;
694468e3 544 bless \$name, $class;
545 }
546 }
547
548Now with the new C<named> method, we can build a horse:
549
550 my $talking = Horse->named("Mr. Ed");
551
552Notice we're back to a class method, so the two arguments to
553C<Horse::named> are C<Horse> and C<Mr. Ed>. The C<bless> operator
554not only blesses C<$name>, it also returns the reference to C<$name>,
555so that's fine as a return value. And that's how to build a horse.
556
dbe48302 557We've called the constructor C<named> here, so that it quickly denotes
558the constructor's argument as the name for this particular C<Horse>.
559You can use different constructors with different names for different
560ways of "giving birth" to the object (like maybe recording its
561pedigree or date of birth). However, you'll find that most people
562coming to Perl from more limited languages use a single constructor
563named C<new>, with various ways of interpreting the arguments to
564C<new>. Either style is fine, as long as you document your particular
565way of giving birth to an object. (And you I<were> going to do that,
566right?)
567
694468e3 568=head2 Inheriting the constructor
569
570But was there anything specific to C<Horse> in that method? No. Therefore,
571it's also the same recipe for building anything else that inherited from
572C<Animal>, so let's put it there:
573
84f709e7 574 { package Animal;
694468e3 575 sub speak {
576 my $class = shift;
577 print "a $class goes ", $class->sound, "!\n"
578 }
579 sub name {
580 my $self = shift;
581 $$self;
582 }
583 sub named {
584 my $class = shift;
84f709e7 585 my $name = shift;
694468e3 586 bless \$name, $class;
587 }
588 }
84f709e7 589 { package Horse;
590 @ISA = qw(Animal);
694468e3 591 sub sound { "neigh" }
592 }
593
594Ahh, but what happens if we invoke C<speak> on an instance?
595
596 my $talking = Horse->named("Mr. Ed");
597 $talking->speak;
598
599We get a debugging value:
600
601 a Horse=SCALAR(0xaca42ac) goes neigh!
602
603Why? Because the C<Animal::speak> routine is expecting a classname as
604its first parameter, not an instance. When the instance is passed in,
605we'll end up using a blessed scalar reference as a string, and that
606shows up as we saw it just now.
607
608=head2 Making a method work with either classes or instances
609
610All we need is for a method to detect if it is being called on a class
611or called on an instance. The most straightforward way is with the
612C<ref> operator. This returns a string (the classname) when used on a
613blessed reference, and C<undef> when used on a string (like a
614classname). Let's modify the C<name> method first to notice the change:
615
616 sub name {
617 my $either = shift;
618 ref $either
84f709e7 619 ? $$either # it's an instance, return name
694468e3 620 : "an unnamed $either"; # it's a class, return generic
621 }
622
623Here, the C<?:> operator comes in handy to select either the
624dereference or a derived string. Now we can use this with either an
625instance or a class. Note that I've changed the first parameter
626holder to C<$either> to show that this is intended:
627
628 my $talking = Horse->named("Mr. Ed");
84f709e7 629 print Horse->name, "\n"; # prints "an unnamed Horse\n"
694468e3 630 print $talking->name, "\n"; # prints "Mr Ed.\n"
631
632and now we'll fix C<speak> to use this:
633
634 sub speak {
635 my $either = shift;
636 print $either->name, " goes ", $either->sound, "\n";
637 }
638
639And since C<sound> already worked with either a class or an instance,
640we're done!
641
642=head2 Adding parameters to a method
643
644Let's train our animals to eat:
645
84f709e7 646 { package Animal;
694468e3 647 sub named {
648 my $class = shift;
84f709e7 649 my $name = shift;
694468e3 650 bless \$name, $class;
651 }
652 sub name {
653 my $either = shift;
654 ref $either
84f709e7 655 ? $$either # it's an instance, return name
694468e3 656 : "an unnamed $either"; # it's a class, return generic
657 }
658 sub speak {
659 my $either = shift;
660 print $either->name, " goes ", $either->sound, "\n";
661 }
662 sub eat {
663 my $either = shift;
84f709e7 664 my $food = shift;
694468e3 665 print $either->name, " eats $food.\n";
666 }
667 }
84f709e7 668 { package Horse;
669 @ISA = qw(Animal);
694468e3 670 sub sound { "neigh" }
671 }
84f709e7 672 { package Sheep;
673 @ISA = qw(Animal);
694468e3 674 sub sound { "baaaah" }
675 }
676
677And now try it out:
678
679 my $talking = Horse->named("Mr. Ed");
680 $talking->eat("hay");
681 Sheep->eat("grass");
682
683which prints:
684
685 Mr. Ed eats hay.
686 an unnamed Sheep eats grass.
687
688An instance method with parameters gets invoked with the instance,
689and then the list of parameters. So that first invocation is like:
690
691 Animal::eat($talking, "hay");
692
693=head2 More interesting instances
694
695What if an instance needs more data? Most interesting instances are
696made of many items, each of which can in turn be a reference or even
697another object. The easiest way to store these is often in a hash.
698The keys of the hash serve as the names of parts of the object (often
699called "instance variables" or "member variables"), and the
700corresponding values are, well, the values.
701
702But how do we turn the horse into a hash? Recall that an object was
703any blessed reference. We can just as easily make it a blessed hash
704reference as a blessed scalar reference, as long as everything that
705looks at the reference is changed accordingly.
706
707Let's make a sheep that has a name and a color:
708
84f709e7 709 my $bad = bless { Name => "Evil", Color => "black" }, Sheep;
694468e3 710
c47ff5f1 711so C<< $bad->{Name} >> has C<Evil>, and C<< $bad->{Color} >> has
712C<black>. But we want to make C<< $bad->name >> access the name, and
694468e3 713that's now messed up because it's expecting a scalar reference. Not
714to worry, because that's pretty easy to fix up:
715
716 ## in Animal
717 sub name {
718 my $either = shift;
719 ref $either ?
720 $either->{Name} :
721 "an unnamed $either";
722 }
723
724And of course C<named> still builds a scalar sheep, so let's fix that
725as well:
726
727 ## in Animal
728 sub named {
729 my $class = shift;
84f709e7 730 my $name = shift;
731 my $self = { Name => $name, Color => $class->default_color };
694468e3 732 bless $self, $class;
733 }
734
735What's this C<default_color>? Well, if C<named> has only the name,
736we still need to set a color, so we'll have a class-specific initial color.
737For a sheep, we might define it as white:
738
739 ## in Sheep
740 sub default_color { "white" }
741
742And then to keep from having to define one for each additional class,
743we'll define a "backstop" method that serves as the "default default",
744directly in C<Animal>:
745
746 ## in Animal
747 sub default_color { "brown" }
748
749Now, because C<name> and C<named> were the only methods that
750referenced the "structure" of the object, the rest of the methods can
751remain the same, so C<speak> still works as before.
752
753=head2 A horse of a different color
754
755But having all our horses be brown would be boring. So let's add a
756method or two to get and set the color.
757
758 ## in Animal
759 sub color {
760 $_[0]->{Color}
761 }
762 sub set_color {
763 $_[0]->{Color} = $_[1];
764 }
765
766Note the alternate way of accessing the arguments: C<$_[0]> is used
767in-place, rather than with a C<shift>. (This saves us a bit of time
768for something that may be invoked frequently.) And now we can fix
769that color for Mr. Ed:
770
771 my $talking = Horse->named("Mr. Ed");
772 $talking->set_color("black-and-white");
773 print $talking->name, " is colored ", $talking->color, "\n";
774
775which results in:
776
777 Mr. Ed is colored black-and-white
778
779=head2 Summary
780
781So, now we have class methods, constructors, instance methods,
782instance data, and even accessors. But that's still just the
783beginning of what Perl has to offer. We haven't even begun to talk
784about accessors that double as getters and setters, destructors,
785indirect object notation, subclasses that add instance data, per-class
786data, overloading, "isa" and "can" tests, C<UNIVERSAL> class, and so
787on. That's for the rest of the Perl documentation to cover.
788Hopefully, this gets you started, though.
789
790=head1 SEE ALSO
791
792For more information, see L<perlobj> (for all the gritty details about
793Perl objects, now that you've seen the basics), L<perltoot> (the
890a53b9 794tutorial for those who already know objects), L<perltooc> (dealing
8257a158 795with class data), L<perlbot> (for some more tricks), and books such as
796Damian Conway's excellent I<Object Oriented Perl>.
797
798Some modules which might prove interesting are Class::Accessor,
799Class::Class, Class::Contract, Class::Data::Inheritable,
800Class::MethodMaker and Tie::SecureHash
694468e3 801
802=head1 COPYRIGHT
803
804Copyright (c) 1999, 2000 by Randal L. Schwartz and Stonehenge
805Consulting Services, Inc. Permission is hereby granted to distribute
806this document intact with the Perl distribution, and in accordance
807with the licenses of the Perl distribution; derived documents must
808include this copyright notice intact.
809
810Portions of this text have been derived from Perl Training materials
811originally appearing in the I<Packages, References, Objects, and
812Modules> course taught by instructors for Stonehenge Consulting
813Services, Inc. and used with permission.
814
815Portions of this text have been derived from materials originally
816appearing in I<Linux Magazine> and used with permission.