Integrate perlio:
[p5sagit/p5-mst-13.2.git] / pod / perlobj.pod
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a0d0e21e 1=head1 NAME
2
3perlobj - Perl objects
4
5=head1 DESCRIPTION
6
14218588 7First you need to understand what references are in Perl.
5f05dabc 8See L<perlref> for that. Second, if you still find the following
9reference work too complicated, a tutorial on object-oriented programming
890a53b9 10in Perl can be found in L<perltoot> and L<perltooc>.
a0d0e21e 11
54310121 12If you're still with us, then
5f05dabc 13here are three very simple definitions that you should find reassuring.
a0d0e21e 14
15=over 4
16
17=item 1.
18
19An object is simply a reference that happens to know which class it
20belongs to.
21
22=item 2.
23
24A class is simply a package that happens to provide methods to deal
25with object references.
26
27=item 3.
28
29A method is simply a subroutine that expects an object reference (or
55497cff 30a package name, for class methods) as the first argument.
a0d0e21e 31
32=back
33
34We'll cover these points now in more depth.
35
36=head2 An Object is Simply a Reference
37
38Unlike say C++, Perl doesn't provide any special syntax for
39constructors. A constructor is merely a subroutine that returns a
cb1a09d0 40reference to something "blessed" into a class, generally the
a0d0e21e 41class that the subroutine is defined in. Here is a typical
42constructor:
43
44 package Critter;
45 sub new { bless {} }
46
5a964f20 47That word C<new> isn't special. You could have written
48a construct this way, too:
49
50 package Critter;
51 sub spawn { bless {} }
52
14218588 53This might even be preferable, because the C++ programmers won't
5a964f20 54be tricked into thinking that C<new> works in Perl as it does in C++.
55It doesn't. We recommend that you name your constructors whatever
56makes sense in the context of the problem you're solving. For example,
57constructors in the Tk extension to Perl are named after the widgets
58they create.
59
60One thing that's different about Perl constructors compared with those in
61C++ is that in Perl, they have to allocate their own memory. (The other
62things is that they don't automatically call overridden base-class
63constructors.) The C<{}> allocates an anonymous hash containing no
64key/value pairs, and returns it The bless() takes that reference and
65tells the object it references that it's now a Critter, and returns
66the reference. This is for convenience, because the referenced object
67itself knows that it has been blessed, and the reference to it could
68have been returned directly, like this:
a0d0e21e 69
70 sub new {
71 my $self = {};
72 bless $self;
73 return $self;
74 }
75
14218588 76You often see such a thing in more complicated constructors
a0d0e21e 77that wish to call methods in the class as part of the construction:
78
79 sub new {
5a964f20 80 my $self = {};
a0d0e21e 81 bless $self;
82 $self->initialize();
cb1a09d0 83 return $self;
84 }
85
1fef88e7 86If you care about inheritance (and you should; see
b687b08b 87L<perlmodlib/"Modules: Creation, Use, and Abuse">),
1fef88e7 88then you want to use the two-arg form of bless
cb1a09d0 89so that your constructors may be inherited:
90
91 sub new {
92 my $class = shift;
93 my $self = {};
5a964f20 94 bless $self, $class;
cb1a09d0 95 $self->initialize();
96 return $self;
97 }
98
c47ff5f1 99Or if you expect people to call not just C<< CLASS->new() >> but also
100C<< $obj->new() >>, then use something like this. The initialize()
54310121 101method used will be of whatever $class we blessed the
cb1a09d0 102object into:
103
104 sub new {
105 my $this = shift;
106 my $class = ref($this) || $this;
107 my $self = {};
5a964f20 108 bless $self, $class;
cb1a09d0 109 $self->initialize();
110 return $self;
a0d0e21e 111 }
112
113Within the class package, the methods will typically deal with the
114reference as an ordinary reference. Outside the class package,
115the reference is generally treated as an opaque value that may
5f05dabc 116be accessed only through the class's methods.
a0d0e21e 117
14218588 118Although a constructor can in theory re-bless a referenced object
19799a22 119currently belonging to another class, this is almost certainly going
120to get you into trouble. The new class is responsible for all
121cleanup later. The previous blessing is forgotten, as an object
122may belong to only one class at a time. (Although of course it's
123free to inherit methods from many classes.) If you find yourself
124having to do this, the parent class is probably misbehaving, though.
a0d0e21e 125
126A clarification: Perl objects are blessed. References are not. Objects
127know which package they belong to. References do not. The bless()
5f05dabc 128function uses the reference to find the object. Consider
a0d0e21e 129the following example:
130
131 $a = {};
132 $b = $a;
133 bless $a, BLAH;
134 print "\$b is a ", ref($b), "\n";
135
54310121 136This reports $b as being a BLAH, so obviously bless()
a0d0e21e 137operated on the object and not on the reference.
138
139=head2 A Class is Simply a Package
140
141Unlike say C++, Perl doesn't provide any special syntax for class
5f05dabc 142definitions. You use a package as a class by putting method
a0d0e21e 143definitions into the class.
144
5a964f20 145There is a special array within each package called @ISA, which says
a0d0e21e 146where else to look for a method if you can't find it in the current
147package. This is how Perl implements inheritance. Each element of the
148@ISA array is just the name of another package that happens to be a
149class package. The classes are searched (depth first) for missing
150methods in the order that they occur in @ISA. The classes accessible
54310121 151through @ISA are known as base classes of the current class.
a0d0e21e 152
5a964f20 153All classes implicitly inherit from class C<UNIVERSAL> as their
154last base class. Several commonly used methods are automatically
155supplied in the UNIVERSAL class; see L<"Default UNIVERSAL methods"> for
156more details.
157
14218588 158If a missing method is found in a base class, it is cached
a0d0e21e 159in the current class for efficiency. Changing @ISA or defining new
160subroutines invalidates the cache and causes Perl to do the lookup again.
161
5a964f20 162If neither the current class, its named base classes, nor the UNIVERSAL
163class contains the requested method, these three places are searched
164all over again, this time looking for a method named AUTOLOAD(). If an
165AUTOLOAD is found, this method is called on behalf of the missing method,
166setting the package global $AUTOLOAD to be the fully qualified name of
167the method that was intended to be called.
168
169If none of that works, Perl finally gives up and complains.
170
ed850460 171If you want to stop the AUTOLOAD inheritance say simply
172
173 sub AUTOLOAD;
174
175and the call will die using the name of the sub being called.
176
5a964f20 177Perl classes do method inheritance only. Data inheritance is left up
178to the class itself. By and large, this is not a problem in Perl,
179because most classes model the attributes of their object using an
180anonymous hash, which serves as its own little namespace to be carved up
181by the various classes that might want to do something with the object.
182The only problem with this is that you can't sure that you aren't using
183a piece of the hash that isn't already used. A reasonable workaround
184is to prepend your fieldname in the hash with the package name.
185
186 sub bump {
187 my $self = shift;
188 $self->{ __PACKAGE__ . ".count"}++;
189 }
a0d0e21e 190
191=head2 A Method is Simply a Subroutine
192
193Unlike say C++, Perl doesn't provide any special syntax for method
194definition. (It does provide a little syntax for method invocation
195though. More on that later.) A method expects its first argument
19799a22 196to be the object (reference) or package (string) it is being invoked
197on. There are two ways of calling methods, which we'll call class
198methods and instance methods.
a0d0e21e 199
55497cff 200A class method expects a class name as the first argument. It
19799a22 201provides functionality for the class as a whole, not for any
202individual object belonging to the class. Constructors are often
890a53b9 203class methods, but see L<perltoot> and L<perltooc> for alternatives.
19799a22 204Many class methods simply ignore their first argument, because they
205already know what package they're in and don't care what package
5f05dabc 206they were invoked via. (These aren't necessarily the same, because
55497cff 207class methods follow the inheritance tree just like ordinary instance
208methods.) Another typical use for class methods is to look up an
a0d0e21e 209object by name:
210
211 sub find {
212 my ($class, $name) = @_;
213 $objtable{$name};
214 }
215
55497cff 216An instance method expects an object reference as its first argument.
a0d0e21e 217Typically it shifts the first argument into a "self" or "this" variable,
218and then uses that as an ordinary reference.
219
220 sub display {
221 my $self = shift;
222 my @keys = @_ ? @_ : sort keys %$self;
223 foreach $key (@keys) {
224 print "\t$key => $self->{$key}\n";
225 }
226 }
227
228=head2 Method Invocation
229
5d9f8747 230For various historical and other reasons, Perl offers two equivalent
231ways to write a method call. The simpler and more common way is to use
232the arrow notation:
a0d0e21e 233
5d9f8747 234 my $fred = Critter->find("Fred");
235 $fred->display("Height", "Weight");
a0d0e21e 236
5f7b1de2 237You should already be familiar with the use of the C<< -> >> operator with
5d9f8747 238references. In fact, since C<$fred> above is a reference to an object,
239you could think of the method call as just another form of
240dereferencing.
a0d0e21e 241
5d9f8747 242Whatever is on the left side of the arrow, whether a reference or a
243class name, is passed to the method subroutine as its first argument.
244So the above code is mostly equivalent to:
a0d0e21e 245
5d9f8747 246 my $fred = Critter::find("Critter", "Fred");
247 Critter::display($fred, "Height", "Weight");
a0d0e21e 248
5d9f8747 249How does Perl know which package the subroutine is in? By looking at
250the left side of the arrow, which must be either a package name or a
251reference to an object, i.e. something that has been blessed to a
5f7b1de2 252package. Either way, that's the package where Perl starts looking. If
5d9f8747 253that package has no subroutine with that name, Perl starts looking for
254it in any base classes of that package, and so on.
a0d0e21e 255
5f7b1de2 256If you need to, you I<can> force Perl to start looking in some other package:
a0d0e21e 257
5d9f8747 258 my $barney = MyCritter->Critter::find("Barney");
259 $barney->Critter::display("Height", "Weight");
a0d0e21e 260
5d9f8747 261Here C<MyCritter> is presumably a subclass of C<Critter> that defines
262its own versions of find() and display(). We haven't specified what
263those methods do, but that doesn't matter above since we've forced Perl
264to start looking for the subroutines in C<Critter>.
a0d0e21e 265
5d9f8747 266As a special case of the above, you may use the C<SUPER> pseudo-class to
5f7b1de2 267tell Perl to start looking for the method in the packages named in the
268current class's C<@ISA> list.
a0d0e21e 269
5d9f8747 270 package MyCritter;
271 use base 'Critter'; # sets @MyCritter::ISA = ('Critter');
a0d0e21e 272
5d9f8747 273 sub display {
274 my ($self, @args) = @_;
275 $self->SUPER::display("Name", @args);
276 }
a0d0e21e 277
5d9f8747 278Instead of a class name or an object reference, you can also use any
279expression that returns either of those on the left side of the arrow.
280So the following statement is valid:
a0d0e21e 281
5d9f8747 282 Critter->find("Fred")->display("Height", "Weight");
a0d0e21e 283
5f7b1de2 284and so is the following:
cb1a09d0 285
5d9f8747 286 my $fred = (reverse "rettirC")->find(reverse "derF");
cb1a09d0 287
5d9f8747 288=head2 Indirect Object Syntax
cb1a09d0 289
5f7b1de2 290The other way to invoke a method is by using the so-called "indirect
291object" notation. This syntax was available in Perl 4 long before
292objects were introduced, and is still used with filehandles like this:
748a9306 293
5d9f8747 294 print STDERR "help!!!\n";
19799a22 295
5d9f8747 296The same syntax can be used to call either object or class methods.
19799a22 297
5d9f8747 298 my $fred = find Critter "Fred";
299 display $fred "Height", "Weight";
19799a22 300
5d9f8747 301Notice that there is no comma between the object or class name and the
302parameters. This is how Perl can tell you want an indirect method call
303instead of an ordinary subroutine call.
19799a22 304
5d9f8747 305But what if there are no arguments? In that case, Perl must guess what
5f7b1de2 306you want. Even worse, it must make that guess I<at compile time>.
307Usually Perl gets it right, but when it doesn't you get a function
308call compiled as a method, or vice versa. This can introduce subtle bugs
309that are hard to detect.
5d9f8747 310
5f7b1de2 311For example, a call to a method C<new> in indirect notation -- as C++
312programmers are wont to make -- can be miscompiled into a subroutine
5d9f8747 313call if there's already a C<new> function in scope. You'd end up
314calling the current package's C<new> as a subroutine, rather than the
315desired class's method. The compiler tries to cheat by remembering
5f7b1de2 316bareword C<require>s, but the grief when it messes up just isn't worth the
317years of debugging it will take you to track down such subtle bugs.
5d9f8747 318
319There is another problem with this syntax: the indirect object is
320limited to a name, a scalar variable, or a block, because it would have
321to do too much lookahead otherwise, just like any other postfix
322dereference in the language. (These are the same quirky rules as are
323used for the filehandle slot in functions like C<print> and C<printf>.)
324This can lead to horribly confusing precedence problems, as in these
325next two lines:
19799a22 326
327 move $obj->{FIELD}; # probably wrong!
328 move $ary[$i]; # probably wrong!
329
330Those actually parse as the very surprising:
331
332 $obj->move->{FIELD}; # Well, lookee here
4f298f32 333 $ary->move([$i]); # Didn't expect this one, eh?
19799a22 334
335Rather than what you might have expected:
336
337 $obj->{FIELD}->move(); # You should be so lucky.
338 $ary[$i]->move; # Yeah, sure.
339
5d9f8747 340To get the correct behavior with indirect object syntax, you would have
341to use a block around the indirect object:
19799a22 342
5d9f8747 343 move {$obj->{FIELD}};
344 move {$ary[$i]};
345
346Even then, you still have the same potential problem if there happens to
347be a function named C<move> in the current package. B<The C<< -> >>
348notation suffers from neither of these disturbing ambiguities, so we
349recommend you use it exclusively.> However, you may still end up having
350to read code using the indirect object notation, so it's important to be
351familiar with it.
748a9306 352
a2bdc9a5 353=head2 Default UNIVERSAL methods
354
355The C<UNIVERSAL> package automatically contains the following methods that
356are inherited by all other classes:
357
358=over 4
359
71be2cbc 360=item isa(CLASS)
a2bdc9a5 361
68dc0745 362C<isa> returns I<true> if its object is blessed into a subclass of C<CLASS>
a2bdc9a5 363
3189d65a 364You can also call C<UNIVERSAL::isa> as a subroutine with two arguments.
365The first does not need to be an object or even a reference. This
366allows you to check what a reference points to, or whether
38242c00 367something is a reference of a given type. Example
a2bdc9a5 368
38242c00 369 if(UNIVERSAL::isa($ref, 'ARRAY')) {
5a964f20 370 #...
a2bdc9a5 371 }
372
3189d65a 373To determine if a reference is a blessed object, you can write
374
375 print "It's an object\n" if UNIVERSAL::isa($val, 'UNIVERSAL');
376
71be2cbc 377=item can(METHOD)
a2bdc9a5 378
379C<can> checks to see if its object has a method called C<METHOD>,
380if it does then a reference to the sub is returned, if it does not then
381I<undef> is returned.
382
3189d65a 383C<UNIVERSAL::can> can also be called as a subroutine with two arguments.
384It'll always return I<undef> if its first argument isn't an object or a
385class name. So here's another way to check if a reference is a
386blessed object
387
388 print "It's still an object\n" if UNIVERSAL::can($val, 'can');
389
b32b0a5d 390You can also use the C<blessed> function of Scalar::Util:
391
392 use Scalar::Util 'blessed';
393
394 my $blessing = blessed $suspected_object;
395
396C<blessed> returns the name of the package the argument has been
397blessed into, or C<undef>.
398
71be2cbc 399=item VERSION( [NEED] )
760ac839 400
71be2cbc 401C<VERSION> returns the version number of the class (package). If the
402NEED argument is given then it will check that the current version (as
403defined by the $VERSION variable in the given package) not less than
404NEED; it will die if this is not the case. This method is normally
405called as a class method. This method is called automatically by the
406C<VERSION> form of C<use>.
a2bdc9a5 407
a2bdc9a5 408 use A 1.2 qw(some imported subs);
71be2cbc 409 # implies:
410 A->VERSION(1.2);
a2bdc9a5 411
a2bdc9a5 412=back
413
414B<NOTE:> C<can> directly uses Perl's internal code for method lookup, and
415C<isa> uses a very similar method and cache-ing strategy. This may cause
416strange effects if the Perl code dynamically changes @ISA in any package.
417
418You may add other methods to the UNIVERSAL class via Perl or XS code.
14218588 419You do not need to C<use UNIVERSAL> to make these methods
38242c00 420available to your program (and you should not do so).
a2bdc9a5 421
54310121 422=head2 Destructors
a0d0e21e 423
424When the last reference to an object goes away, the object is
425automatically destroyed. (This may even be after you exit, if you've
426stored references in global variables.) If you want to capture control
427just before the object is freed, you may define a DESTROY method in
428your class. It will automatically be called at the appropriate moment,
4e8e7886 429and you can do any extra cleanup you need to do. Perl passes a reference
430to the object under destruction as the first (and only) argument. Beware
431that the reference is a read-only value, and cannot be modified by
432manipulating C<$_[0]> within the destructor. The object itself (i.e.
433the thingy the reference points to, namely C<${$_[0]}>, C<@{$_[0]}>,
434C<%{$_[0]}> etc.) is not similarly constrained.
435
436If you arrange to re-bless the reference before the destructor returns,
437perl will again call the DESTROY method for the re-blessed object after
438the current one returns. This can be used for clean delegation of
439object destruction, or for ensuring that destructors in the base classes
440of your choosing get called. Explicitly calling DESTROY is also possible,
441but is usually never needed.
442
14218588 443Do not confuse the previous discussion with how objects I<CONTAINED> in the current
4e8e7886 444one are destroyed. Such objects will be freed and destroyed automatically
445when the current object is freed, provided no other references to them exist
446elsewhere.
a0d0e21e 447
448=head2 Summary
449
5f05dabc 450That's about all there is to it. Now you need just to go off and buy a
a0d0e21e 451book about object-oriented design methodology, and bang your forehead
452with it for the next six months or so.
453
cb1a09d0 454=head2 Two-Phased Garbage Collection
455
14218588 456For most purposes, Perl uses a fast and simple, reference-based
457garbage collection system. That means there's an extra
cb1a09d0 458dereference going on at some level, so if you haven't built
459your Perl executable using your C compiler's C<-O> flag, performance
460will suffer. If you I<have> built Perl with C<cc -O>, then this
461probably won't matter.
462
463A more serious concern is that unreachable memory with a non-zero
464reference count will not normally get freed. Therefore, this is a bad
54310121 465idea:
cb1a09d0 466
467 {
468 my $a;
469 $a = \$a;
54310121 470 }
cb1a09d0 471
472Even thought $a I<should> go away, it can't. When building recursive data
473structures, you'll have to break the self-reference yourself explicitly
474if you don't care to leak. For example, here's a self-referential
475node such as one might use in a sophisticated tree structure:
476
477 sub new_node {
478 my $self = shift;
479 my $class = ref($self) || $self;
480 my $node = {};
481 $node->{LEFT} = $node->{RIGHT} = $node;
482 $node->{DATA} = [ @_ ];
483 return bless $node => $class;
54310121 484 }
cb1a09d0 485
486If you create nodes like that, they (currently) won't go away unless you
487break their self reference yourself. (In other words, this is not to be
488construed as a feature, and you shouldn't depend on it.)
489
490Almost.
491
492When an interpreter thread finally shuts down (usually when your program
493exits), then a rather costly but complete mark-and-sweep style of garbage
494collection is performed, and everything allocated by that thread gets
495destroyed. This is essential to support Perl as an embedded or a
54310121 496multithreadable language. For example, this program demonstrates Perl's
cb1a09d0 497two-phased garbage collection:
498
54310121 499 #!/usr/bin/perl
cb1a09d0 500 package Subtle;
501
502 sub new {
503 my $test;
504 $test = \$test;
505 warn "CREATING " . \$test;
506 return bless \$test;
54310121 507 }
cb1a09d0 508
509 sub DESTROY {
510 my $self = shift;
511 warn "DESTROYING $self";
54310121 512 }
cb1a09d0 513
514 package main;
515
516 warn "starting program";
517 {
518 my $a = Subtle->new;
519 my $b = Subtle->new;
520 $$a = 0; # break selfref
521 warn "leaving block";
54310121 522 }
cb1a09d0 523
524 warn "just exited block";
525 warn "time to die...";
526 exit;
527
528When run as F</tmp/test>, the following output is produced:
529
530 starting program at /tmp/test line 18.
531 CREATING SCALAR(0x8e5b8) at /tmp/test line 7.
532 CREATING SCALAR(0x8e57c) at /tmp/test line 7.
533 leaving block at /tmp/test line 23.
534 DESTROYING Subtle=SCALAR(0x8e5b8) at /tmp/test line 13.
535 just exited block at /tmp/test line 26.
536 time to die... at /tmp/test line 27.
537 DESTROYING Subtle=SCALAR(0x8e57c) during global destruction.
538
539Notice that "global destruction" bit there? That's the thread
54310121 540garbage collector reaching the unreachable.
cb1a09d0 541
14218588 542Objects are always destructed, even when regular refs aren't. Objects
543are destructed in a separate pass before ordinary refs just to
cb1a09d0 544prevent object destructors from using refs that have been themselves
5f05dabc 545destructed. Plain refs are only garbage-collected if the destruct level
cb1a09d0 546is greater than 0. You can test the higher levels of global destruction
547by setting the PERL_DESTRUCT_LEVEL environment variable, presuming
548C<-DDEBUGGING> was enabled during perl build time.
64cea5fd 549See L<perlhack/PERL_DESTRUCT_LEVEL> for more information.
cb1a09d0 550
551A more complete garbage collection strategy will be implemented
552at a future date.
553
5a964f20 554In the meantime, the best solution is to create a non-recursive container
555class that holds a pointer to the self-referential data structure.
556Define a DESTROY method for the containing object's class that manually
557breaks the circularities in the self-referential structure.
558
a0d0e21e 559=head1 SEE ALSO
560
8257a158 561A kinder, gentler tutorial on object-oriented programming in Perl can
890a53b9 562be found in L<perltoot>, L<perlboot> and L<perltooc>. You should
8257a158 563also check out L<perlbot> for other object tricks, traps, and tips, as
564well as L<perlmodlib> for some style guides on constructing both
565modules and classes.