7 First of all, you need to understand what references are in Perl.
8 See L<perlref> for that. Second, if you still find the following
9 reference work too complicated, a tutorial on object-oriented programming
10 in Perl can be found in L<perltoot>.
12 If you're still with us, then
13 here are three very simple definitions that you should find reassuring.
19 An object is simply a reference that happens to know which class it
24 A class is simply a package that happens to provide methods to deal
25 with object references.
29 A method is simply a subroutine that expects an object reference (or
30 a package name, for class methods) as the first argument.
34 We'll cover these points now in more depth.
36 =head2 An Object is Simply a Reference
38 Unlike say C++, Perl doesn't provide any special syntax for
39 constructors. A constructor is merely a subroutine that returns a
40 reference to something "blessed" into a class, generally the
41 class that the subroutine is defined in. Here is a typical
47 That word C<new> isn't special. You could have written
48 a construct this way, too:
51 sub spawn { bless {} }
53 In fact, this might even be preferable, because the C++ programmers won't
54 be tricked into thinking that C<new> works in Perl as it does in C++.
55 It doesn't. We recommend that you name your constructors whatever
56 makes sense in the context of the problem you're solving. For example,
57 constructors in the Tk extension to Perl are named after the widgets
60 One thing that's different about Perl constructors compared with those in
61 C++ is that in Perl, they have to allocate their own memory. (The other
62 things is that they don't automatically call overridden base-class
63 constructors.) The C<{}> allocates an anonymous hash containing no
64 key/value pairs, and returns it The bless() takes that reference and
65 tells the object it references that it's now a Critter, and returns
66 the reference. This is for convenience, because the referenced object
67 itself knows that it has been blessed, and the reference to it could
68 have been returned directly, like this:
76 In fact, you often see such a thing in more complicated constructors
77 that wish to call methods in the class as part of the construction:
86 If you care about inheritance (and you should; see
87 L<perlmodlib/"Modules: Creation, Use, and Abuse">),
88 then you want to use the two-arg form of bless
89 so that your constructors may be inherited:
99 Or if you expect people to call not just C<CLASS-E<gt>new()> but also
100 C<$obj-E<gt>new()>, then use something like this. The initialize()
101 method used will be of whatever $class we blessed the
106 my $class = ref($this) || $this;
113 Within the class package, the methods will typically deal with the
114 reference as an ordinary reference. Outside the class package,
115 the reference is generally treated as an opaque value that may
116 be accessed only through the class's methods.
118 A constructor may re-bless a referenced object currently belonging to
119 another class, but then the new class is responsible for all cleanup
120 later. The previous blessing is forgotten, as an object may belong
121 to only one class at a time. (Although of course it's free to
122 inherit methods from many classes.) If you find yourself having to
123 do this, the parent class is probably misbehaving, though.
125 A clarification: Perl objects are blessed. References are not. Objects
126 know which package they belong to. References do not. The bless()
127 function uses the reference to find the object. Consider
128 the following example:
133 print "\$b is a ", ref($b), "\n";
135 This reports $b as being a BLAH, so obviously bless()
136 operated on the object and not on the reference.
138 =head2 A Class is Simply a Package
140 Unlike say C++, Perl doesn't provide any special syntax for class
141 definitions. You use a package as a class by putting method
142 definitions into the class.
144 There is a special array within each package called @ISA, which says
145 where else to look for a method if you can't find it in the current
146 package. This is how Perl implements inheritance. Each element of the
147 @ISA array is just the name of another package that happens to be a
148 class package. The classes are searched (depth first) for missing
149 methods in the order that they occur in @ISA. The classes accessible
150 through @ISA are known as base classes of the current class.
152 All classes implicitly inherit from class C<UNIVERSAL> as their
153 last base class. Several commonly used methods are automatically
154 supplied in the UNIVERSAL class; see L<"Default UNIVERSAL methods"> for
157 If a missing method is found in one of the base classes, it is cached
158 in the current class for efficiency. Changing @ISA or defining new
159 subroutines invalidates the cache and causes Perl to do the lookup again.
161 If neither the current class, its named base classes, nor the UNIVERSAL
162 class contains the requested method, these three places are searched
163 all over again, this time looking for a method named AUTOLOAD(). If an
164 AUTOLOAD is found, this method is called on behalf of the missing method,
165 setting the package global $AUTOLOAD to be the fully qualified name of
166 the method that was intended to be called.
168 If none of that works, Perl finally gives up and complains.
170 Perl classes do method inheritance only. Data inheritance is left up
171 to the class itself. By and large, this is not a problem in Perl,
172 because most classes model the attributes of their object using an
173 anonymous hash, which serves as its own little namespace to be carved up
174 by the various classes that might want to do something with the object.
175 The only problem with this is that you can't sure that you aren't using
176 a piece of the hash that isn't already used. A reasonable workaround
177 is to prepend your fieldname in the hash with the package name.
181 $self->{ __PACKAGE__ . ".count"}++;
184 =head2 A Method is Simply a Subroutine
186 Unlike say C++, Perl doesn't provide any special syntax for method
187 definition. (It does provide a little syntax for method invocation
188 though. More on that later.) A method expects its first argument
189 to be the object (reference) or package (string) it is being invoked on. There are just two
190 types of methods, which we'll call class and instance.
191 (Sometimes you'll hear these called static and virtual, in honor of
192 the two C++ method types they most closely resemble.)
194 A class method expects a class name as the first argument. It
195 provides functionality for the class as a whole, not for any individual
196 object belonging to the class. Constructors are typically class
197 methods. Many class methods simply ignore their first argument, because
198 they already know what package they're in, and don't care what package
199 they were invoked via. (These aren't necessarily the same, because
200 class methods follow the inheritance tree just like ordinary instance
201 methods.) Another typical use for class methods is to look up an
205 my ($class, $name) = @_;
209 An instance method expects an object reference as its first argument.
210 Typically it shifts the first argument into a "self" or "this" variable,
211 and then uses that as an ordinary reference.
215 my @keys = @_ ? @_ : sort keys %$self;
216 foreach $key (@keys) {
217 print "\t$key => $self->{$key}\n";
221 =head2 Method Invocation
223 There are two ways to invoke a method, one of which you're already
224 familiar with, and the other of which will look familiar. Perl 4
225 already had an "indirect object" syntax that you use when you say
227 print STDERR "help!!!\n";
229 This same syntax can be used to call either class or instance methods.
230 We'll use the two methods defined above, the class method to lookup
231 an object reference and the instance method to print out its attributes.
233 $fred = find Critter "Fred";
234 display $fred 'Height', 'Weight';
236 These could be combined into one statement by using a BLOCK in the
237 indirect object slot:
239 display {find Critter "Fred"} 'Height', 'Weight';
241 For C++ fans, there's also a syntax using -E<gt> notation that does exactly
242 the same thing. The parentheses are required if there are any arguments.
244 $fred = Critter->find("Fred");
245 $fred->display('Height', 'Weight');
249 Critter->find("Fred")->display('Height', 'Weight');
251 There are times when one syntax is more readable, and times when the
252 other syntax is more readable. The indirect object syntax is less
253 cluttered, but it has the same ambiguity as ordinary list operators.
254 Indirect object method calls are parsed using the same rule as list
255 operators: "If it looks like a function, it is a function". (Presuming
256 for the moment that you think two words in a row can look like a
257 function name. C++ programmers seem to think so with some regularity,
258 especially when the first word is "new".) Thus, the parentheses of
260 new Critter ('Barney', 1.5, 70)
262 are assumed to surround ALL the arguments of the method call, regardless
263 of what comes after. Saying
265 new Critter ('Bam' x 2), 1.4, 45
267 would be equivalent to
269 Critter->new('Bam' x 2), 1.4, 45
271 which is unlikely to do what you want.
273 There are times when you wish to specify which class's method to use.
274 In this case, you can call your method as an ordinary subroutine
275 call, being sure to pass the requisite first argument explicitly:
277 $fred = MyCritter::find("Critter", "Fred");
278 MyCritter::display($fred, 'Height', 'Weight');
280 Note however, that this does not do any inheritance. If you wish
281 merely to specify that Perl should I<START> looking for a method in a
282 particular package, use an ordinary method call, but qualify the method
283 name with the package like this:
285 $fred = Critter->MyCritter::find("Fred");
286 $fred->MyCritter::display('Height', 'Weight');
288 If you're trying to control where the method search begins I<and> you're
289 executing in the class itself, then you may use the SUPER pseudo class,
290 which says to start looking in your base class's @ISA list without having
291 to name it explicitly:
293 $self->SUPER::display('Height', 'Weight');
295 Please note that the C<SUPER::> construct is meaningful I<only> within the
298 Sometimes you want to call a method when you don't know the method name
299 ahead of time. You can use the arrow form, replacing the method name
300 with a simple scalar variable containing the method name:
302 $method = $fast ? "findfirst" : "findbest";
303 $fred->$method(@args);
305 =head2 Default UNIVERSAL methods
307 The C<UNIVERSAL> package automatically contains the following methods that
308 are inherited by all other classes:
314 C<isa> returns I<true> if its object is blessed into a subclass of C<CLASS>
316 C<isa> is also exportable and can be called as a sub with two arguments. This
317 allows the ability to check what a reference points to. Example
319 use UNIVERSAL qw(isa);
321 if(isa($ref, 'ARRAY')) {
327 C<can> checks to see if its object has a method called C<METHOD>,
328 if it does then a reference to the sub is returned, if it does not then
329 I<undef> is returned.
331 =item VERSION( [NEED] )
333 C<VERSION> returns the version number of the class (package). If the
334 NEED argument is given then it will check that the current version (as
335 defined by the $VERSION variable in the given package) not less than
336 NEED; it will die if this is not the case. This method is normally
337 called as a class method. This method is called automatically by the
338 C<VERSION> form of C<use>.
340 use A 1.2 qw(some imported subs);
346 B<NOTE:> C<can> directly uses Perl's internal code for method lookup, and
347 C<isa> uses a very similar method and cache-ing strategy. This may cause
348 strange effects if the Perl code dynamically changes @ISA in any package.
350 You may add other methods to the UNIVERSAL class via Perl or XS code.
351 You do not need to C<use UNIVERSAL> in order to make these methods
352 available to your program. This is necessary only if you wish to
353 have C<isa> available as a plain subroutine in the current package.
357 When the last reference to an object goes away, the object is
358 automatically destroyed. (This may even be after you exit, if you've
359 stored references in global variables.) If you want to capture control
360 just before the object is freed, you may define a DESTROY method in
361 your class. It will automatically be called at the appropriate moment,
362 and you can do any extra cleanup you need to do. Perl passes a reference
363 to the object under destruction as the first (and only) argument. Beware
364 that the reference is a read-only value, and cannot be modified by
365 manipulating C<$_[0]> within the destructor. The object itself (i.e.
366 the thingy the reference points to, namely C<${$_[0]}>, C<@{$_[0]}>,
367 C<%{$_[0]}> etc.) is not similarly constrained.
369 If you arrange to re-bless the reference before the destructor returns,
370 perl will again call the DESTROY method for the re-blessed object after
371 the current one returns. This can be used for clean delegation of
372 object destruction, or for ensuring that destructors in the base classes
373 of your choosing get called. Explicitly calling DESTROY is also possible,
374 but is usually never needed.
376 Do not confuse the foregoing with how objects I<CONTAINED> in the current
377 one are destroyed. Such objects will be freed and destroyed automatically
378 when the current object is freed, provided no other references to them exist
383 While indirect object syntax may well be appealing to English speakers and
384 to C++ programmers, be not seduced! It suffers from two grave problems.
386 The first problem is that an indirect object is limited to a name,
387 a scalar variable, or a block, because it would have to do too much
388 lookahead otherwise, just like any other postfix dereference in the
389 language. (These are the same quirky rules as are used for the filehandle
390 slot in functions like C<print> and C<printf>.) This can lead to horribly
391 confusing precedence problems, as in these next two lines:
393 move $obj->{FIELD}; # probably wrong!
394 move $ary[$i]; # probably wrong!
396 Those actually parse as the very surprising:
398 $obj->move->{FIELD}; # Well, lookee here
399 $ary->move->[$i]; # Didn't expect this one, eh?
401 Rather than what you might have expected:
403 $obj->{FIELD}->move(); # You should be so lucky.
404 $ary[$i]->move; # Yeah, sure.
406 The left side of ``-E<gt>'' is not so limited, because it's an infix operator,
407 not a postfix operator.
409 As if that weren't bad enough, think about this: Perl must guess I<at
410 compile time> whether C<name> and C<move> above are functions or methods.
411 Usually Perl gets it right, but when it doesn't it, you get a function
412 call compiled as a method, or vice versa. This can introduce subtle
413 bugs that are hard to unravel. For example, calling a method C<new>
414 in indirect notation--as C++ programmers are so wont to do--can
415 be miscompiled into a subroutine call if there's already a C<new>
416 function in scope. You'd end up calling the current package's C<new>
417 as a subroutine, rather than the desired class's method. The compiler
418 tries to cheat by remembering bareword C<require>s, but the grief if it
419 messes up just isn't worth the years of debugging it would likely take
420 you to to track such subtle bugs down.
422 The infix arrow notation using ``C<-E<gt>>'' doesn't suffer from either
423 of these disturbing ambiguities, so we recommend you use it exclusively.
427 That's about all there is to it. Now you need just to go off and buy a
428 book about object-oriented design methodology, and bang your forehead
429 with it for the next six months or so.
431 =head2 Two-Phased Garbage Collection
433 For most purposes, Perl uses a fast and simple reference-based
434 garbage collection system. For this reason, there's an extra
435 dereference going on at some level, so if you haven't built
436 your Perl executable using your C compiler's C<-O> flag, performance
437 will suffer. If you I<have> built Perl with C<cc -O>, then this
438 probably won't matter.
440 A more serious concern is that unreachable memory with a non-zero
441 reference count will not normally get freed. Therefore, this is a bad
449 Even thought $a I<should> go away, it can't. When building recursive data
450 structures, you'll have to break the self-reference yourself explicitly
451 if you don't care to leak. For example, here's a self-referential
452 node such as one might use in a sophisticated tree structure:
456 my $class = ref($self) || $self;
458 $node->{LEFT} = $node->{RIGHT} = $node;
459 $node->{DATA} = [ @_ ];
460 return bless $node => $class;
463 If you create nodes like that, they (currently) won't go away unless you
464 break their self reference yourself. (In other words, this is not to be
465 construed as a feature, and you shouldn't depend on it.)
469 When an interpreter thread finally shuts down (usually when your program
470 exits), then a rather costly but complete mark-and-sweep style of garbage
471 collection is performed, and everything allocated by that thread gets
472 destroyed. This is essential to support Perl as an embedded or a
473 multithreadable language. For example, this program demonstrates Perl's
474 two-phased garbage collection:
482 warn "CREATING " . \$test;
488 warn "DESTROYING $self";
493 warn "starting program";
497 $$a = 0; # break selfref
498 warn "leaving block";
501 warn "just exited block";
502 warn "time to die...";
505 When run as F</tmp/test>, the following output is produced:
507 starting program at /tmp/test line 18.
508 CREATING SCALAR(0x8e5b8) at /tmp/test line 7.
509 CREATING SCALAR(0x8e57c) at /tmp/test line 7.
510 leaving block at /tmp/test line 23.
511 DESTROYING Subtle=SCALAR(0x8e5b8) at /tmp/test line 13.
512 just exited block at /tmp/test line 26.
513 time to die... at /tmp/test line 27.
514 DESTROYING Subtle=SCALAR(0x8e57c) during global destruction.
516 Notice that "global destruction" bit there? That's the thread
517 garbage collector reaching the unreachable.
519 Objects are always destructed, even when regular refs aren't and in fact
520 are destructed in a separate pass before ordinary refs just to try to
521 prevent object destructors from using refs that have been themselves
522 destructed. Plain refs are only garbage-collected if the destruct level
523 is greater than 0. You can test the higher levels of global destruction
524 by setting the PERL_DESTRUCT_LEVEL environment variable, presuming
525 C<-DDEBUGGING> was enabled during perl build time.
527 A more complete garbage collection strategy will be implemented
530 In the meantime, the best solution is to create a non-recursive container
531 class that holds a pointer to the self-referential data structure.
532 Define a DESTROY method for the containing object's class that manually
533 breaks the circularities in the self-referential structure.
537 A kinder, gentler tutorial on object-oriented programming in Perl can
538 be found in L<perltoot>.
539 You should also check out L<perlbot> for other object tricks, traps, and tips,
540 as well as L<perlmodlib> for some style guides on constructing both modules