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 The C<{}> constructs a reference to an anonymous hash containing no
48 key/value pairs. The bless() takes that reference and tells the object
49 it references that it's now a Critter, and returns the reference.
50 This is for convenience, because the referenced object itself knows that
51 it has been blessed, and the reference to it could have been returned
60 In fact, you often see such a thing in more complicated constructors
61 that wish to call methods in the class as part of the construction:
70 If you care about inheritance (and you should; see
71 L<perlmod/"Modules: Creation, Use, and Abuse">),
72 then you want to use the two-arg form of bless
73 so that your constructors may be inherited:
83 Or if you expect people to call not just C<CLASS-E<gt>new()> but also
84 C<$obj-E<gt>new()>, then use something like this. The initialize()
85 method used will be of whatever $class we blessed the
90 my $class = ref($this) || $this;
97 Within the class package, the methods will typically deal with the
98 reference as an ordinary reference. Outside the class package,
99 the reference is generally treated as an opaque value that may
100 be accessed only through the class's methods.
102 A constructor may re-bless a referenced object currently belonging to
103 another class, but then the new class is responsible for all cleanup
104 later. The previous blessing is forgotten, as an object may belong
105 to only one class at a time. (Although of course it's free to
106 inherit methods from many classes.)
108 A clarification: Perl objects are blessed. References are not. Objects
109 know which package they belong to. References do not. The bless()
110 function uses the reference to find the object. Consider
111 the following example:
116 print "\$b is a ", ref($b), "\n";
118 This reports $b as being a BLAH, so obviously bless()
119 operated on the object and not on the reference.
121 =head2 A Class is Simply a Package
123 Unlike say C++, Perl doesn't provide any special syntax for class
124 definitions. You use a package as a class by putting method
125 definitions into the class.
127 There is a special array within each package called @ISA which says
128 where else to look for a method if you can't find it in the current
129 package. This is how Perl implements inheritance. Each element of the
130 @ISA array is just the name of another package that happens to be a
131 class package. The classes are searched (depth first) for missing
132 methods in the order that they occur in @ISA. The classes accessible
133 through @ISA are known as base classes of the current class.
135 If a missing method is found in one of the base classes, it is cached
136 in the current class for efficiency. Changing @ISA or defining new
137 subroutines invalidates the cache and causes Perl to do the lookup again.
139 If a method isn't found, but an AUTOLOAD routine is found, then
140 that is called on behalf of the missing method.
142 If neither a method nor an AUTOLOAD routine is found in @ISA, then one
143 last try is made for the method (or an AUTOLOAD routine) in a class
144 called UNIVERSAL. (Several commonly used methods are automatically
145 supplied in the UNIVERSAL class; see L<"Default UNIVERSAL methods"> for
146 more details.) If that doesn't work, Perl finally gives up and
149 Perl classes do only method inheritance. Data inheritance is left
150 up to the class itself. By and large, this is not a problem in Perl,
151 because most classes model the attributes of their object using
152 an anonymous hash, which serves as its own little namespace to be
153 carved up by the various classes that might want to do something
156 =head2 A Method is Simply a Subroutine
158 Unlike say C++, Perl doesn't provide any special syntax for method
159 definition. (It does provide a little syntax for method invocation
160 though. More on that later.) A method expects its first argument
161 to be the object or package it is being invoked on. There are just two
162 types of methods, which we'll call class and instance.
163 (Sometimes you'll hear these called static and virtual, in honor of
164 the two C++ method types they most closely resemble.)
166 A class method expects a class name as the first argument. It
167 provides functionality for the class as a whole, not for any individual
168 object belonging to the class. Constructors are typically class
169 methods. Many class methods simply ignore their first argument, because
170 they already know what package they're in, and don't care what package
171 they were invoked via. (These aren't necessarily the same, because
172 class methods follow the inheritance tree just like ordinary instance
173 methods.) Another typical use for class methods is to look up an
177 my ($class, $name) = @_;
181 An instance method expects an object reference as its first argument.
182 Typically it shifts the first argument into a "self" or "this" variable,
183 and then uses that as an ordinary reference.
187 my @keys = @_ ? @_ : sort keys %$self;
188 foreach $key (@keys) {
189 print "\t$key => $self->{$key}\n";
193 =head2 Method Invocation
195 There are two ways to invoke a method, one of which you're already
196 familiar with, and the other of which will look familiar. Perl 4
197 already had an "indirect object" syntax that you use when you say
199 print STDERR "help!!!\n";
201 This same syntax can be used to call either class or instance methods.
202 We'll use the two methods defined above, the class method to lookup
203 an object reference and the instance method to print out its attributes.
205 $fred = find Critter "Fred";
206 display $fred 'Height', 'Weight';
208 These could be combined into one statement by using a BLOCK in the
209 indirect object slot:
211 display {find Critter "Fred"} 'Height', 'Weight';
213 For C++ fans, there's also a syntax using -E<gt> notation that does exactly
214 the same thing. The parentheses are required if there are any arguments.
216 $fred = Critter->find("Fred");
217 $fred->display('Height', 'Weight');
221 Critter->find("Fred")->display('Height', 'Weight');
223 There are times when one syntax is more readable, and times when the
224 other syntax is more readable. The indirect object syntax is less
225 cluttered, but it has the same ambiguity as ordinary list operators.
226 Indirect object method calls are parsed using the same rule as list
227 operators: "If it looks like a function, it is a function". (Presuming
228 for the moment that you think two words in a row can look like a
229 function name. C++ programmers seem to think so with some regularity,
230 especially when the first word is "new".) Thus, the parentheses of
232 new Critter ('Barney', 1.5, 70)
234 are assumed to surround ALL the arguments of the method call, regardless
235 of what comes after. Saying
237 new Critter ('Bam' x 2), 1.4, 45
239 would be equivalent to
241 Critter->new('Bam' x 2), 1.4, 45
243 which is unlikely to do what you want.
245 There are times when you wish to specify which class's method to use.
246 In this case, you can call your method as an ordinary subroutine
247 call, being sure to pass the requisite first argument explicitly:
249 $fred = MyCritter::find("Critter", "Fred");
250 MyCritter::display($fred, 'Height', 'Weight');
252 Note however, that this does not do any inheritance. If you wish
253 merely to specify that Perl should I<START> looking for a method in a
254 particular package, use an ordinary method call, but qualify the method
255 name with the package like this:
257 $fred = Critter->MyCritter::find("Fred");
258 $fred->MyCritter::display('Height', 'Weight');
260 If you're trying to control where the method search begins I<and> you're
261 executing in the class itself, then you may use the SUPER pseudo class,
262 which says to start looking in your base class's @ISA list without having
263 to name it explicitly:
265 $self->SUPER::display('Height', 'Weight');
267 Please note that the C<SUPER::> construct is meaningful I<only> within the
270 Sometimes you want to call a method when you don't know the method name
271 ahead of time. You can use the arrow form, replacing the method name
272 with a simple scalar variable containing the method name:
274 $method = $fast ? "findfirst" : "findbest";
275 $fred->$method(@args);
277 =head2 Default UNIVERSAL methods
279 The C<UNIVERSAL> package automatically contains the following methods that
280 are inherited by all other classes:
286 C<isa> returns I<true> if its object is blessed into a subclass of C<CLASS>
288 C<isa> is also exportable and can be called as a sub with two arguments. This
289 allows the ability to check what a reference points to. Example
291 use UNIVERSAL qw(isa);
293 if(isa($ref, 'ARRAY')) {
299 C<can> checks to see if its object has a method called C<METHOD>,
300 if it does then a reference to the sub is returned, if it does not then
301 I<undef> is returned.
303 =item VERSION( [NEED] )
305 C<VERSION> returns the version number of the class (package). If the
306 NEED argument is given then it will check that the current version (as
307 defined by the $VERSION variable in the given package) not less than
308 NEED; it will die if this is not the case. This method is normally
309 called as a class method. This method is called automatically by the
310 C<VERSION> form of C<use>.
312 use A 1.2 qw(some imported subs);
318 B<NOTE:> C<can> directly uses Perl's internal code for method lookup, and
319 C<isa> uses a very similar method and cache-ing strategy. This may cause
320 strange effects if the Perl code dynamically changes @ISA in any package.
322 You may add other methods to the UNIVERSAL class via Perl or XS code.
323 You do not need to C<use UNIVERSAL> in order to make these methods
324 available to your program. This is necessary only if you wish to
325 have C<isa> available as a plain subroutine in the current package.
329 When the last reference to an object goes away, the object is
330 automatically destroyed. (This may even be after you exit, if you've
331 stored references in global variables.) If you want to capture control
332 just before the object is freed, you may define a DESTROY method in
333 your class. It will automatically be called at the appropriate moment,
334 and you can do any extra cleanup you need to do. Perl passes a reference
335 to the object under destruction as the first (and only) argument. Beware
336 that the reference is a read-only value, and cannot be modified by
337 manipulating C<$_[0]> within the destructor. The object itself (i.e.
338 the thingy the reference points to, namely C<${$_[0]}>, C<@{$_[0]}>,
339 C<%{$_[0]}> etc.) is not similarly constrained.
341 If you arrange to re-bless the reference before the destructor returns,
342 perl will again call the DESTROY method for the re-blessed object after
343 the current one returns. This can be used for clean delegation of
344 object destruction, or for ensuring that destructors in the base classes
345 of your choosing get called. Explicitly calling DESTROY is also possible,
346 but is usually never needed.
348 Do not confuse the foregoing with how objects I<CONTAINED> in the current
349 one are destroyed. Such objects will be freed and destroyed automatically
350 when the current object is freed, provided no other references to them exist
355 An indirect object is limited to a name, a scalar variable, or a block,
356 because it would have to do too much lookahead otherwise, just like any
357 other postfix dereference in the language. The left side of -E<gt> is not so
358 limited, because it's an infix operator, not a postfix operator.
360 That means that in the following, A and B are equivalent to each other, and
361 C and D are equivalent, but A/B and C/D are different:
363 A: method $obref->{"fieldname"}
364 B: (method $obref)->{"fieldname"}
365 C: $obref->{"fieldname"}->method()
366 D: method {$obref->{"fieldname"}}
370 That's about all there is to it. Now you need just to go off and buy a
371 book about object-oriented design methodology, and bang your forehead
372 with it for the next six months or so.
374 =head2 Two-Phased Garbage Collection
376 For most purposes, Perl uses a fast and simple reference-based
377 garbage collection system. For this reason, there's an extra
378 dereference going on at some level, so if you haven't built
379 your Perl executable using your C compiler's C<-O> flag, performance
380 will suffer. If you I<have> built Perl with C<cc -O>, then this
381 probably won't matter.
383 A more serious concern is that unreachable memory with a non-zero
384 reference count will not normally get freed. Therefore, this is a bad
392 Even thought $a I<should> go away, it can't. When building recursive data
393 structures, you'll have to break the self-reference yourself explicitly
394 if you don't care to leak. For example, here's a self-referential
395 node such as one might use in a sophisticated tree structure:
399 my $class = ref($self) || $self;
401 $node->{LEFT} = $node->{RIGHT} = $node;
402 $node->{DATA} = [ @_ ];
403 return bless $node => $class;
406 If you create nodes like that, they (currently) won't go away unless you
407 break their self reference yourself. (In other words, this is not to be
408 construed as a feature, and you shouldn't depend on it.)
412 When an interpreter thread finally shuts down (usually when your program
413 exits), then a rather costly but complete mark-and-sweep style of garbage
414 collection is performed, and everything allocated by that thread gets
415 destroyed. This is essential to support Perl as an embedded or a
416 multithreadable language. For example, this program demonstrates Perl's
417 two-phased garbage collection:
425 warn "CREATING " . \$test;
431 warn "DESTROYING $self";
436 warn "starting program";
440 $$a = 0; # break selfref
441 warn "leaving block";
444 warn "just exited block";
445 warn "time to die...";
448 When run as F</tmp/test>, the following output is produced:
450 starting program at /tmp/test line 18.
451 CREATING SCALAR(0x8e5b8) at /tmp/test line 7.
452 CREATING SCALAR(0x8e57c) at /tmp/test line 7.
453 leaving block at /tmp/test line 23.
454 DESTROYING Subtle=SCALAR(0x8e5b8) at /tmp/test line 13.
455 just exited block at /tmp/test line 26.
456 time to die... at /tmp/test line 27.
457 DESTROYING Subtle=SCALAR(0x8e57c) during global destruction.
459 Notice that "global destruction" bit there? That's the thread
460 garbage collector reaching the unreachable.
462 Objects are always destructed, even when regular refs aren't and in fact
463 are destructed in a separate pass before ordinary refs just to try to
464 prevent object destructors from using refs that have been themselves
465 destructed. Plain refs are only garbage-collected if the destruct level
466 is greater than 0. You can test the higher levels of global destruction
467 by setting the PERL_DESTRUCT_LEVEL environment variable, presuming
468 C<-DDEBUGGING> was enabled during perl build time.
470 A more complete garbage collection strategy will be implemented
475 A kinder, gentler tutorial on object-oriented programming in Perl can
476 be found in L<perltoot>.
477 You should also check out L<perlbot> for other object tricks, traps, and tips,
478 as well as L<perlmodlib> for some style guides on constructing both modules