7 First of all, you need to understand what references are in Perl. See
10 Here are three very simple definitions that you should find reassuring.
16 An object is simply a reference that happens to know which class it
21 A class is simply a package that happens to provide methods to deal
22 with object references.
26 A method is simply a subroutine that expects an object reference (or
27 a package name, for static methods) as the first argument.
31 We'll cover these points now in more depth.
33 =head2 An Object is Simply a Reference
35 Unlike say C++, Perl doesn't provide any special syntax for
36 constructors. A constructor is merely a subroutine that returns a
37 reference to something "blessed" into a class, generally the
38 class that the subroutine is defined in. Here is a typical
44 The C<{}> constructs a reference to an anonymous hash containing no
45 key/value pairs. The bless() takes that reference and tells the object
46 it references that it's now a Critter, and returns the reference.
47 This is for convenience, since the referenced object itself knows that
48 it has been blessed, and its reference to it could have been returned
57 In fact, you often see such a thing in more complicated constructors
58 that wish to call methods in the class as part of the construction:
67 If you care about inheritance (and you should; see L<perlmod/"Modules:
68 Creation, Use and Abuse">), then you want to use the two-arg form of bless
69 so that your constructors may be inherited:
79 Or if you expect people to call not just C<CLASS-E<gt>new()> but also
80 C<$obj-E<gt>new()>, then use something like this. The initialize()
81 method used will be of whatever $class we blessed the
86 my $class = ref($this) || $this;
93 Within the class package, the methods will typically deal with the
94 reference as an ordinary reference. Outside the class package,
95 the reference is generally treated as an opaque value that may
96 only be accessed through the class's methods.
98 A constructor may re-bless a referenced object currently belonging to
99 another class, but then the new class is responsible for all cleanup
100 later. The previous blessing is forgotten, as an object may only
101 belong to one class at a time. (Although of course it's free to
102 inherit methods from many classes.)
104 A clarification: Perl objects are blessed. References are not. Objects
105 know which package they belong to. References do not. The bless()
106 function simply uses the reference in order to find the object. Consider
107 the following example:
112 print "\$b is a ", ref($b), "\n";
114 This reports $b as being a BLAH, so obviously bless()
115 operated on the object and not on the reference.
117 =head2 A Class is Simply a Package
119 Unlike say C++, Perl doesn't provide any special syntax for class
120 definitions. You just use a package as a class by putting method
121 definitions into the class.
123 There is a special array within each package called @ISA which says
124 where else to look for a method if you can't find it in the current
125 package. This is how Perl implements inheritance. Each element of the
126 @ISA array is just the name of another package that happens to be a
127 class package. The classes are searched (depth first) for missing
128 methods in the order that they occur in @ISA. The classes accessible
129 through @ISA are known as base classes of the current class.
131 If a missing method is found in one of the base classes, it is cached
132 in the current class for efficiency. Changing @ISA or defining new
133 subroutines invalidates the cache and causes Perl to do the lookup again.
135 If a method isn't found, but an AUTOLOAD routine is found, then
136 that is called on behalf of the missing method.
138 If neither a method nor an AUTOLOAD routine is found in @ISA, then one
139 last try is made for the method (or an AUTOLOAD routine) in a class
140 called UNIVERSAL. (Several commonly used methods are automatically
141 supplied in the UNIVERSAL class; see L<"Default UNIVERSAL methods"> for
142 more details.) If that doesn't work, Perl finally gives up and
145 Perl classes only do method inheritance. Data inheritance is left
146 up to the class itself. By and large, this is not a problem in Perl,
147 because most classes model the attributes of their object using
148 an anonymous hash, which serves as its own little namespace to be
149 carved up by the various classes that might want to do something
152 =head2 A Method is Simply a Subroutine
154 Unlike say C++, Perl doesn't provide any special syntax for method
155 definition. (It does provide a little syntax for method invocation
156 though. More on that later.) A method expects its first argument
157 to be the object or package it is being invoked on. There are just two
158 types of methods, which we'll call static and virtual, in honor of
159 the two C++ method types they most closely resemble.
161 A static method expects a class name as the first argument. It
162 provides functionality for the class as a whole, not for any individual
163 object belonging to the class. Constructors are typically static
164 methods. Many static methods simply ignore their first argument, since
165 they already know what package they're in, and don't care what package
166 they were invoked via. (These aren't necessarily the same, since
167 static methods follow the inheritance tree just like ordinary virtual
168 methods.) Another typical use for static methods is to look up an
172 my ($class, $name) = @_;
176 A virtual method expects an object reference as its first argument.
177 Typically it shifts the first argument into a "self" or "this" variable,
178 and then uses that as an ordinary reference.
182 my @keys = @_ ? @_ : sort keys %$self;
183 foreach $key (@keys) {
184 print "\t$key => $self->{$key}\n";
188 =head2 Method Invocation
190 There are two ways to invoke a method, one of which you're already
191 familiar with, and the other of which will look familiar. Perl 4
192 already had an "indirect object" syntax that you use when you say
194 print STDERR "help!!!\n";
196 This same syntax can be used to call either static or virtual methods.
197 We'll use the two methods defined above, the static method to lookup
198 an object reference and the virtual method to print out its attributes.
200 $fred = find Critter "Fred";
201 display $fred 'Height', 'Weight';
203 These could be combined into one statement by using a BLOCK in the
204 indirect object slot:
206 display {find Critter "Fred"} 'Height', 'Weight';
208 For C++ fans, there's also a syntax using -E<gt> notation that does exactly
209 the same thing. The parentheses are required if there are any arguments.
211 $fred = Critter->find("Fred");
212 $fred->display('Height', 'Weight');
216 Critter->find("Fred")->display('Height', 'Weight');
218 There are times when one syntax is more readable, and times when the
219 other syntax is more readable. The indirect object syntax is less
220 cluttered, but it has the same ambiguity as ordinary list operators.
221 Indirect object method calls are parsed using the same rule as list
222 operators: "If it looks like a function, it is a function". (Presuming
223 for the moment that you think two words in a row can look like a
224 function name. C++ programmers seem to think so with some regularity,
225 especially when the first word is "new".) Thus, the parens of
227 new Critter ('Barney', 1.5, 70)
229 are assumed to surround ALL the arguments of the method call, regardless
230 of what comes after. Saying
232 new Critter ('Bam' x 2), 1.4, 45
234 would be equivalent to
236 Critter->new('Bam' x 2), 1.4, 45
238 which is unlikely to do what you want.
240 There are times when you wish to specify which class's method to use.
241 In this case, you can call your method as an ordinary subroutine
242 call, being sure to pass the requisite first argument explicitly:
244 $fred = MyCritter::find("Critter", "Fred");
245 MyCritter::display($fred, 'Height', 'Weight');
247 Note however, that this does not do any inheritance. If you merely
248 wish to specify that Perl should I<START> looking for a method in a
249 particular package, use an ordinary method call, but qualify the method
250 name with the package like this:
252 $fred = Critter->MyCritter::find("Fred");
253 $fred->MyCritter::display('Height', 'Weight');
255 If you're trying to control where the method search begins I<and> you're
256 executing in the class itself, then you may use the SUPER pseudoclass,
257 which says to start looking in your base class's @ISA list without having
258 to explicitly name it:
260 $self->SUPER::display('Height', 'Weight');
262 Please note that the C<SUPER::> construct is I<only> meaningful within the
265 Sometimes you want to call a method when you don't know the method name
266 ahead of time. You can use the arrow form, replacing the method name
267 with a simple scalar variable containing the method name:
269 $method = $fast ? "findfirst" : "findbest";
270 $fred->$method(@args);
272 =head2 Default UNIVERSAL methods
274 The C<UNIVERSAL> package automatically contains the following methods that
275 are inherited by all other classes:
281 C<isa> returns I<true> if its object is blessed into a sub-class of C<CLASS>
283 C<isa> is also exportable and can be called as a sub with two arguments. This
284 allows the ability to check what a reference points to. Example
286 use UNIVERSAL qw(isa);
288 if(isa($ref, 'ARRAY')) {
294 C<can> checks to see if its object has a method called C<METHOD>,
295 if it does then a reference to the sub is returned, if it does not then
296 I<undef> is returned.
298 =item VERSION ( [ VERSION ] )
300 C<VERSION> returns the VERSION number of the class (package). If
301 an argument is given then it will check that the current version is not
302 less that the given argument. This method is normally called as a static
303 method. This method is also called when the C<VERSION> form of C<use> is
307 use A 1.2 qw(some imported subs);
309 A->require_version( 1.2 );
313 C<class> returns the class name of its object.
317 C<is_instance> returns true if its object is an instance of some
318 class, false if its object is the class (package) itself. Example
320 A->is_instance(); # False
323 $var->is_instance(); # False
325 $ref = bless [], 'A';
326 $ref->is_instance(); # True
330 B<NOTE:> C<can> directly uses Perl's internal code for method lookup, and
331 C<isa> uses a very similar method and cache-ing strategy. This may cause
332 strange effects if the Perl code dynamically changes @ISA in any package.
334 You may add other methods to the UNIVERSAL class via Perl or XS code.
338 When the last reference to an object goes away, the object is
339 automatically destroyed. (This may even be after you exit, if you've
340 stored references in global variables.) If you want to capture control
341 just before the object is freed, you may define a DESTROY method in
342 your class. It will automatically be called at the appropriate moment,
343 and you can do any extra cleanup you need to do.
345 Perl doesn't do nested destruction for you. If your constructor
346 reblessed a reference from one of your base classes, your DESTROY may
347 need to call DESTROY for any base classes that need it. But this only
348 applies to reblessed objects--an object reference that is merely
349 I<CONTAINED> in the current object will be freed and destroyed
350 automatically when the current object is freed.
354 An indirect object is limited to a name, a scalar variable, or a block,
355 because it would have to do too much lookahead otherwise, just like any
356 other postfix dereference in the language. The left side of -E<gt> is not so
357 limited, because it's an infix operator, not a postfix operator.
359 That means that below, A and B are equivalent to each other, and C and D
360 are equivalent, but AB and CD are different:
362 A: method $obref->{"fieldname"}
363 B: (method $obref)->{"fieldname"}
364 C: $obref->{"fieldname"}->method()
365 D: method {$obref->{"fieldname"}}
369 That's about all there is to it. Now you just need to go off and buy a
370 book about object-oriented design methodology, and bang your forehead
371 with it for the next six months or so.
373 =head2 Two-Phased Garbage Collection
375 For most purposes, Perl uses a fast and simple reference-based
376 garbage collection system. For this reason, there's an extra
377 dereference going on at some level, so if you haven't built
378 your Perl executable using your C compiler's C<-O> flag, performance
379 will suffer. If you I<have> built Perl with C<cc -O>, then this
380 probably won't matter.
382 A more serious concern is that unreachable memory with a non-zero
383 reference count will not normally get freed. Therefore, this is a bad
391 Even thought $a I<should> go away, it can't. When building recursive data
392 structures, you'll have to break the self-reference yourself explicitly
393 if you don't care to leak. For example, here's a self-referential
394 node such as one might use in a sophisticated tree structure:
398 my $class = ref($self) || $self;
400 $node->{LEFT} = $node->{RIGHT} = $node;
401 $node->{DATA} = [ @_ ];
402 return bless $node => $class;
405 If you create nodes like that, they (currently) won't go away unless you
406 break their self reference yourself. (In other words, this is not to be
407 construed as a feature, and you shouldn't depend on it.)
411 When an interpreter thread finally shuts down (usually when your program
412 exits), then a rather costly but complete mark-and-sweep style of garbage
413 collection is performed, and everything allocated by that thread gets
414 destroyed. This is essential to support Perl as an embedded or a
415 multithreadable language. For example, this program demonstrates Perl's
416 two-phased garbage collection:
424 warn "CREATING " . \$test;
430 warn "DESTROYING $self";
435 warn "starting program";
439 $$a = 0; # break selfref
440 warn "leaving block";
443 warn "just exited block";
444 warn "time to die...";
447 When run as F</tmp/test>, the following output is produced:
449 starting program at /tmp/test line 18.
450 CREATING SCALAR(0x8e5b8) at /tmp/test line 7.
451 CREATING SCALAR(0x8e57c) at /tmp/test line 7.
452 leaving block at /tmp/test line 23.
453 DESTROYING Subtle=SCALAR(0x8e5b8) at /tmp/test line 13.
454 just exited block at /tmp/test line 26.
455 time to die... at /tmp/test line 27.
456 DESTROYING Subtle=SCALAR(0x8e57c) during global destruction.
458 Notice that "global destruction" bit there? That's the thread
459 garbage collector reaching the unreachable.
461 Objects are always destructed, even when regular refs aren't and in fact
462 are destructed in a separate pass before ordinary refs just to try to
463 prevent object destructors from using refs that have been themselves
464 destructed. Plain refs are only garbage collected if the destruct level
465 is greater than 0. You can test the higher levels of global destruction
466 by setting the PERL_DESTRUCT_LEVEL environment variable, presuming
467 C<-DDEBUGGING> was enabled during perl build time.
469 A more complete garbage collection strategy will be implemented
474 You should also check out L<perlbot> for other object tricks, traps, and tips,
475 as well as L<perlmod> for some style guides on constructing both modules