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. If that doesn't work, Perl finally gives up and
143 Perl classes only do method inheritance. Data inheritance is left
144 up to the class itself. By and large, this is not a problem in Perl,
145 because most classes model the attributes of their object using
146 an anonymous hash, which serves as its own little namespace to be
147 carved up by the various classes that might want to do something
150 =head2 A Method is Simply a Subroutine
152 Unlike say C++, Perl doesn't provide any special syntax for method
153 definition. (It does provide a little syntax for method invocation
154 though. More on that later.) A method expects its first argument
155 to be the object or package it is being invoked on. There are just two
156 types of methods, which we'll call static and virtual, in honor of
157 the two C++ method types they most closely resemble.
159 A static method expects a class name as the first argument. It
160 provides functionality for the class as a whole, not for any individual
161 object belonging to the class. Constructors are typically static
162 methods. Many static methods simply ignore their first argument, since
163 they already know what package they're in, and don't care what package
164 they were invoked via. (These aren't necessarily the same, since
165 static methods follow the inheritance tree just like ordinary virtual
166 methods.) Another typical use for static methods is to look up an
170 my ($class, $name) = @_;
174 A virtual method expects an object reference as its first argument.
175 Typically it shifts the first argument into a "self" or "this" variable,
176 and then uses that as an ordinary reference.
180 my @keys = @_ ? @_ : sort keys %$self;
181 foreach $key (@keys) {
182 print "\t$key => $self->{$key}\n";
186 =head2 Method Invocation
188 There are two ways to invoke a method, one of which you're already
189 familiar with, and the other of which will look familiar. Perl 4
190 already had an "indirect object" syntax that you use when you say
192 print STDERR "help!!!\n";
194 This same syntax can be used to call either static or virtual methods.
195 We'll use the two methods defined above, the static method to lookup
196 an object reference and the virtual method to print out its attributes.
198 $fred = find Critter "Fred";
199 display $fred 'Height', 'Weight';
201 These could be combined into one statement by using a BLOCK in the
202 indirect object slot:
204 display {find Critter "Fred"} 'Height', 'Weight';
206 For C++ fans, there's also a syntax using -E<gt> notation that does exactly
207 the same thing. The parentheses are required if there are any arguments.
209 $fred = Critter->find("Fred");
210 $fred->display('Height', 'Weight');
214 Critter->find("Fred")->display('Height', 'Weight');
216 There are times when one syntax is more readable, and times when the
217 other syntax is more readable. The indirect object syntax is less
218 cluttered, but it has the same ambiguity as ordinary list operators.
219 Indirect object method calls are parsed using the same rule as list
220 operators: "If it looks like a function, it is a function". (Presuming
221 for the moment that you think two words in a row can look like a
222 function name. C++ programmers seem to think so with some regularity,
223 especially when the first word is "new".) Thus, the parens of
225 new Critter ('Barney', 1.5, 70)
227 are assumed to surround ALL the arguments of the method call, regardless
228 of what comes after. Saying
230 new Critter ('Bam' x 2), 1.4, 45
232 would be equivalent to
234 Critter->new('Bam' x 2), 1.4, 45
236 which is unlikely to do what you want.
238 There are times when you wish to specify which class's method to use.
239 In this case, you can call your method as an ordinary subroutine
240 call, being sure to pass the requisite first argument explicitly:
242 $fred = MyCritter::find("Critter", "Fred");
243 MyCritter::display($fred, 'Height', 'Weight');
245 Note however, that this does not do any inheritance. If you merely
246 wish to specify that Perl should I<START> looking for a method in a
247 particular package, use an ordinary method call, but qualify the method
248 name with the package like this:
250 $fred = Critter->MyCritter::find("Fred");
251 $fred->MyCritter::display('Height', 'Weight');
253 If you're trying to control where the method search begins I<and> you're
254 executing in the class itself, then you may use the SUPER pseudoclass,
255 which says to start looking in your base class's @ISA list without having
256 to explicitly name it:
258 $self->SUPER::display('Height', 'Weight');
260 Please note that the C<SUPER::> construct is I<only> meaningful within the
263 Sometimes you want to call a method when you don't know the method name
264 ahead of time. You can use the arrow form, replacing the method name
265 with a simple scalar variable containing the method name:
267 $method = $fast ? "findfirst" : "findbest";
268 $fred->$method(@args);
272 When the last reference to an object goes away, the object is
273 automatically destroyed. (This may even be after you exit, if you've
274 stored references in global variables.) If you want to capture control
275 just before the object is freed, you may define a DESTROY method in
276 your class. It will automatically be called at the appropriate moment,
277 and you can do any extra cleanup you need to do.
279 Perl doesn't do nested destruction for you. If your constructor
280 reblessed a reference from one of your base classes, your DESTROY may
281 need to call DESTROY for any base classes that need it. But this only
282 applies to reblessed objects--an object reference that is merely
283 I<CONTAINED> in the current object will be freed and destroyed
284 automatically when the current object is freed.
288 An indirect object is limited to a name, a scalar variable, or a block,
289 because it would have to do too much lookahead otherwise, just like any
290 other postfix dereference in the language. The left side of -E<gt> is not so
291 limited, because it's an infix operator, not a postfix operator.
293 That means that below, A and B are equivalent to each other, and C and D
294 are equivalent, but AB and CD are different:
296 A: method $obref->{"fieldname"}
297 B: (method $obref)->{"fieldname"}
298 C: $obref->{"fieldname"}->method()
299 D: method {$obref->{"fieldname"}}
303 That's about all there is to it. Now you just need to go off and buy a
304 book about object-oriented design methodology, and bang your forehead
305 with it for the next six months or so.
307 =head2 Two-Phased Garbage Collection
309 For most purposes, Perl uses a fast and simple reference-based
310 garbage collection system. For this reason, there's an extra
311 dereference going on at some level, so if you haven't built
312 your Perl executable using your C compiler's C<-O> flag, performance
313 will suffer. If you I<have> built Perl with C<cc -O>, then this
314 probably won't matter.
316 A more serious concern is that unreachable memory with a non-zero
317 reference count will not normally get freed. Therefore, this is a bad
325 Even thought $a I<should> go away, it can't. When building recursive data
326 structures, you'll have to break the self-reference yourself explicitly
327 if you don't care to leak. For example, here's a self-referential
328 node such as one might use in a sophisticated tree structure:
332 my $class = ref($self) || $self;
334 $node->{LEFT} = $node->{RIGHT} = $node;
335 $node->{DATA} = [ @_ ];
336 return bless $node => $class;
339 If you create nodes like that, they (currently) won't go away unless you
340 break their self reference yourself. (In other words, this is not to be
341 construed as a feature, and you shouldn't depend on it.)
345 When an interpreter thread finally shuts down (usually when your program
346 exits), then a rather costly but complete mark-and-sweep style of garbage
347 collection is performed, and everything allocated by that thread gets
348 destroyed. This is essential to support Perl as an embedded or a
349 multithreadable language. For example, this program demonstrates Perl's
350 two-phased garbage collection:
358 warn "CREATING " . \$test;
364 warn "DESTROYING $self";
369 warn "starting program";
373 $$a = 0; # break selfref
374 warn "leaving block";
377 warn "just exited block";
378 warn "time to die...";
381 When run as F</tmp/test>, the following output is produced:
383 starting program at /tmp/test line 18.
384 CREATING SCALAR(0x8e5b8) at /tmp/test line 7.
385 CREATING SCALAR(0x8e57c) at /tmp/test line 7.
386 leaving block at /tmp/test line 23.
387 DESTROYING Subtle=SCALAR(0x8e5b8) at /tmp/test line 13.
388 just exited block at /tmp/test line 26.
389 time to die... at /tmp/test line 27.
390 DESTROYING Subtle=SCALAR(0x8e57c) during global destruction.
392 Notice that "global destruction" bit there? That's the thread
393 garbage collector reaching the unreachable.
395 Objects are always destructed, even when regular refs aren't and in fact
396 are destructed in a separate pass before ordinary refs just to try to
397 prevent object destructors from using refs that have been themselves
398 destructed. Plain refs are only garbage collected if the destruct level
399 is greater than 0. You can test the higher levels of global destruction
400 by setting the PERL_DESTRUCT_LEVEL environment variable, presuming
401 C<-DDEBUGGING> was enabled during perl build time.
403 A more complete garbage collection strategy will be implemented
408 You should also check out L<perlbot> for other object tricks, traps, and tips,
409 as well as L<perlmod> for some style guides on constructing both modules