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129 | .\" ======================================================================== | |
130 | .\" | |
131 | .IX Title "PERLOBJ 1" | |
132 | .TH PERLOBJ 1 "2002-06-08" "perl v5.8.0" "Perl Programmers Reference Guide" | |
133 | .SH "NAME" | |
134 | perlobj \- Perl objects | |
135 | .SH "DESCRIPTION" | |
136 | .IX Header "DESCRIPTION" | |
137 | First you need to understand what references are in Perl. | |
138 | See perlref for that. Second, if you still find the following | |
139 | reference work too complicated, a tutorial on object-oriented programming | |
140 | in Perl can be found in perltoot and perltooc. | |
141 | .PP | |
142 | If you're still with us, then | |
143 | here are three very simple definitions that you should find reassuring. | |
144 | .IP "1." 4 | |
145 | An object is simply a reference that happens to know which class it | |
146 | belongs to. | |
147 | .IP "2." 4 | |
148 | A class is simply a package that happens to provide methods to deal | |
149 | with object references. | |
150 | .IP "3." 4 | |
151 | A method is simply a subroutine that expects an object reference (or | |
152 | a package name, for class methods) as the first argument. | |
153 | .PP | |
154 | We'll cover these points now in more depth. | |
155 | .Sh "An Object is Simply a Reference" | |
156 | .IX Subsection "An Object is Simply a Reference" | |
157 | Unlike say \*(C+, Perl doesn't provide any special syntax for | |
158 | constructors. A constructor is merely a subroutine that returns a | |
159 | reference to something \*(L"blessed\*(R" into a class, generally the | |
160 | class that the subroutine is defined in. Here is a typical | |
161 | constructor: | |
162 | .PP | |
163 | .Vb 2 | |
164 | \& package Critter; | |
165 | \& sub new { bless {} } | |
166 | .Ve | |
167 | .PP | |
168 | That word \f(CW\*(C`new\*(C'\fR isn't special. You could have written | |
169 | a construct this way, too: | |
170 | .PP | |
171 | .Vb 2 | |
172 | \& package Critter; | |
173 | \& sub spawn { bless {} } | |
174 | .Ve | |
175 | .PP | |
176 | This might even be preferable, because the \*(C+ programmers won't | |
177 | be tricked into thinking that \f(CW\*(C`new\*(C'\fR works in Perl as it does in \*(C+. | |
178 | It doesn't. We recommend that you name your constructors whatever | |
179 | makes sense in the context of the problem you're solving. For example, | |
180 | constructors in the Tk extension to Perl are named after the widgets | |
181 | they create. | |
182 | .PP | |
183 | One thing that's different about Perl constructors compared with those in | |
184 | \&\*(C+ is that in Perl, they have to allocate their own memory. (The other | |
185 | things is that they don't automatically call overridden base-class | |
186 | constructors.) The \f(CW\*(C`{}\*(C'\fR allocates an anonymous hash containing no | |
187 | key/value pairs, and returns it The \fIbless()\fR takes that reference and | |
188 | tells the object it references that it's now a Critter, and returns | |
189 | the reference. This is for convenience, because the referenced object | |
190 | itself knows that it has been blessed, and the reference to it could | |
191 | have been returned directly, like this: | |
192 | .PP | |
193 | .Vb 5 | |
194 | \& sub new { | |
195 | \& my $self = {}; | |
196 | \& bless $self; | |
197 | \& return $self; | |
198 | \& } | |
199 | .Ve | |
200 | .PP | |
201 | You often see such a thing in more complicated constructors | |
202 | that wish to call methods in the class as part of the construction: | |
203 | .PP | |
204 | .Vb 6 | |
205 | \& sub new { | |
206 | \& my $self = {}; | |
207 | \& bless $self; | |
208 | \& $self->initialize(); | |
209 | \& return $self; | |
210 | \& } | |
211 | .Ve | |
212 | .PP | |
213 | If you care about inheritance (and you should; see | |
214 | \&\*(L"Modules: Creation, Use, and Abuse\*(R" in perlmodlib), | |
215 | then you want to use the two-arg form of bless | |
216 | so that your constructors may be inherited: | |
217 | .PP | |
218 | .Vb 7 | |
219 | \& sub new { | |
220 | \& my $class = shift; | |
221 | \& my $self = {}; | |
222 | \& bless $self, $class; | |
223 | \& $self->initialize(); | |
224 | \& return $self; | |
225 | \& } | |
226 | .Ve | |
227 | .PP | |
228 | Or if you expect people to call not just \f(CW\*(C`CLASS\->new()\*(C'\fR but also | |
229 | \&\f(CW\*(C`$obj\->new()\*(C'\fR, then use something like this. The \fIinitialize()\fR | |
230 | method used will be of whatever \f(CW$class\fR we blessed the | |
231 | object into: | |
232 | .PP | |
233 | .Vb 8 | |
234 | \& sub new { | |
235 | \& my $this = shift; | |
236 | \& my $class = ref($this) || $this; | |
237 | \& my $self = {}; | |
238 | \& bless $self, $class; | |
239 | \& $self->initialize(); | |
240 | \& return $self; | |
241 | \& } | |
242 | .Ve | |
243 | .PP | |
244 | Within the class package, the methods will typically deal with the | |
245 | reference as an ordinary reference. Outside the class package, | |
246 | the reference is generally treated as an opaque value that may | |
247 | be accessed only through the class's methods. | |
248 | .PP | |
249 | Although a constructor can in theory re-bless a referenced object | |
250 | currently belonging to another class, this is almost certainly going | |
251 | to get you into trouble. The new class is responsible for all | |
252 | cleanup later. The previous blessing is forgotten, as an object | |
253 | may belong to only one class at a time. (Although of course it's | |
254 | free to inherit methods from many classes.) If you find yourself | |
255 | having to do this, the parent class is probably misbehaving, though. | |
256 | .PP | |
257 | A clarification: Perl objects are blessed. References are not. Objects | |
258 | know which package they belong to. References do not. The \fIbless()\fR | |
259 | function uses the reference to find the object. Consider | |
260 | the following example: | |
261 | .PP | |
262 | .Vb 4 | |
263 | \& $a = {}; | |
264 | \& $b = $a; | |
265 | \& bless $a, BLAH; | |
266 | \& print "\e$b is a ", ref($b), "\en"; | |
267 | .Ve | |
268 | .PP | |
269 | This reports \f(CW$b\fR as being a \s-1BLAH\s0, so obviously \fIbless()\fR | |
270 | operated on the object and not on the reference. | |
271 | .Sh "A Class is Simply a Package" | |
272 | .IX Subsection "A Class is Simply a Package" | |
273 | Unlike say \*(C+, Perl doesn't provide any special syntax for class | |
274 | definitions. You use a package as a class by putting method | |
275 | definitions into the class. | |
276 | .PP | |
277 | There is a special array within each package called \f(CW@ISA\fR, which says | |
278 | where else to look for a method if you can't find it in the current | |
279 | package. This is how Perl implements inheritance. Each element of the | |
280 | \&\f(CW@ISA\fR array is just the name of another package that happens to be a | |
281 | class package. The classes are searched (depth first) for missing | |
282 | methods in the order that they occur in \f(CW@ISA\fR. The classes accessible | |
283 | through \f(CW@ISA\fR are known as base classes of the current class. | |
284 | .PP | |
285 | All classes implicitly inherit from class \f(CW\*(C`UNIVERSAL\*(C'\fR as their | |
286 | last base class. Several commonly used methods are automatically | |
287 | supplied in the \s-1UNIVERSAL\s0 class; see \*(L"Default \s-1UNIVERSAL\s0 methods\*(R" for | |
288 | more details. | |
289 | .PP | |
290 | If a missing method is found in a base class, it is cached | |
291 | in the current class for efficiency. Changing \f(CW@ISA\fR or defining new | |
292 | subroutines invalidates the cache and causes Perl to do the lookup again. | |
293 | .PP | |
294 | If neither the current class, its named base classes, nor the \s-1UNIVERSAL\s0 | |
295 | class contains the requested method, these three places are searched | |
296 | all over again, this time looking for a method named \s-1\fIAUTOLOAD\s0()\fR. If an | |
297 | \&\s-1AUTOLOAD\s0 is found, this method is called on behalf of the missing method, | |
298 | setting the package global \f(CW$AUTOLOAD\fR to be the fully qualified name of | |
299 | the method that was intended to be called. | |
300 | .PP | |
301 | If none of that works, Perl finally gives up and complains. | |
302 | .PP | |
303 | If you want to stop the \s-1AUTOLOAD\s0 inheritance say simply | |
304 | .PP | |
305 | .Vb 1 | |
306 | \& sub AUTOLOAD; | |
307 | .Ve | |
308 | .PP | |
309 | and the call will die using the name of the sub being called. | |
310 | .PP | |
311 | Perl classes do method inheritance only. Data inheritance is left up | |
312 | to the class itself. By and large, this is not a problem in Perl, | |
313 | because most classes model the attributes of their object using an | |
314 | anonymous hash, which serves as its own little namespace to be carved up | |
315 | by the various classes that might want to do something with the object. | |
316 | The only problem with this is that you can't sure that you aren't using | |
317 | a piece of the hash that isn't already used. A reasonable workaround | |
318 | is to prepend your fieldname in the hash with the package name. | |
319 | .PP | |
320 | .Vb 4 | |
321 | \& sub bump { | |
322 | \& my $self = shift; | |
323 | \& $self->{ __PACKAGE__ . ".count"}++; | |
324 | \& } | |
325 | .Ve | |
326 | .Sh "A Method is Simply a Subroutine" | |
327 | .IX Subsection "A Method is Simply a Subroutine" | |
328 | Unlike say \*(C+, Perl doesn't provide any special syntax for method | |
329 | definition. (It does provide a little syntax for method invocation | |
330 | though. More on that later.) A method expects its first argument | |
331 | to be the object (reference) or package (string) it is being invoked | |
332 | on. There are two ways of calling methods, which we'll call class | |
333 | methods and instance methods. | |
334 | .PP | |
335 | A class method expects a class name as the first argument. It | |
336 | provides functionality for the class as a whole, not for any | |
337 | individual object belonging to the class. Constructors are often | |
338 | class methods, but see perltoot and perltooc for alternatives. | |
339 | Many class methods simply ignore their first argument, because they | |
340 | already know what package they're in and don't care what package | |
341 | they were invoked via. (These aren't necessarily the same, because | |
342 | class methods follow the inheritance tree just like ordinary instance | |
343 | methods.) Another typical use for class methods is to look up an | |
344 | object by name: | |
345 | .PP | |
346 | .Vb 4 | |
347 | \& sub find { | |
348 | \& my ($class, $name) = @_; | |
349 | \& $objtable{$name}; | |
350 | \& } | |
351 | .Ve | |
352 | .PP | |
353 | An instance method expects an object reference as its first argument. | |
354 | Typically it shifts the first argument into a \*(L"self\*(R" or \*(L"this\*(R" variable, | |
355 | and then uses that as an ordinary reference. | |
356 | .PP | |
357 | .Vb 7 | |
358 | \& sub display { | |
359 | \& my $self = shift; | |
360 | \& my @keys = @_ ? @_ : sort keys %$self; | |
361 | \& foreach $key (@keys) { | |
362 | \& print "\et$key => $self->{$key}\en"; | |
363 | \& } | |
364 | \& } | |
365 | .Ve | |
366 | .Sh "Method Invocation" | |
367 | .IX Subsection "Method Invocation" | |
368 | For various historical and other reasons, Perl offers two equivalent | |
369 | ways to write a method call. The simpler and more common way is to use | |
370 | the arrow notation: | |
371 | .PP | |
372 | .Vb 2 | |
373 | \& my $fred = Critter->find("Fred"); | |
374 | \& $fred->display("Height", "Weight"); | |
375 | .Ve | |
376 | .PP | |
377 | You should already be familiar with the use of the \f(CW\*(C`\->\*(C'\fR operator with | |
378 | references. In fact, since \f(CW$fred\fR above is a reference to an object, | |
379 | you could think of the method call as just another form of | |
380 | dereferencing. | |
381 | .PP | |
382 | Whatever is on the left side of the arrow, whether a reference or a | |
383 | class name, is passed to the method subroutine as its first argument. | |
384 | So the above code is mostly equivalent to: | |
385 | .PP | |
386 | .Vb 2 | |
387 | \& my $fred = Critter::find("Critter", "Fred"); | |
388 | \& Critter::display($fred, "Height", "Weight"); | |
389 | .Ve | |
390 | .PP | |
391 | How does Perl know which package the subroutine is in? By looking at | |
392 | the left side of the arrow, which must be either a package name or a | |
393 | reference to an object, i.e. something that has been blessed to a | |
394 | package. Either way, that's the package where Perl starts looking. If | |
395 | that package has no subroutine with that name, Perl starts looking for | |
396 | it in any base classes of that package, and so on. | |
397 | .PP | |
398 | If you need to, you \fIcan\fR force Perl to start looking in some other package: | |
399 | .PP | |
400 | .Vb 2 | |
401 | \& my $barney = MyCritter->Critter::find("Barney"); | |
402 | \& $barney->Critter::display("Height", "Weight"); | |
403 | .Ve | |
404 | .PP | |
405 | Here \f(CW\*(C`MyCritter\*(C'\fR is presumably a subclass of \f(CW\*(C`Critter\*(C'\fR that defines | |
406 | its own versions of \fIfind()\fR and \fIdisplay()\fR. We haven't specified what | |
407 | those methods do, but that doesn't matter above since we've forced Perl | |
408 | to start looking for the subroutines in \f(CW\*(C`Critter\*(C'\fR. | |
409 | .PP | |
410 | As a special case of the above, you may use the \f(CW\*(C`SUPER\*(C'\fR pseudo-class to | |
411 | tell Perl to start looking for the method in the packages named in the | |
412 | current class's \f(CW@ISA\fR list. | |
413 | .PP | |
414 | .Vb 2 | |
415 | \& package MyCritter; | |
416 | \& use base 'Critter'; # sets @MyCritter::ISA = ('Critter'); | |
417 | .Ve | |
418 | .PP | |
419 | .Vb 4 | |
420 | \& sub display { | |
421 | \& my ($self, @args) = @_; | |
422 | \& $self->SUPER::display("Name", @args); | |
423 | \& } | |
424 | .Ve | |
425 | .PP | |
426 | Instead of a class name or an object reference, you can also use any | |
427 | expression that returns either of those on the left side of the arrow. | |
428 | So the following statement is valid: | |
429 | .PP | |
430 | .Vb 1 | |
431 | \& Critter->find("Fred")->display("Height", "Weight"); | |
432 | .Ve | |
433 | .PP | |
434 | and so is the following: | |
435 | .PP | |
436 | .Vb 1 | |
437 | \& my $fred = (reverse "rettirC")->find(reverse "derF"); | |
438 | .Ve | |
439 | .Sh "Indirect Object Syntax" | |
440 | .IX Subsection "Indirect Object Syntax" | |
441 | The other way to invoke a method is by using the so-called \*(L"indirect | |
442 | object\*(R" notation. This syntax was available in Perl 4 long before | |
443 | objects were introduced, and is still used with filehandles like this: | |
444 | .PP | |
445 | .Vb 1 | |
446 | \& print STDERR "help!!!\en"; | |
447 | .Ve | |
448 | .PP | |
449 | The same syntax can be used to call either object or class methods. | |
450 | .PP | |
451 | .Vb 2 | |
452 | \& my $fred = find Critter "Fred"; | |
453 | \& display $fred "Height", "Weight"; | |
454 | .Ve | |
455 | .PP | |
456 | Notice that there is no comma between the object or class name and the | |
457 | parameters. This is how Perl can tell you want an indirect method call | |
458 | instead of an ordinary subroutine call. | |
459 | .PP | |
460 | But what if there are no arguments? In that case, Perl must guess what | |
461 | you want. Even worse, it must make that guess \fIat compile time\fR. | |
462 | Usually Perl gets it right, but when it doesn't you get a function | |
463 | call compiled as a method, or vice versa. This can introduce subtle bugs | |
464 | that are hard to detect. | |
465 | .PP | |
466 | For example, a call to a method \f(CW\*(C`new\*(C'\fR in indirect notation \*(-- as \*(C+ | |
467 | programmers are wont to make \*(-- can be miscompiled into a subroutine | |
468 | call if there's already a \f(CW\*(C`new\*(C'\fR function in scope. You'd end up | |
469 | calling the current package's \f(CW\*(C`new\*(C'\fR as a subroutine, rather than the | |
470 | desired class's method. The compiler tries to cheat by remembering | |
471 | bareword \f(CW\*(C`require\*(C'\fRs, but the grief when it messes up just isn't worth the | |
472 | years of debugging it will take you to track down such subtle bugs. | |
473 | .PP | |
474 | There is another problem with this syntax: the indirect object is | |
475 | limited to a name, a scalar variable, or a block, because it would have | |
476 | to do too much lookahead otherwise, just like any other postfix | |
477 | dereference in the language. (These are the same quirky rules as are | |
478 | used for the filehandle slot in functions like \f(CW\*(C`print\*(C'\fR and \f(CW\*(C`printf\*(C'\fR.) | |
479 | This can lead to horribly confusing precedence problems, as in these | |
480 | next two lines: | |
481 | .PP | |
482 | .Vb 2 | |
483 | \& move $obj->{FIELD}; # probably wrong! | |
484 | \& move $ary[$i]; # probably wrong! | |
485 | .Ve | |
486 | .PP | |
487 | Those actually parse as the very surprising: | |
488 | .PP | |
489 | .Vb 2 | |
490 | \& $obj->move->{FIELD}; # Well, lookee here | |
491 | \& $ary->move([$i]); # Didn't expect this one, eh? | |
492 | .Ve | |
493 | .PP | |
494 | Rather than what you might have expected: | |
495 | .PP | |
496 | .Vb 2 | |
497 | \& $obj->{FIELD}->move(); # You should be so lucky. | |
498 | \& $ary[$i]->move; # Yeah, sure. | |
499 | .Ve | |
500 | .PP | |
501 | To get the correct behavior with indirect object syntax, you would have | |
502 | to use a block around the indirect object: | |
503 | .PP | |
504 | .Vb 2 | |
505 | \& move {$obj->{FIELD}}; | |
506 | \& move {$ary[$i]}; | |
507 | .Ve | |
508 | .PP | |
509 | Even then, you still have the same potential problem if there happens to | |
510 | be a function named \f(CW\*(C`move\*(C'\fR in the current package. \fBThe \f(CB\*(C`\->\*(C'\fB | |
511 | notation suffers from neither of these disturbing ambiguities, so we | |
512 | recommend you use it exclusively.\fR However, you may still end up having | |
513 | to read code using the indirect object notation, so it's important to be | |
514 | familiar with it. | |
515 | .Sh "Default \s-1UNIVERSAL\s0 methods" | |
516 | .IX Subsection "Default UNIVERSAL methods" | |
517 | The \f(CW\*(C`UNIVERSAL\*(C'\fR package automatically contains the following methods that | |
518 | are inherited by all other classes: | |
519 | .IP "isa(\s-1CLASS\s0)" 4 | |
520 | .IX Item "isa(CLASS)" | |
521 | \&\f(CW\*(C`isa\*(C'\fR returns \fItrue\fR if its object is blessed into a subclass of \f(CW\*(C`CLASS\*(C'\fR | |
522 | .Sp | |
523 | You can also call \f(CW\*(C`UNIVERSAL::isa\*(C'\fR as a subroutine with two arguments. | |
524 | The first does not need to be an object or even a reference. This | |
525 | allows you to check what a reference points to, or whether | |
526 | something is a reference of a given type. Example | |
527 | .Sp | |
528 | .Vb 3 | |
529 | \& if(UNIVERSAL::isa($ref, 'ARRAY')) { | |
530 | \& #... | |
531 | \& } | |
532 | .Ve | |
533 | .Sp | |
534 | To determine if a reference is a blessed object, you can write | |
535 | .Sp | |
536 | .Vb 1 | |
537 | \& print "It's an object\en" if UNIVERSAL::isa($val, 'UNIVERSAL'); | |
538 | .Ve | |
539 | .IP "can(\s-1METHOD\s0)" 4 | |
540 | .IX Item "can(METHOD)" | |
541 | \&\f(CW\*(C`can\*(C'\fR checks to see if its object has a method called \f(CW\*(C`METHOD\*(C'\fR, | |
542 | if it does then a reference to the sub is returned, if it does not then | |
543 | \&\fIundef\fR is returned. | |
544 | .Sp | |
545 | \&\f(CW\*(C`UNIVERSAL::can\*(C'\fR can also be called as a subroutine with two arguments. | |
546 | It'll always return \fIundef\fR if its first argument isn't an object or a | |
547 | class name. So here's another way to check if a reference is a | |
548 | blessed object | |
549 | .Sp | |
550 | .Vb 1 | |
551 | \& print "It's still an object\en" if UNIVERSAL::can($val, 'can'); | |
552 | .Ve | |
553 | .Sp | |
554 | You can also use the \f(CW\*(C`blessed\*(C'\fR function of Scalar::Util: | |
555 | .Sp | |
556 | .Vb 1 | |
557 | \& use Scalar::Util 'blessed'; | |
558 | .Ve | |
559 | .Sp | |
560 | .Vb 1 | |
561 | \& my $blessing = blessed $suspected_object; | |
562 | .Ve | |
563 | .Sp | |
564 | \&\f(CW\*(C`blessed\*(C'\fR returns the name of the package the argument has been | |
565 | blessed into, or \f(CW\*(C`undef\*(C'\fR. | |
566 | .IP "\s-1VERSION\s0( [\s-1NEED\s0] )" 4 | |
567 | .IX Item "VERSION( [NEED] )" | |
568 | \&\f(CW\*(C`VERSION\*(C'\fR returns the version number of the class (package). If the | |
569 | \&\s-1NEED\s0 argument is given then it will check that the current version (as | |
570 | defined by the \f(CW$VERSION\fR variable in the given package) not less than | |
571 | \&\s-1NEED\s0; it will die if this is not the case. This method is normally | |
572 | called as a class method. This method is called automatically by the | |
573 | \&\f(CW\*(C`VERSION\*(C'\fR form of \f(CW\*(C`use\*(C'\fR. | |
574 | .Sp | |
575 | .Vb 3 | |
576 | \& use A 1.2 qw(some imported subs); | |
577 | \& # implies: | |
578 | \& A->VERSION(1.2); | |
579 | .Ve | |
580 | .PP | |
581 | \&\fB\s-1NOTE:\s0\fR \f(CW\*(C`can\*(C'\fR directly uses Perl's internal code for method lookup, and | |
582 | \&\f(CW\*(C`isa\*(C'\fR uses a very similar method and cache-ing strategy. This may cause | |
583 | strange effects if the Perl code dynamically changes \f(CW@ISA\fR in any package. | |
584 | .PP | |
585 | You may add other methods to the \s-1UNIVERSAL\s0 class via Perl or \s-1XS\s0 code. | |
586 | You do not need to \f(CW\*(C`use UNIVERSAL\*(C'\fR to make these methods | |
587 | available to your program (and you should not do so). | |
588 | .Sh "Destructors" | |
589 | .IX Subsection "Destructors" | |
590 | When the last reference to an object goes away, the object is | |
591 | automatically destroyed. (This may even be after you exit, if you've | |
592 | stored references in global variables.) If you want to capture control | |
593 | just before the object is freed, you may define a \s-1DESTROY\s0 method in | |
594 | your class. It will automatically be called at the appropriate moment, | |
595 | and you can do any extra cleanup you need to do. Perl passes a reference | |
596 | to the object under destruction as the first (and only) argument. Beware | |
597 | that the reference is a read-only value, and cannot be modified by | |
598 | manipulating \f(CW$_[0]\fR within the destructor. The object itself (i.e. | |
599 | the thingy the reference points to, namely \f(CW\*(C`${$_[0]}\*(C'\fR, \f(CW\*(C`@{$_[0]}\*(C'\fR, | |
600 | \&\f(CW\*(C`%{$_[0]}\*(C'\fR etc.) is not similarly constrained. | |
601 | .PP | |
602 | If you arrange to re-bless the reference before the destructor returns, | |
603 | perl will again call the \s-1DESTROY\s0 method for the re-blessed object after | |
604 | the current one returns. This can be used for clean delegation of | |
605 | object destruction, or for ensuring that destructors in the base classes | |
606 | of your choosing get called. Explicitly calling \s-1DESTROY\s0 is also possible, | |
607 | but is usually never needed. | |
608 | .PP | |
609 | Do not confuse the previous discussion with how objects \fI\s-1CONTAINED\s0\fR in the current | |
610 | one are destroyed. Such objects will be freed and destroyed automatically | |
611 | when the current object is freed, provided no other references to them exist | |
612 | elsewhere. | |
613 | .Sh "Summary" | |
614 | .IX Subsection "Summary" | |
615 | That's about all there is to it. Now you need just to go off and buy a | |
616 | book about object-oriented design methodology, and bang your forehead | |
617 | with it for the next six months or so. | |
618 | .Sh "Two-Phased Garbage Collection" | |
619 | .IX Subsection "Two-Phased Garbage Collection" | |
620 | For most purposes, Perl uses a fast and simple, reference-based | |
621 | garbage collection system. That means there's an extra | |
622 | dereference going on at some level, so if you haven't built | |
623 | your Perl executable using your C compiler's \f(CW\*(C`\-O\*(C'\fR flag, performance | |
624 | will suffer. If you \fIhave\fR built Perl with \f(CW\*(C`cc \-O\*(C'\fR, then this | |
625 | probably won't matter. | |
626 | .PP | |
627 | A more serious concern is that unreachable memory with a non-zero | |
628 | reference count will not normally get freed. Therefore, this is a bad | |
629 | idea: | |
630 | .PP | |
631 | .Vb 4 | |
632 | \& { | |
633 | \& my $a; | |
634 | \& $a = \e$a; | |
635 | \& } | |
636 | .Ve | |
637 | .PP | |
638 | Even thought \f(CW$a\fR \fIshould\fR go away, it can't. When building recursive data | |
639 | structures, you'll have to break the self-reference yourself explicitly | |
640 | if you don't care to leak. For example, here's a self-referential | |
641 | node such as one might use in a sophisticated tree structure: | |
642 | .PP | |
643 | .Vb 8 | |
644 | \& sub new_node { | |
645 | \& my $self = shift; | |
646 | \& my $class = ref($self) || $self; | |
647 | \& my $node = {}; | |
648 | \& $node->{LEFT} = $node->{RIGHT} = $node; | |
649 | \& $node->{DATA} = [ @_ ]; | |
650 | \& return bless $node => $class; | |
651 | \& } | |
652 | .Ve | |
653 | .PP | |
654 | If you create nodes like that, they (currently) won't go away unless you | |
655 | break their self reference yourself. (In other words, this is not to be | |
656 | construed as a feature, and you shouldn't depend on it.) | |
657 | .PP | |
658 | Almost. | |
659 | .PP | |
660 | When an interpreter thread finally shuts down (usually when your program | |
661 | exits), then a rather costly but complete mark-and-sweep style of garbage | |
662 | collection is performed, and everything allocated by that thread gets | |
663 | destroyed. This is essential to support Perl as an embedded or a | |
664 | multithreadable language. For example, this program demonstrates Perl's | |
665 | two-phased garbage collection: | |
666 | .PP | |
667 | .Vb 2 | |
668 | \& #!/usr/bin/perl | |
669 | \& package Subtle; | |
670 | .Ve | |
671 | .PP | |
672 | .Vb 6 | |
673 | \& sub new { | |
674 | \& my $test; | |
675 | \& $test = \e$test; | |
676 | \& warn "CREATING " . \e$test; | |
677 | \& return bless \e$test; | |
678 | \& } | |
679 | .Ve | |
680 | .PP | |
681 | .Vb 4 | |
682 | \& sub DESTROY { | |
683 | \& my $self = shift; | |
684 | \& warn "DESTROYING $self"; | |
685 | \& } | |
686 | .Ve | |
687 | .PP | |
688 | .Vb 1 | |
689 | \& package main; | |
690 | .Ve | |
691 | .PP | |
692 | .Vb 7 | |
693 | \& warn "starting program"; | |
694 | \& { | |
695 | \& my $a = Subtle->new; | |
696 | \& my $b = Subtle->new; | |
697 | \& $$a = 0; # break selfref | |
698 | \& warn "leaving block"; | |
699 | \& } | |
700 | .Ve | |
701 | .PP | |
702 | .Vb 3 | |
703 | \& warn "just exited block"; | |
704 | \& warn "time to die..."; | |
705 | \& exit; | |
706 | .Ve | |
707 | .PP | |
708 | When run as \fI/tmp/test\fR, the following output is produced: | |
709 | .PP | |
710 | .Vb 8 | |
711 | \& starting program at /tmp/test line 18. | |
712 | \& CREATING SCALAR(0x8e5b8) at /tmp/test line 7. | |
713 | \& CREATING SCALAR(0x8e57c) at /tmp/test line 7. | |
714 | \& leaving block at /tmp/test line 23. | |
715 | \& DESTROYING Subtle=SCALAR(0x8e5b8) at /tmp/test line 13. | |
716 | \& just exited block at /tmp/test line 26. | |
717 | \& time to die... at /tmp/test line 27. | |
718 | \& DESTROYING Subtle=SCALAR(0x8e57c) during global destruction. | |
719 | .Ve | |
720 | .PP | |
721 | Notice that \*(L"global destruction\*(R" bit there? That's the thread | |
722 | garbage collector reaching the unreachable. | |
723 | .PP | |
724 | Objects are always destructed, even when regular refs aren't. Objects | |
725 | are destructed in a separate pass before ordinary refs just to | |
726 | prevent object destructors from using refs that have been themselves | |
727 | destructed. Plain refs are only garbage-collected if the destruct level | |
728 | is greater than 0. You can test the higher levels of global destruction | |
729 | by setting the \s-1PERL_DESTRUCT_LEVEL\s0 environment variable, presuming | |
730 | \&\f(CW\*(C`\-DDEBUGGING\*(C'\fR was enabled during perl build time. | |
731 | See \*(L"\s-1PERL_DESTRUCT_LEVEL\s0\*(R" in perlhack for more information. | |
732 | .PP | |
733 | A more complete garbage collection strategy will be implemented | |
734 | at a future date. | |
735 | .PP | |
736 | In the meantime, the best solution is to create a non-recursive container | |
737 | class that holds a pointer to the self-referential data structure. | |
738 | Define a \s-1DESTROY\s0 method for the containing object's class that manually | |
739 | breaks the circularities in the self-referential structure. | |
740 | .SH "SEE ALSO" | |
741 | .IX Header "SEE ALSO" | |
742 | A kinder, gentler tutorial on object-oriented programming in Perl can | |
743 | be found in perltoot, perlboot and perltooc. You should | |
744 | also check out perlbot for other object tricks, traps, and tips, as | |
745 | well as perlmodlib for some style guides on constructing both | |
746 | modules and classes. |