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129 | .\" ======================================================================== | |
130 | .\" | |
131 | .IX Title "PERLBOT 1" | |
132 | .TH PERLBOT 1 "2006-01-07" "perl v5.8.8" "Perl Programmers Reference Guide" | |
133 | .SH "NAME" | |
134 | perlbot \- Bag'o Object Tricks (the BOT) | |
135 | .SH "DESCRIPTION" | |
136 | .IX Header "DESCRIPTION" | |
137 | The following collection of tricks and hints is intended to whet curious | |
138 | appetites about such things as the use of instance variables and the | |
139 | mechanics of object and class relationships. The reader is encouraged to | |
140 | consult relevant textbooks for discussion of Object Oriented definitions and | |
141 | methodology. This is not intended as a tutorial for object-oriented | |
142 | programming or as a comprehensive guide to Perl's object oriented features, | |
143 | nor should it be construed as a style guide. If you're looking for tutorials, | |
144 | be sure to read perlboot, perltoot, and perltooc. | |
145 | .PP | |
146 | The Perl motto still holds: There's more than one way to do it. | |
147 | .SH "OO SCALING TIPS" | |
148 | .IX Header "OO SCALING TIPS" | |
149 | .IP "1" 5 | |
150 | .IX Item "1" | |
151 | Do not attempt to verify the type of \f(CW$self\fR. That'll break if the class is | |
152 | inherited, when the type of \f(CW$self\fR is valid but its package isn't what you | |
153 | expect. See rule 5. | |
154 | .IP "2" 5 | |
155 | .IX Item "2" | |
156 | If an object-oriented (\s-1OO\s0) or indirect-object (\s-1IO\s0) syntax was used, then the | |
157 | object is probably the correct type and there's no need to become paranoid | |
158 | about it. Perl isn't a paranoid language anyway. If people subvert the \s-1OO\s0 | |
159 | or \s-1IO\s0 syntax then they probably know what they're doing and you should let | |
160 | them do it. See rule 1. | |
161 | .IP "3" 5 | |
162 | .IX Item "3" | |
163 | Use the two-argument form of \fIbless()\fR. Let a subclass use your constructor. | |
164 | See \*(L"\s-1INHERITING\s0 A \s-1CONSTRUCTOR\s0\*(R". | |
165 | .IP "4" 5 | |
166 | .IX Item "4" | |
167 | The subclass is allowed to know things about its immediate superclass, the | |
168 | superclass is allowed to know nothing about a subclass. | |
169 | .IP "5" 5 | |
170 | .IX Item "5" | |
171 | Don't be trigger happy with inheritance. A \*(L"using\*(R", \*(L"containing\*(R", or | |
172 | \&\*(L"delegation\*(R" relationship (some sort of aggregation, at least) is often more | |
173 | appropriate. See \*(L"\s-1OBJECT\s0 \s-1RELATIONSHIPS\s0\*(R", \*(L"\s-1USING\s0 \s-1RELATIONSHIP\s0 \s-1WITH\s0 \s-1SDBM\s0\*(R", | |
174 | and \*(L"\s-1DELEGATION\s0\*(R". | |
175 | .IP "6" 5 | |
176 | .IX Item "6" | |
177 | The object is the namespace. Make package globals accessible via the | |
178 | object. This will remove the guess work about the symbol's home package. | |
179 | See \*(L"\s-1CLASS\s0 \s-1CONTEXT\s0 \s-1AND\s0 \s-1THE\s0 \s-1OBJECT\s0\*(R". | |
180 | .IP "7" 5 | |
181 | .IX Item "7" | |
182 | \&\s-1IO\s0 syntax is certainly less noisy, but it is also prone to ambiguities that | |
183 | can cause difficult-to-find bugs. Allow people to use the sure-thing \s-1OO\s0 | |
184 | syntax, even if you don't like it. | |
185 | .IP "8" 5 | |
186 | .IX Item "8" | |
187 | Do not use function-call syntax on a method. You're going to be bitten | |
188 | someday. Someone might move that method into a superclass and your code | |
189 | will be broken. On top of that you're feeding the paranoia in rule 2. | |
190 | .IP "9" 5 | |
191 | .IX Item "9" | |
192 | Don't assume you know the home package of a method. You're making it | |
193 | difficult for someone to override that method. See \*(L"\s-1THINKING\s0 \s-1OF\s0 \s-1CODE\s0 \s-1REUSE\s0\*(R". | |
194 | .SH "INSTANCE VARIABLES" | |
195 | .IX Header "INSTANCE VARIABLES" | |
196 | An anonymous array or anonymous hash can be used to hold instance | |
197 | variables. Named parameters are also demonstrated. | |
198 | .PP | |
199 | .Vb 1 | |
200 | \& package Foo; | |
201 | .Ve | |
202 | .PP | |
203 | .Vb 8 | |
204 | \& sub new { | |
205 | \& my $type = shift; | |
206 | \& my %params = @_; | |
207 | \& my $self = {}; | |
208 | \& $self->{'High'} = $params{'High'}; | |
209 | \& $self->{'Low'} = $params{'Low'}; | |
210 | \& bless $self, $type; | |
211 | \& } | |
212 | .Ve | |
213 | .PP | |
214 | .Vb 1 | |
215 | \& package Bar; | |
216 | .Ve | |
217 | .PP | |
218 | .Vb 8 | |
219 | \& sub new { | |
220 | \& my $type = shift; | |
221 | \& my %params = @_; | |
222 | \& my $self = []; | |
223 | \& $self->[0] = $params{'Left'}; | |
224 | \& $self->[1] = $params{'Right'}; | |
225 | \& bless $self, $type; | |
226 | \& } | |
227 | .Ve | |
228 | .PP | |
229 | .Vb 1 | |
230 | \& package main; | |
231 | .Ve | |
232 | .PP | |
233 | .Vb 3 | |
234 | \& $a = Foo->new( 'High' => 42, 'Low' => 11 ); | |
235 | \& print "High=$a->{'High'}\en"; | |
236 | \& print "Low=$a->{'Low'}\en"; | |
237 | .Ve | |
238 | .PP | |
239 | .Vb 3 | |
240 | \& $b = Bar->new( 'Left' => 78, 'Right' => 40 ); | |
241 | \& print "Left=$b->[0]\en"; | |
242 | \& print "Right=$b->[1]\en"; | |
243 | .Ve | |
244 | .SH "SCALAR INSTANCE VARIABLES" | |
245 | .IX Header "SCALAR INSTANCE VARIABLES" | |
246 | An anonymous scalar can be used when only one instance variable is needed. | |
247 | .PP | |
248 | .Vb 1 | |
249 | \& package Foo; | |
250 | .Ve | |
251 | .PP | |
252 | .Vb 6 | |
253 | \& sub new { | |
254 | \& my $type = shift; | |
255 | \& my $self; | |
256 | \& $self = shift; | |
257 | \& bless \e$self, $type; | |
258 | \& } | |
259 | .Ve | |
260 | .PP | |
261 | .Vb 1 | |
262 | \& package main; | |
263 | .Ve | |
264 | .PP | |
265 | .Vb 2 | |
266 | \& $a = Foo->new( 42 ); | |
267 | \& print "a=$$a\en"; | |
268 | .Ve | |
269 | .SH "INSTANCE VARIABLE INHERITANCE" | |
270 | .IX Header "INSTANCE VARIABLE INHERITANCE" | |
271 | This example demonstrates how one might inherit instance variables from a | |
272 | superclass for inclusion in the new class. This requires calling the | |
273 | superclass's constructor and adding one's own instance variables to the new | |
274 | object. | |
275 | .PP | |
276 | .Vb 1 | |
277 | \& package Bar; | |
278 | .Ve | |
279 | .PP | |
280 | .Vb 6 | |
281 | \& sub new { | |
282 | \& my $type = shift; | |
283 | \& my $self = {}; | |
284 | \& $self->{'buz'} = 42; | |
285 | \& bless $self, $type; | |
286 | \& } | |
287 | .Ve | |
288 | .PP | |
289 | .Vb 2 | |
290 | \& package Foo; | |
291 | \& @ISA = qw( Bar ); | |
292 | .Ve | |
293 | .PP | |
294 | .Vb 6 | |
295 | \& sub new { | |
296 | \& my $type = shift; | |
297 | \& my $self = Bar->new; | |
298 | \& $self->{'biz'} = 11; | |
299 | \& bless $self, $type; | |
300 | \& } | |
301 | .Ve | |
302 | .PP | |
303 | .Vb 1 | |
304 | \& package main; | |
305 | .Ve | |
306 | .PP | |
307 | .Vb 3 | |
308 | \& $a = Foo->new; | |
309 | \& print "buz = ", $a->{'buz'}, "\en"; | |
310 | \& print "biz = ", $a->{'biz'}, "\en"; | |
311 | .Ve | |
312 | .SH "OBJECT RELATIONSHIPS" | |
313 | .IX Header "OBJECT RELATIONSHIPS" | |
314 | The following demonstrates how one might implement \*(L"containing\*(R" and \*(L"using\*(R" | |
315 | relationships between objects. | |
316 | .PP | |
317 | .Vb 1 | |
318 | \& package Bar; | |
319 | .Ve | |
320 | .PP | |
321 | .Vb 6 | |
322 | \& sub new { | |
323 | \& my $type = shift; | |
324 | \& my $self = {}; | |
325 | \& $self->{'buz'} = 42; | |
326 | \& bless $self, $type; | |
327 | \& } | |
328 | .Ve | |
329 | .PP | |
330 | .Vb 1 | |
331 | \& package Foo; | |
332 | .Ve | |
333 | .PP | |
334 | .Vb 7 | |
335 | \& sub new { | |
336 | \& my $type = shift; | |
337 | \& my $self = {}; | |
338 | \& $self->{'Bar'} = Bar->new; | |
339 | \& $self->{'biz'} = 11; | |
340 | \& bless $self, $type; | |
341 | \& } | |
342 | .Ve | |
343 | .PP | |
344 | .Vb 1 | |
345 | \& package main; | |
346 | .Ve | |
347 | .PP | |
348 | .Vb 3 | |
349 | \& $a = Foo->new; | |
350 | \& print "buz = ", $a->{'Bar'}->{'buz'}, "\en"; | |
351 | \& print "biz = ", $a->{'biz'}, "\en"; | |
352 | .Ve | |
353 | .SH "OVERRIDING SUPERCLASS METHODS" | |
354 | .IX Header "OVERRIDING SUPERCLASS METHODS" | |
355 | The following example demonstrates how to override a superclass method and | |
356 | then call the overridden method. The \fB\s-1SUPER\s0\fR pseudo-class allows the | |
357 | programmer to call an overridden superclass method without actually knowing | |
358 | where that method is defined. | |
359 | .PP | |
360 | .Vb 2 | |
361 | \& package Buz; | |
362 | \& sub goo { print "here's the goo\en" } | |
363 | .Ve | |
364 | .PP | |
365 | .Vb 2 | |
366 | \& package Bar; @ISA = qw( Buz ); | |
367 | \& sub google { print "google here\en" } | |
368 | .Ve | |
369 | .PP | |
370 | .Vb 2 | |
371 | \& package Baz; | |
372 | \& sub mumble { print "mumbling\en" } | |
373 | .Ve | |
374 | .PP | |
375 | .Vb 2 | |
376 | \& package Foo; | |
377 | \& @ISA = qw( Bar Baz ); | |
378 | .Ve | |
379 | .PP | |
380 | .Vb 17 | |
381 | \& sub new { | |
382 | \& my $type = shift; | |
383 | \& bless [], $type; | |
384 | \& } | |
385 | \& sub grr { print "grumble\en" } | |
386 | \& sub goo { | |
387 | \& my $self = shift; | |
388 | \& $self->SUPER::goo(); | |
389 | \& } | |
390 | \& sub mumble { | |
391 | \& my $self = shift; | |
392 | \& $self->SUPER::mumble(); | |
393 | \& } | |
394 | \& sub google { | |
395 | \& my $self = shift; | |
396 | \& $self->SUPER::google(); | |
397 | \& } | |
398 | .Ve | |
399 | .PP | |
400 | .Vb 1 | |
401 | \& package main; | |
402 | .Ve | |
403 | .PP | |
404 | .Vb 5 | |
405 | \& $foo = Foo->new; | |
406 | \& $foo->mumble; | |
407 | \& $foo->grr; | |
408 | \& $foo->goo; | |
409 | \& $foo->google; | |
410 | .Ve | |
411 | .PP | |
412 | Note that \f(CW\*(C`SUPER\*(C'\fR refers to the superclasses of the current package | |
413 | (\f(CW\*(C`Foo\*(C'\fR), not to the superclasses of \f(CW$self\fR. | |
414 | .SH "USING RELATIONSHIP WITH SDBM" | |
415 | .IX Header "USING RELATIONSHIP WITH SDBM" | |
416 | This example demonstrates an interface for the \s-1SDBM\s0 class. This creates a | |
417 | \&\*(L"using\*(R" relationship between the \s-1SDBM\s0 class and the new class Mydbm. | |
418 | .PP | |
419 | .Vb 1 | |
420 | \& package Mydbm; | |
421 | .Ve | |
422 | .PP | |
423 | .Vb 3 | |
424 | \& require SDBM_File; | |
425 | \& require Tie::Hash; | |
426 | \& @ISA = qw( Tie::Hash ); | |
427 | .Ve | |
428 | .PP | |
429 | .Vb 19 | |
430 | \& sub TIEHASH { | |
431 | \& my $type = shift; | |
432 | \& my $ref = SDBM_File->new(@_); | |
433 | \& bless {'dbm' => $ref}, $type; | |
434 | \& } | |
435 | \& sub FETCH { | |
436 | \& my $self = shift; | |
437 | \& my $ref = $self->{'dbm'}; | |
438 | \& $ref->FETCH(@_); | |
439 | \& } | |
440 | \& sub STORE { | |
441 | \& my $self = shift; | |
442 | \& if (defined $_[0]){ | |
443 | \& my $ref = $self->{'dbm'}; | |
444 | \& $ref->STORE(@_); | |
445 | \& } else { | |
446 | \& die "Cannot STORE an undefined key in Mydbm\en"; | |
447 | \& } | |
448 | \& } | |
449 | .Ve | |
450 | .PP | |
451 | .Vb 2 | |
452 | \& package main; | |
453 | \& use Fcntl qw( O_RDWR O_CREAT ); | |
454 | .Ve | |
455 | .PP | |
456 | .Vb 3 | |
457 | \& tie %foo, "Mydbm", "Sdbm", O_RDWR|O_CREAT, 0640; | |
458 | \& $foo{'bar'} = 123; | |
459 | \& print "foo-bar = $foo{'bar'}\en"; | |
460 | .Ve | |
461 | .PP | |
462 | .Vb 3 | |
463 | \& tie %bar, "Mydbm", "Sdbm2", O_RDWR|O_CREAT, 0640; | |
464 | \& $bar{'Cathy'} = 456; | |
465 | \& print "bar-Cathy = $bar{'Cathy'}\en"; | |
466 | .Ve | |
467 | .SH "THINKING OF CODE REUSE" | |
468 | .IX Header "THINKING OF CODE REUSE" | |
469 | One strength of Object-Oriented languages is the ease with which old code | |
470 | can use new code. The following examples will demonstrate first how one can | |
471 | hinder code reuse and then how one can promote code reuse. | |
472 | .PP | |
473 | This first example illustrates a class which uses a fully-qualified method | |
474 | call to access the \*(L"private\*(R" method \s-1\fIBAZ\s0()\fR. The second example will show | |
475 | that it is impossible to override the \s-1\fIBAZ\s0()\fR method. | |
476 | .PP | |
477 | .Vb 1 | |
478 | \& package FOO; | |
479 | .Ve | |
480 | .PP | |
481 | .Vb 8 | |
482 | \& sub new { | |
483 | \& my $type = shift; | |
484 | \& bless {}, $type; | |
485 | \& } | |
486 | \& sub bar { | |
487 | \& my $self = shift; | |
488 | \& $self->FOO::private::BAZ; | |
489 | \& } | |
490 | .Ve | |
491 | .PP | |
492 | .Vb 1 | |
493 | \& package FOO::private; | |
494 | .Ve | |
495 | .PP | |
496 | .Vb 3 | |
497 | \& sub BAZ { | |
498 | \& print "in BAZ\en"; | |
499 | \& } | |
500 | .Ve | |
501 | .PP | |
502 | .Vb 1 | |
503 | \& package main; | |
504 | .Ve | |
505 | .PP | |
506 | .Vb 2 | |
507 | \& $a = FOO->new; | |
508 | \& $a->bar; | |
509 | .Ve | |
510 | .PP | |
511 | Now we try to override the \s-1\fIBAZ\s0()\fR method. We would like \fIFOO::bar()\fR to call | |
512 | \&\s-1\fIGOOP::BAZ\s0()\fR, but this cannot happen because \fIFOO::bar()\fR explicitly calls | |
513 | \&\fIFOO::private::BAZ()\fR. | |
514 | .PP | |
515 | .Vb 1 | |
516 | \& package FOO; | |
517 | .Ve | |
518 | .PP | |
519 | .Vb 8 | |
520 | \& sub new { | |
521 | \& my $type = shift; | |
522 | \& bless {}, $type; | |
523 | \& } | |
524 | \& sub bar { | |
525 | \& my $self = shift; | |
526 | \& $self->FOO::private::BAZ; | |
527 | \& } | |
528 | .Ve | |
529 | .PP | |
530 | .Vb 1 | |
531 | \& package FOO::private; | |
532 | .Ve | |
533 | .PP | |
534 | .Vb 3 | |
535 | \& sub BAZ { | |
536 | \& print "in BAZ\en"; | |
537 | \& } | |
538 | .Ve | |
539 | .PP | |
540 | .Vb 6 | |
541 | \& package GOOP; | |
542 | \& @ISA = qw( FOO ); | |
543 | \& sub new { | |
544 | \& my $type = shift; | |
545 | \& bless {}, $type; | |
546 | \& } | |
547 | .Ve | |
548 | .PP | |
549 | .Vb 3 | |
550 | \& sub BAZ { | |
551 | \& print "in GOOP::BAZ\en"; | |
552 | \& } | |
553 | .Ve | |
554 | .PP | |
555 | .Vb 1 | |
556 | \& package main; | |
557 | .Ve | |
558 | .PP | |
559 | .Vb 2 | |
560 | \& $a = GOOP->new; | |
561 | \& $a->bar; | |
562 | .Ve | |
563 | .PP | |
564 | To create reusable code we must modify class \s-1FOO\s0, flattening class | |
565 | FOO::private. The next example shows a reusable class \s-1FOO\s0 which allows the | |
566 | method \s-1\fIGOOP::BAZ\s0()\fR to be used in place of \s-1\fIFOO::BAZ\s0()\fR. | |
567 | .PP | |
568 | .Vb 1 | |
569 | \& package FOO; | |
570 | .Ve | |
571 | .PP | |
572 | .Vb 8 | |
573 | \& sub new { | |
574 | \& my $type = shift; | |
575 | \& bless {}, $type; | |
576 | \& } | |
577 | \& sub bar { | |
578 | \& my $self = shift; | |
579 | \& $self->BAZ; | |
580 | \& } | |
581 | .Ve | |
582 | .PP | |
583 | .Vb 3 | |
584 | \& sub BAZ { | |
585 | \& print "in BAZ\en"; | |
586 | \& } | |
587 | .Ve | |
588 | .PP | |
589 | .Vb 2 | |
590 | \& package GOOP; | |
591 | \& @ISA = qw( FOO ); | |
592 | .Ve | |
593 | .PP | |
594 | .Vb 7 | |
595 | \& sub new { | |
596 | \& my $type = shift; | |
597 | \& bless {}, $type; | |
598 | \& } | |
599 | \& sub BAZ { | |
600 | \& print "in GOOP::BAZ\en"; | |
601 | \& } | |
602 | .Ve | |
603 | .PP | |
604 | .Vb 1 | |
605 | \& package main; | |
606 | .Ve | |
607 | .PP | |
608 | .Vb 2 | |
609 | \& $a = GOOP->new; | |
610 | \& $a->bar; | |
611 | .Ve | |
612 | .SH "CLASS CONTEXT AND THE OBJECT" | |
613 | .IX Header "CLASS CONTEXT AND THE OBJECT" | |
614 | Use the object to solve package and class context problems. Everything a | |
615 | method needs should be available via the object or should be passed as a | |
616 | parameter to the method. | |
617 | .PP | |
618 | A class will sometimes have static or global data to be used by the | |
619 | methods. A subclass may want to override that data and replace it with new | |
620 | data. When this happens the superclass may not know how to find the new | |
621 | copy of the data. | |
622 | .PP | |
623 | This problem can be solved by using the object to define the context of the | |
624 | method. Let the method look in the object for a reference to the data. The | |
625 | alternative is to force the method to go hunting for the data (\*(L"Is it in my | |
626 | class, or in a subclass? Which subclass?\*(R"), and this can be inconvenient | |
627 | and will lead to hackery. It is better just to let the object tell the | |
628 | method where that data is located. | |
629 | .PP | |
630 | .Vb 1 | |
631 | \& package Bar; | |
632 | .Ve | |
633 | .PP | |
634 | .Vb 1 | |
635 | \& %fizzle = ( 'Password' => 'XYZZY' ); | |
636 | .Ve | |
637 | .PP | |
638 | .Vb 6 | |
639 | \& sub new { | |
640 | \& my $type = shift; | |
641 | \& my $self = {}; | |
642 | \& $self->{'fizzle'} = \e%fizzle; | |
643 | \& bless $self, $type; | |
644 | \& } | |
645 | .Ve | |
646 | .PP | |
647 | .Vb 2 | |
648 | \& sub enter { | |
649 | \& my $self = shift; | |
650 | .Ve | |
651 | .PP | |
652 | .Vb 5 | |
653 | \& # Don't try to guess if we should use %Bar::fizzle | |
654 | \& # or %Foo::fizzle. The object already knows which | |
655 | \& # we should use, so just ask it. | |
656 | \& # | |
657 | \& my $fizzle = $self->{'fizzle'}; | |
658 | .Ve | |
659 | .PP | |
660 | .Vb 2 | |
661 | \& print "The word is ", $fizzle->{'Password'}, "\en"; | |
662 | \& } | |
663 | .Ve | |
664 | .PP | |
665 | .Vb 2 | |
666 | \& package Foo; | |
667 | \& @ISA = qw( Bar ); | |
668 | .Ve | |
669 | .PP | |
670 | .Vb 1 | |
671 | \& %fizzle = ( 'Password' => 'Rumple' ); | |
672 | .Ve | |
673 | .PP | |
674 | .Vb 6 | |
675 | \& sub new { | |
676 | \& my $type = shift; | |
677 | \& my $self = Bar->new; | |
678 | \& $self->{'fizzle'} = \e%fizzle; | |
679 | \& bless $self, $type; | |
680 | \& } | |
681 | .Ve | |
682 | .PP | |
683 | .Vb 1 | |
684 | \& package main; | |
685 | .Ve | |
686 | .PP | |
687 | .Vb 4 | |
688 | \& $a = Bar->new; | |
689 | \& $b = Foo->new; | |
690 | \& $a->enter; | |
691 | \& $b->enter; | |
692 | .Ve | |
693 | .SH "INHERITING A CONSTRUCTOR" | |
694 | .IX Header "INHERITING A CONSTRUCTOR" | |
695 | An inheritable constructor should use the second form of \fIbless()\fR which allows | |
696 | blessing directly into a specified class. Notice in this example that the | |
697 | object will be a \s-1BAR\s0 not a \s-1FOO\s0, even though the constructor is in class \s-1FOO\s0. | |
698 | .PP | |
699 | .Vb 1 | |
700 | \& package FOO; | |
701 | .Ve | |
702 | .PP | |
703 | .Vb 5 | |
704 | \& sub new { | |
705 | \& my $type = shift; | |
706 | \& my $self = {}; | |
707 | \& bless $self, $type; | |
708 | \& } | |
709 | .Ve | |
710 | .PP | |
711 | .Vb 3 | |
712 | \& sub baz { | |
713 | \& print "in FOO::baz()\en"; | |
714 | \& } | |
715 | .Ve | |
716 | .PP | |
717 | .Vb 2 | |
718 | \& package BAR; | |
719 | \& @ISA = qw(FOO); | |
720 | .Ve | |
721 | .PP | |
722 | .Vb 3 | |
723 | \& sub baz { | |
724 | \& print "in BAR::baz()\en"; | |
725 | \& } | |
726 | .Ve | |
727 | .PP | |
728 | .Vb 1 | |
729 | \& package main; | |
730 | .Ve | |
731 | .PP | |
732 | .Vb 2 | |
733 | \& $a = BAR->new; | |
734 | \& $a->baz; | |
735 | .Ve | |
736 | .SH "DELEGATION" | |
737 | .IX Header "DELEGATION" | |
738 | Some classes, such as SDBM_File, cannot be effectively subclassed because | |
739 | they create foreign objects. Such a class can be extended with some sort of | |
740 | aggregation technique such as the \*(L"using\*(R" relationship mentioned earlier or | |
741 | by delegation. | |
742 | .PP | |
743 | The following example demonstrates delegation using an \s-1\fIAUTOLOAD\s0()\fR function to | |
744 | perform message\-forwarding. This will allow the Mydbm object to behave | |
745 | exactly like an SDBM_File object. The Mydbm class could now extend the | |
746 | behavior by adding custom \s-1\fIFETCH\s0()\fR and \s-1\fISTORE\s0()\fR methods, if this is desired. | |
747 | .PP | |
748 | .Vb 1 | |
749 | \& package Mydbm; | |
750 | .Ve | |
751 | .PP | |
752 | .Vb 3 | |
753 | \& require SDBM_File; | |
754 | \& require Tie::Hash; | |
755 | \& @ISA = qw(Tie::Hash); | |
756 | .Ve | |
757 | .PP | |
758 | .Vb 5 | |
759 | \& sub TIEHASH { | |
760 | \& my $type = shift; | |
761 | \& my $ref = SDBM_File->new(@_); | |
762 | \& bless {'delegate' => $ref}; | |
763 | \& } | |
764 | .Ve | |
765 | .PP | |
766 | .Vb 2 | |
767 | \& sub AUTOLOAD { | |
768 | \& my $self = shift; | |
769 | .Ve | |
770 | .PP | |
771 | .Vb 2 | |
772 | \& # The Perl interpreter places the name of the | |
773 | \& # message in a variable called $AUTOLOAD. | |
774 | .Ve | |
775 | .PP | |
776 | .Vb 2 | |
777 | \& # DESTROY messages should never be propagated. | |
778 | \& return if $AUTOLOAD =~ /::DESTROY$/; | |
779 | .Ve | |
780 | .PP | |
781 | .Vb 2 | |
782 | \& # Remove the package name. | |
783 | \& $AUTOLOAD =~ s/^Mydbm:://; | |
784 | .Ve | |
785 | .PP | |
786 | .Vb 3 | |
787 | \& # Pass the message to the delegate. | |
788 | \& $self->{'delegate'}->$AUTOLOAD(@_); | |
789 | \& } | |
790 | .Ve | |
791 | .PP | |
792 | .Vb 2 | |
793 | \& package main; | |
794 | \& use Fcntl qw( O_RDWR O_CREAT ); | |
795 | .Ve | |
796 | .PP | |
797 | .Vb 3 | |
798 | \& tie %foo, "Mydbm", "adbm", O_RDWR|O_CREAT, 0640; | |
799 | \& $foo{'bar'} = 123; | |
800 | \& print "foo-bar = $foo{'bar'}\en"; | |
801 | .Ve | |
802 | .SH "SEE ALSO" | |
803 | .IX Header "SEE ALSO" | |
804 | perlboot, perltoot, perltooc. |