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1 | =head1 NAME |
2 | ||
3 | perlbot - Bag'o Object Tricks (the BOT) | |
4 | ||
5 | =head1 DESCRIPTION | |
6 | ||
7 | The following collection of tricks and hints is intended to whet curious | |
8 | appetites about such things as the use of instance variables and the | |
9 | mechanics of object and class relationships. The reader is encouraged to | |
10 | consult relevant textbooks for discussion of Object Oriented definitions and | |
11 | methodology. This is not intended as a tutorial for object-oriented | |
12 | programming or as a comprehensive guide to Perl's object oriented features, | |
13 | nor should it be construed as a style guide. If you're looking for tutorials, | |
14 | be sure to read L<perlboot>, L<perltoot>, and L<perltooc>. | |
15 | ||
16 | The Perl motto still holds: There's more than one way to do it. | |
17 | ||
18 | =head1 OO SCALING TIPS | |
19 | ||
20 | =over 5 | |
21 | ||
22 | =item 1 | |
23 | ||
24 | Do not attempt to verify the type of $self. That'll break if the class is | |
25 | inherited, when the type of $self is valid but its package isn't what you | |
26 | expect. See rule 5. | |
27 | ||
28 | =item 2 | |
29 | ||
30 | If an object-oriented (OO) or indirect-object (IO) syntax was used, then the | |
31 | object is probably the correct type and there's no need to become paranoid | |
32 | about it. Perl isn't a paranoid language anyway. If people subvert the OO | |
33 | or IO syntax then they probably know what they're doing and you should let | |
34 | them do it. See rule 1. | |
35 | ||
36 | =item 3 | |
37 | ||
38 | Use the two-argument form of bless(). Let a subclass use your constructor. | |
39 | See L<INHERITING A CONSTRUCTOR>. | |
40 | ||
41 | =item 4 | |
42 | ||
43 | The subclass is allowed to know things about its immediate superclass, the | |
44 | superclass is allowed to know nothing about a subclass. | |
45 | ||
46 | =item 5 | |
47 | ||
48 | Don't be trigger happy with inheritance. A "using", "containing", or | |
49 | "delegation" relationship (some sort of aggregation, at least) is often more | |
50 | appropriate. See L<OBJECT RELATIONSHIPS>, L<USING RELATIONSHIP WITH SDBM>, | |
51 | and L<"DELEGATION">. | |
52 | ||
53 | =item 6 | |
54 | ||
55 | The object is the namespace. Make package globals accessible via the | |
56 | object. This will remove the guess work about the symbol's home package. | |
57 | See L<CLASS CONTEXT AND THE OBJECT>. | |
58 | ||
59 | =item 7 | |
60 | ||
61 | IO syntax is certainly less noisy, but it is also prone to ambiguities that | |
62 | can cause difficult-to-find bugs. Allow people to use the sure-thing OO | |
63 | syntax, even if you don't like it. | |
64 | ||
65 | =item 8 | |
66 | ||
67 | Do not use function-call syntax on a method. You're going to be bitten | |
68 | someday. Someone might move that method into a superclass and your code | |
69 | will be broken. On top of that you're feeding the paranoia in rule 2. | |
70 | ||
71 | =item 9 | |
72 | ||
73 | Don't assume you know the home package of a method. You're making it | |
74 | difficult for someone to override that method. See L<THINKING OF CODE REUSE>. | |
75 | ||
76 | =back | |
77 | ||
78 | =head1 INSTANCE VARIABLES | |
79 | ||
80 | An anonymous array or anonymous hash can be used to hold instance | |
81 | variables. Named parameters are also demonstrated. | |
82 | ||
83 | package Foo; | |
84 | ||
85 | sub new { | |
86 | my $type = shift; | |
87 | my %params = @_; | |
88 | my $self = {}; | |
89 | $self->{'High'} = $params{'High'}; | |
90 | $self->{'Low'} = $params{'Low'}; | |
91 | bless $self, $type; | |
92 | } | |
93 | ||
94 | ||
95 | package Bar; | |
96 | ||
97 | sub new { | |
98 | my $type = shift; | |
99 | my %params = @_; | |
100 | my $self = []; | |
101 | $self->[0] = $params{'Left'}; | |
102 | $self->[1] = $params{'Right'}; | |
103 | bless $self, $type; | |
104 | } | |
105 | ||
106 | package main; | |
107 | ||
108 | $a = Foo->new( 'High' => 42, 'Low' => 11 ); | |
109 | print "High=$a->{'High'}\n"; | |
110 | print "Low=$a->{'Low'}\n"; | |
111 | ||
112 | $b = Bar->new( 'Left' => 78, 'Right' => 40 ); | |
113 | print "Left=$b->[0]\n"; | |
114 | print "Right=$b->[1]\n"; | |
115 | ||
116 | =head1 SCALAR INSTANCE VARIABLES | |
117 | ||
118 | An anonymous scalar can be used when only one instance variable is needed. | |
119 | ||
120 | package Foo; | |
121 | ||
122 | sub new { | |
123 | my $type = shift; | |
124 | my $self; | |
125 | $self = shift; | |
126 | bless \$self, $type; | |
127 | } | |
128 | ||
129 | package main; | |
130 | ||
131 | $a = Foo->new( 42 ); | |
132 | print "a=$$a\n"; | |
133 | ||
134 | ||
135 | =head1 INSTANCE VARIABLE INHERITANCE | |
136 | ||
137 | This example demonstrates how one might inherit instance variables from a | |
138 | superclass for inclusion in the new class. This requires calling the | |
139 | superclass's constructor and adding one's own instance variables to the new | |
140 | object. | |
141 | ||
142 | package Bar; | |
143 | ||
144 | sub new { | |
145 | my $type = shift; | |
146 | my $self = {}; | |
147 | $self->{'buz'} = 42; | |
148 | bless $self, $type; | |
149 | } | |
150 | ||
151 | package Foo; | |
152 | @ISA = qw( Bar ); | |
153 | ||
154 | sub new { | |
155 | my $type = shift; | |
156 | my $self = Bar->new; | |
157 | $self->{'biz'} = 11; | |
158 | bless $self, $type; | |
159 | } | |
160 | ||
161 | package main; | |
162 | ||
163 | $a = Foo->new; | |
164 | print "buz = ", $a->{'buz'}, "\n"; | |
165 | print "biz = ", $a->{'biz'}, "\n"; | |
166 | ||
167 | ||
168 | ||
169 | =head1 OBJECT RELATIONSHIPS | |
170 | ||
171 | The following demonstrates how one might implement "containing" and "using" | |
172 | relationships between objects. | |
173 | ||
174 | package Bar; | |
175 | ||
176 | sub new { | |
177 | my $type = shift; | |
178 | my $self = {}; | |
179 | $self->{'buz'} = 42; | |
180 | bless $self, $type; | |
181 | } | |
182 | ||
183 | package Foo; | |
184 | ||
185 | sub new { | |
186 | my $type = shift; | |
187 | my $self = {}; | |
188 | $self->{'Bar'} = Bar->new; | |
189 | $self->{'biz'} = 11; | |
190 | bless $self, $type; | |
191 | } | |
192 | ||
193 | package main; | |
194 | ||
195 | $a = Foo->new; | |
196 | print "buz = ", $a->{'Bar'}->{'buz'}, "\n"; | |
197 | print "biz = ", $a->{'biz'}, "\n"; | |
198 | ||
199 | ||
200 | ||
201 | =head1 OVERRIDING SUPERCLASS METHODS | |
202 | ||
203 | The following example demonstrates how to override a superclass method and | |
204 | then call the overridden method. The B<SUPER> pseudo-class allows the | |
205 | programmer to call an overridden superclass method without actually knowing | |
206 | where that method is defined. | |
207 | ||
208 | package Buz; | |
209 | sub goo { print "here's the goo\n" } | |
210 | ||
211 | package Bar; @ISA = qw( Buz ); | |
212 | sub google { print "google here\n" } | |
213 | ||
214 | package Baz; | |
215 | sub mumble { print "mumbling\n" } | |
216 | ||
217 | package Foo; | |
218 | @ISA = qw( Bar Baz ); | |
219 | ||
220 | sub new { | |
221 | my $type = shift; | |
222 | bless [], $type; | |
223 | } | |
224 | sub grr { print "grumble\n" } | |
225 | sub goo { | |
226 | my $self = shift; | |
227 | $self->SUPER::goo(); | |
228 | } | |
229 | sub mumble { | |
230 | my $self = shift; | |
231 | $self->SUPER::mumble(); | |
232 | } | |
233 | sub google { | |
234 | my $self = shift; | |
235 | $self->SUPER::google(); | |
236 | } | |
237 | ||
238 | package main; | |
239 | ||
240 | $foo = Foo->new; | |
241 | $foo->mumble; | |
242 | $foo->grr; | |
243 | $foo->goo; | |
244 | $foo->google; | |
245 | ||
246 | Note that C<SUPER> refers to the superclasses of the current package | |
247 | (C<Foo>), not to the superclasses of C<$self>. | |
248 | ||
249 | ||
250 | =head1 USING RELATIONSHIP WITH SDBM | |
251 | ||
252 | This example demonstrates an interface for the SDBM class. This creates a | |
253 | "using" relationship between the SDBM class and the new class Mydbm. | |
254 | ||
255 | package Mydbm; | |
256 | ||
257 | require SDBM_File; | |
258 | require Tie::Hash; | |
259 | @ISA = qw( Tie::Hash ); | |
260 | ||
261 | sub TIEHASH { | |
262 | my $type = shift; | |
263 | my $ref = SDBM_File->new(@_); | |
264 | bless {'dbm' => $ref}, $type; | |
265 | } | |
266 | sub FETCH { | |
267 | my $self = shift; | |
268 | my $ref = $self->{'dbm'}; | |
269 | $ref->FETCH(@_); | |
270 | } | |
271 | sub STORE { | |
272 | my $self = shift; | |
273 | if (defined $_[0]){ | |
274 | my $ref = $self->{'dbm'}; | |
275 | $ref->STORE(@_); | |
276 | } else { | |
277 | die "Cannot STORE an undefined key in Mydbm\n"; | |
278 | } | |
279 | } | |
280 | ||
281 | package main; | |
282 | use Fcntl qw( O_RDWR O_CREAT ); | |
283 | ||
284 | tie %foo, "Mydbm", "Sdbm", O_RDWR|O_CREAT, 0640; | |
285 | $foo{'bar'} = 123; | |
286 | print "foo-bar = $foo{'bar'}\n"; | |
287 | ||
288 | tie %bar, "Mydbm", "Sdbm2", O_RDWR|O_CREAT, 0640; | |
289 | $bar{'Cathy'} = 456; | |
290 | print "bar-Cathy = $bar{'Cathy'}\n"; | |
291 | ||
292 | =head1 THINKING OF CODE REUSE | |
293 | ||
294 | One strength of Object-Oriented languages is the ease with which old code | |
295 | can use new code. The following examples will demonstrate first how one can | |
296 | hinder code reuse and then how one can promote code reuse. | |
297 | ||
298 | This first example illustrates a class which uses a fully-qualified method | |
299 | call to access the "private" method BAZ(). The second example will show | |
300 | that it is impossible to override the BAZ() method. | |
301 | ||
302 | package FOO; | |
303 | ||
304 | sub new { | |
305 | my $type = shift; | |
306 | bless {}, $type; | |
307 | } | |
308 | sub bar { | |
309 | my $self = shift; | |
310 | $self->FOO::private::BAZ; | |
311 | } | |
312 | ||
313 | package FOO::private; | |
314 | ||
315 | sub BAZ { | |
316 | print "in BAZ\n"; | |
317 | } | |
318 | ||
319 | package main; | |
320 | ||
321 | $a = FOO->new; | |
322 | $a->bar; | |
323 | ||
324 | Now we try to override the BAZ() method. We would like FOO::bar() to call | |
325 | GOOP::BAZ(), but this cannot happen because FOO::bar() explicitly calls | |
326 | FOO::private::BAZ(). | |
327 | ||
328 | package FOO; | |
329 | ||
330 | sub new { | |
331 | my $type = shift; | |
332 | bless {}, $type; | |
333 | } | |
334 | sub bar { | |
335 | my $self = shift; | |
336 | $self->FOO::private::BAZ; | |
337 | } | |
338 | ||
339 | package FOO::private; | |
340 | ||
341 | sub BAZ { | |
342 | print "in BAZ\n"; | |
343 | } | |
344 | ||
345 | package GOOP; | |
346 | @ISA = qw( FOO ); | |
347 | sub new { | |
348 | my $type = shift; | |
349 | bless {}, $type; | |
350 | } | |
351 | ||
352 | sub BAZ { | |
353 | print "in GOOP::BAZ\n"; | |
354 | } | |
355 | ||
356 | package main; | |
357 | ||
358 | $a = GOOP->new; | |
359 | $a->bar; | |
360 | ||
361 | To create reusable code we must modify class FOO, flattening class | |
362 | FOO::private. The next example shows a reusable class FOO which allows the | |
363 | method GOOP::BAZ() to be used in place of FOO::BAZ(). | |
364 | ||
365 | package FOO; | |
366 | ||
367 | sub new { | |
368 | my $type = shift; | |
369 | bless {}, $type; | |
370 | } | |
371 | sub bar { | |
372 | my $self = shift; | |
373 | $self->BAZ; | |
374 | } | |
375 | ||
376 | sub BAZ { | |
377 | print "in BAZ\n"; | |
378 | } | |
379 | ||
380 | package GOOP; | |
381 | @ISA = qw( FOO ); | |
382 | ||
383 | sub new { | |
384 | my $type = shift; | |
385 | bless {}, $type; | |
386 | } | |
387 | sub BAZ { | |
388 | print "in GOOP::BAZ\n"; | |
389 | } | |
390 | ||
391 | package main; | |
392 | ||
393 | $a = GOOP->new; | |
394 | $a->bar; | |
395 | ||
396 | =head1 CLASS CONTEXT AND THE OBJECT | |
397 | ||
398 | Use the object to solve package and class context problems. Everything a | |
399 | method needs should be available via the object or should be passed as a | |
400 | parameter to the method. | |
401 | ||
402 | A class will sometimes have static or global data to be used by the | |
403 | methods. A subclass may want to override that data and replace it with new | |
404 | data. When this happens the superclass may not know how to find the new | |
405 | copy of the data. | |
406 | ||
407 | This problem can be solved by using the object to define the context of the | |
408 | method. Let the method look in the object for a reference to the data. The | |
409 | alternative is to force the method to go hunting for the data ("Is it in my | |
410 | class, or in a subclass? Which subclass?"), and this can be inconvenient | |
411 | and will lead to hackery. It is better just to let the object tell the | |
412 | method where that data is located. | |
413 | ||
414 | package Bar; | |
415 | ||
416 | %fizzle = ( 'Password' => 'XYZZY' ); | |
417 | ||
418 | sub new { | |
419 | my $type = shift; | |
420 | my $self = {}; | |
421 | $self->{'fizzle'} = \%fizzle; | |
422 | bless $self, $type; | |
423 | } | |
424 | ||
425 | sub enter { | |
426 | my $self = shift; | |
427 | ||
428 | # Don't try to guess if we should use %Bar::fizzle | |
429 | # or %Foo::fizzle. The object already knows which | |
430 | # we should use, so just ask it. | |
431 | # | |
432 | my $fizzle = $self->{'fizzle'}; | |
433 | ||
434 | print "The word is ", $fizzle->{'Password'}, "\n"; | |
435 | } | |
436 | ||
437 | package Foo; | |
438 | @ISA = qw( Bar ); | |
439 | ||
440 | %fizzle = ( 'Password' => 'Rumple' ); | |
441 | ||
442 | sub new { | |
443 | my $type = shift; | |
444 | my $self = Bar->new; | |
445 | $self->{'fizzle'} = \%fizzle; | |
446 | bless $self, $type; | |
447 | } | |
448 | ||
449 | package main; | |
450 | ||
451 | $a = Bar->new; | |
452 | $b = Foo->new; | |
453 | $a->enter; | |
454 | $b->enter; | |
455 | ||
456 | =head1 INHERITING A CONSTRUCTOR | |
457 | ||
458 | An inheritable constructor should use the second form of bless() which allows | |
459 | blessing directly into a specified class. Notice in this example that the | |
460 | object will be a BAR not a FOO, even though the constructor is in class FOO. | |
461 | ||
462 | package FOO; | |
463 | ||
464 | sub new { | |
465 | my $type = shift; | |
466 | my $self = {}; | |
467 | bless $self, $type; | |
468 | } | |
469 | ||
470 | sub baz { | |
471 | print "in FOO::baz()\n"; | |
472 | } | |
473 | ||
474 | package BAR; | |
475 | @ISA = qw(FOO); | |
476 | ||
477 | sub baz { | |
478 | print "in BAR::baz()\n"; | |
479 | } | |
480 | ||
481 | package main; | |
482 | ||
483 | $a = BAR->new; | |
484 | $a->baz; | |
485 | ||
486 | =head1 DELEGATION | |
487 | ||
488 | Some classes, such as SDBM_File, cannot be effectively subclassed because | |
489 | they create foreign objects. Such a class can be extended with some sort of | |
490 | aggregation technique such as the "using" relationship mentioned earlier or | |
491 | by delegation. | |
492 | ||
493 | The following example demonstrates delegation using an AUTOLOAD() function to | |
494 | perform message-forwarding. This will allow the Mydbm object to behave | |
495 | exactly like an SDBM_File object. The Mydbm class could now extend the | |
496 | behavior by adding custom FETCH() and STORE() methods, if this is desired. | |
497 | ||
498 | package Mydbm; | |
499 | ||
500 | require SDBM_File; | |
501 | require Tie::Hash; | |
502 | @ISA = qw(Tie::Hash); | |
503 | ||
504 | sub TIEHASH { | |
505 | my $type = shift; | |
506 | my $ref = SDBM_File->new(@_); | |
507 | bless {'delegate' => $ref}; | |
508 | } | |
509 | ||
510 | sub AUTOLOAD { | |
511 | my $self = shift; | |
512 | ||
513 | # The Perl interpreter places the name of the | |
514 | # message in a variable called $AUTOLOAD. | |
515 | ||
516 | # DESTROY messages should never be propagated. | |
517 | return if $AUTOLOAD =~ /::DESTROY$/; | |
518 | ||
519 | # Remove the package name. | |
520 | $AUTOLOAD =~ s/^Mydbm:://; | |
521 | ||
522 | # Pass the message to the delegate. | |
523 | $self->{'delegate'}->$AUTOLOAD(@_); | |
524 | } | |
525 | ||
526 | package main; | |
527 | use Fcntl qw( O_RDWR O_CREAT ); | |
528 | ||
529 | tie %foo, "Mydbm", "adbm", O_RDWR|O_CREAT, 0640; | |
530 | $foo{'bar'} = 123; | |
531 | print "foo-bar = $foo{'bar'}\n"; | |
532 | ||
533 | =head1 SEE ALSO | |
534 | ||
535 | L<perlboot>, L<perltoot>, L<perltooc>. |