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a0d0e21e LW |
1 | =head1 NAME |
2 | ||
c2960299 | 3 | perlbot - Bag'o Object Tricks (the BOT) |
a0d0e21e | 4 | |
cb1a09d0 | 5 | =head1 DESCRIPTION |
a0d0e21e LW |
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 | |
c2960299 AD |
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. | |
a0d0e21e LW |
14 | |
15 | The Perl motto still holds: There's more than one way to do it. | |
16 | ||
c2960299 AD |
17 | =head1 OO SCALING TIPS |
18 | ||
19 | =over 5 | |
20 | ||
21 | =item 1 | |
22 | ||
23 | Do not attempt to verify the type of $self. That'll break if the class is | |
24 | inherited, when the type of $self is valid but its package isn't what you | |
25 | expect. See rule 5. | |
26 | ||
27 | =item 2 | |
28 | ||
29 | If an object-oriented (OO) or indirect-object (IO) syntax was used, then the | |
30 | object is probably the correct type and there's no need to become paranoid | |
31 | about it. Perl isn't a paranoid language anyway. If people subvert the OO | |
32 | or IO syntax then they probably know what they're doing and you should let | |
33 | them do it. See rule 1. | |
34 | ||
35 | =item 3 | |
36 | ||
37 | Use the two-argument form of bless(). Let a subclass use your constructor. | |
38 | See L<INHERITING A CONSTRUCTOR>. | |
39 | ||
40 | =item 4 | |
41 | ||
42 | The subclass is allowed to know things about its immediate superclass, the | |
43 | superclass is allowed to know nothing about a subclass. | |
44 | ||
45 | =item 5 | |
46 | ||
47 | Don't be trigger happy with inheritance. A "using", "containing", or | |
48 | "delegation" relationship (some sort of aggregation, at least) is often more | |
49 | appropriate. See L<OBJECT RELATIONSHIPS>, L<USING RELATIONSHIP WITH SDBM>, | |
50 | and L<"DELEGATION">. | |
51 | ||
52 | =item 6 | |
53 | ||
54 | The object is the namespace. Make package globals accessible via the | |
55 | object. This will remove the guess work about the symbol's home package. | |
56 | See L<CLASS CONTEXT AND THE OBJECT>. | |
57 | ||
58 | =item 7 | |
59 | ||
60 | IO syntax is certainly less noisy, but it is also prone to ambiguities which | |
61 | can cause difficult-to-find bugs. Allow people to use the sure-thing OO | |
62 | syntax, even if you don't like it. | |
63 | ||
64 | =item 8 | |
65 | ||
66 | Do not use function-call syntax on a method. You're going to be bitten | |
67 | someday. Someone might move that method into a superclass and your code | |
68 | will be broken. On top of that you're feeding the paranoia in rule 2. | |
69 | ||
70 | =item 9 | |
71 | ||
72 | Don't assume you know the home package of a method. You're making it | |
73 | difficult for someone to override that method. See L<THINKING OF CODE REUSE>. | |
74 | ||
75 | =back | |
76 | ||
a0d0e21e LW |
77 | =head1 INSTANCE VARIABLES |
78 | ||
79 | An anonymous array or anonymous hash can be used to hold instance | |
80 | variables. Named parameters are also demonstrated. | |
81 | ||
82 | package Foo; | |
83 | ||
84 | sub new { | |
85 | my $type = shift; | |
86 | my %params = @_; | |
87 | my $self = {}; | |
88 | $self->{'High'} = $params{'High'}; | |
89 | $self->{'Low'} = $params{'Low'}; | |
c2960299 | 90 | bless $self, $type; |
a0d0e21e LW |
91 | } |
92 | ||
93 | ||
94 | package Bar; | |
95 | ||
96 | sub new { | |
97 | my $type = shift; | |
98 | my %params = @_; | |
99 | my $self = []; | |
100 | $self->[0] = $params{'Left'}; | |
101 | $self->[1] = $params{'Right'}; | |
c2960299 | 102 | bless $self, $type; |
a0d0e21e LW |
103 | } |
104 | ||
105 | package main; | |
106 | ||
c2960299 | 107 | $a = Foo->new( 'High' => 42, 'Low' => 11 ); |
a0d0e21e LW |
108 | print "High=$a->{'High'}\n"; |
109 | print "Low=$a->{'Low'}\n"; | |
110 | ||
c2960299 | 111 | $b = Bar->new( 'Left' => 78, 'Right' => 40 ); |
a0d0e21e LW |
112 | print "Left=$b->[0]\n"; |
113 | print "Right=$b->[1]\n"; | |
114 | ||
a0d0e21e LW |
115 | =head1 SCALAR INSTANCE VARIABLES |
116 | ||
117 | An anonymous scalar can be used when only one instance variable is needed. | |
118 | ||
119 | package Foo; | |
120 | ||
121 | sub new { | |
122 | my $type = shift; | |
123 | my $self; | |
124 | $self = shift; | |
c2960299 | 125 | bless \$self, $type; |
a0d0e21e LW |
126 | } |
127 | ||
128 | package main; | |
129 | ||
c2960299 | 130 | $a = Foo->new( 42 ); |
a0d0e21e LW |
131 | print "a=$$a\n"; |
132 | ||
133 | ||
134 | =head1 INSTANCE VARIABLE INHERITANCE | |
135 | ||
136 | This example demonstrates how one might inherit instance variables from a | |
137 | superclass for inclusion in the new class. This requires calling the | |
138 | superclass's constructor and adding one's own instance variables to the new | |
139 | object. | |
140 | ||
141 | package Bar; | |
142 | ||
143 | sub new { | |
c2960299 | 144 | my $type = shift; |
a0d0e21e LW |
145 | my $self = {}; |
146 | $self->{'buz'} = 42; | |
c2960299 | 147 | bless $self, $type; |
a0d0e21e LW |
148 | } |
149 | ||
150 | package Foo; | |
151 | @ISA = qw( Bar ); | |
152 | ||
153 | sub new { | |
c2960299 AD |
154 | my $type = shift; |
155 | my $self = Bar->new; | |
a0d0e21e | 156 | $self->{'biz'} = 11; |
c2960299 | 157 | bless $self, $type; |
a0d0e21e LW |
158 | } |
159 | ||
160 | package main; | |
161 | ||
c2960299 | 162 | $a = Foo->new; |
a0d0e21e LW |
163 | print "buz = ", $a->{'buz'}, "\n"; |
164 | print "biz = ", $a->{'biz'}, "\n"; | |
165 | ||
166 | ||
167 | ||
168 | =head1 OBJECT RELATIONSHIPS | |
169 | ||
170 | The following demonstrates how one might implement "containing" and "using" | |
171 | relationships between objects. | |
172 | ||
173 | package Bar; | |
174 | ||
175 | sub new { | |
c2960299 | 176 | my $type = shift; |
a0d0e21e LW |
177 | my $self = {}; |
178 | $self->{'buz'} = 42; | |
c2960299 | 179 | bless $self, $type; |
a0d0e21e LW |
180 | } |
181 | ||
182 | package Foo; | |
183 | ||
184 | sub new { | |
c2960299 | 185 | my $type = shift; |
a0d0e21e | 186 | my $self = {}; |
c2960299 | 187 | $self->{'Bar'} = Bar->new; |
a0d0e21e | 188 | $self->{'biz'} = 11; |
c2960299 | 189 | bless $self, $type; |
a0d0e21e LW |
190 | } |
191 | ||
192 | package main; | |
193 | ||
c2960299 | 194 | $a = Foo->new; |
a0d0e21e LW |
195 | print "buz = ", $a->{'Bar'}->{'buz'}, "\n"; |
196 | print "biz = ", $a->{'biz'}, "\n"; | |
197 | ||
198 | ||
199 | ||
200 | =head1 OVERRIDING SUPERCLASS METHODS | |
201 | ||
4633a7c4 LW |
202 | The following example demonstrates how to override a superclass method and |
203 | then call the overridden method. The B<SUPER> pseudo-class allows the | |
204 | programmer to call an overridden superclass method without actually knowing | |
205 | where that method is defined. | |
a0d0e21e LW |
206 | |
207 | package Buz; | |
208 | sub goo { print "here's the goo\n" } | |
209 | ||
210 | package Bar; @ISA = qw( Buz ); | |
211 | sub google { print "google here\n" } | |
212 | ||
213 | package Baz; | |
214 | sub mumble { print "mumbling\n" } | |
215 | ||
216 | package Foo; | |
217 | @ISA = qw( Bar Baz ); | |
a0d0e21e | 218 | |
c2960299 AD |
219 | sub new { |
220 | my $type = shift; | |
221 | bless [], $type; | |
222 | } | |
a0d0e21e LW |
223 | sub grr { print "grumble\n" } |
224 | sub goo { | |
225 | my $self = shift; | |
4633a7c4 | 226 | $self->SUPER::goo(); |
a0d0e21e LW |
227 | } |
228 | sub mumble { | |
229 | my $self = shift; | |
4633a7c4 | 230 | $self->SUPER::mumble(); |
a0d0e21e LW |
231 | } |
232 | sub google { | |
233 | my $self = shift; | |
4633a7c4 | 234 | $self->SUPER::google(); |
a0d0e21e LW |
235 | } |
236 | ||
237 | package main; | |
238 | ||
c2960299 | 239 | $foo = Foo->new; |
a0d0e21e LW |
240 | $foo->mumble; |
241 | $foo->grr; | |
242 | $foo->goo; | |
243 | $foo->google; | |
244 | ||
245 | ||
c2960299 | 246 | =head1 USING RELATIONSHIP WITH SDBM |
a0d0e21e LW |
247 | |
248 | This example demonstrates an interface for the SDBM class. This creates a | |
249 | "using" relationship between the SDBM class and the new class Mydbm. | |
250 | ||
a0d0e21e LW |
251 | package Mydbm; |
252 | ||
c2960299 AD |
253 | require SDBM_File; |
254 | require TieHash; | |
255 | @ISA = qw( TieHash ); | |
256 | ||
a0d0e21e | 257 | sub TIEHASH { |
c2960299 | 258 | my $type = shift; |
a0d0e21e | 259 | my $ref = SDBM_File->new(@_); |
c2960299 | 260 | bless {'dbm' => $ref}, $type; |
a0d0e21e LW |
261 | } |
262 | sub FETCH { | |
263 | my $self = shift; | |
264 | my $ref = $self->{'dbm'}; | |
265 | $ref->FETCH(@_); | |
266 | } | |
267 | sub STORE { | |
268 | my $self = shift; | |
269 | if (defined $_[0]){ | |
270 | my $ref = $self->{'dbm'}; | |
271 | $ref->STORE(@_); | |
272 | } else { | |
273 | die "Cannot STORE an undefined key in Mydbm\n"; | |
274 | } | |
275 | } | |
276 | ||
277 | package main; | |
c2960299 | 278 | use Fcntl qw( O_RDWR O_CREAT ); |
a0d0e21e LW |
279 | |
280 | tie %foo, Mydbm, "Sdbm", O_RDWR|O_CREAT, 0640; | |
281 | $foo{'bar'} = 123; | |
282 | print "foo-bar = $foo{'bar'}\n"; | |
283 | ||
284 | tie %bar, Mydbm, "Sdbm2", O_RDWR|O_CREAT, 0640; | |
285 | $bar{'Cathy'} = 456; | |
286 | print "bar-Cathy = $bar{'Cathy'}\n"; | |
287 | ||
288 | =head1 THINKING OF CODE REUSE | |
289 | ||
290 | One strength of Object-Oriented languages is the ease with which old code | |
291 | can use new code. The following examples will demonstrate first how one can | |
292 | hinder code reuse and then how one can promote code reuse. | |
293 | ||
294 | This first example illustrates a class which uses a fully-qualified method | |
295 | call to access the "private" method BAZ(). The second example will show | |
296 | that it is impossible to override the BAZ() method. | |
297 | ||
298 | package FOO; | |
299 | ||
c2960299 AD |
300 | sub new { |
301 | my $type = shift; | |
302 | bless {}, $type; | |
303 | } | |
a0d0e21e LW |
304 | sub bar { |
305 | my $self = shift; | |
306 | $self->FOO::private::BAZ; | |
307 | } | |
308 | ||
309 | package FOO::private; | |
310 | ||
311 | sub BAZ { | |
312 | print "in BAZ\n"; | |
313 | } | |
314 | ||
315 | package main; | |
316 | ||
317 | $a = FOO->new; | |
318 | $a->bar; | |
319 | ||
320 | Now we try to override the BAZ() method. We would like FOO::bar() to call | |
d1b91892 | 321 | GOOP::BAZ(), but this cannot happen because FOO::bar() explicitly calls |
a0d0e21e LW |
322 | FOO::private::BAZ(). |
323 | ||
324 | package FOO; | |
325 | ||
c2960299 AD |
326 | sub new { |
327 | my $type = shift; | |
328 | bless {}, $type; | |
329 | } | |
a0d0e21e LW |
330 | sub bar { |
331 | my $self = shift; | |
332 | $self->FOO::private::BAZ; | |
333 | } | |
334 | ||
335 | package FOO::private; | |
336 | ||
337 | sub BAZ { | |
338 | print "in BAZ\n"; | |
339 | } | |
340 | ||
341 | package GOOP; | |
342 | @ISA = qw( FOO ); | |
c2960299 AD |
343 | sub new { |
344 | my $type = shift; | |
345 | bless {}, $type; | |
346 | } | |
a0d0e21e LW |
347 | |
348 | sub BAZ { | |
349 | print "in GOOP::BAZ\n"; | |
350 | } | |
351 | ||
352 | package main; | |
353 | ||
354 | $a = GOOP->new; | |
355 | $a->bar; | |
356 | ||
357 | To create reusable code we must modify class FOO, flattening class | |
358 | FOO::private. The next example shows a reusable class FOO which allows the | |
359 | method GOOP::BAZ() to be used in place of FOO::BAZ(). | |
360 | ||
361 | package FOO; | |
362 | ||
c2960299 AD |
363 | sub new { |
364 | my $type = shift; | |
365 | bless {}, $type; | |
366 | } | |
a0d0e21e LW |
367 | sub bar { |
368 | my $self = shift; | |
369 | $self->BAZ; | |
370 | } | |
371 | ||
372 | sub BAZ { | |
373 | print "in BAZ\n"; | |
374 | } | |
375 | ||
376 | package GOOP; | |
377 | @ISA = qw( FOO ); | |
378 | ||
c2960299 AD |
379 | sub new { |
380 | my $type = shift; | |
381 | bless {}, $type; | |
382 | } | |
a0d0e21e LW |
383 | sub BAZ { |
384 | print "in GOOP::BAZ\n"; | |
385 | } | |
386 | ||
387 | package main; | |
388 | ||
389 | $a = GOOP->new; | |
390 | $a->bar; | |
391 | ||
392 | =head1 CLASS CONTEXT AND THE OBJECT | |
393 | ||
394 | Use the object to solve package and class context problems. Everything a | |
395 | method needs should be available via the object or should be passed as a | |
396 | parameter to the method. | |
397 | ||
398 | A class will sometimes have static or global data to be used by the | |
399 | methods. A subclass may want to override that data and replace it with new | |
400 | data. When this happens the superclass may not know how to find the new | |
401 | copy of the data. | |
402 | ||
403 | This problem can be solved by using the object to define the context of the | |
404 | method. Let the method look in the object for a reference to the data. The | |
405 | alternative is to force the method to go hunting for the data ("Is it in my | |
406 | class, or in a subclass? Which subclass?"), and this can be inconvenient | |
407 | and will lead to hackery. It is better to just let the object tell the | |
408 | method where that data is located. | |
409 | ||
410 | package Bar; | |
411 | ||
412 | %fizzle = ( 'Password' => 'XYZZY' ); | |
413 | ||
414 | sub new { | |
c2960299 | 415 | my $type = shift; |
a0d0e21e LW |
416 | my $self = {}; |
417 | $self->{'fizzle'} = \%fizzle; | |
c2960299 | 418 | bless $self, $type; |
a0d0e21e LW |
419 | } |
420 | ||
421 | sub enter { | |
422 | my $self = shift; | |
423 | ||
424 | # Don't try to guess if we should use %Bar::fizzle | |
425 | # or %Foo::fizzle. The object already knows which | |
426 | # we should use, so just ask it. | |
427 | # | |
428 | my $fizzle = $self->{'fizzle'}; | |
429 | ||
430 | print "The word is ", $fizzle->{'Password'}, "\n"; | |
431 | } | |
432 | ||
433 | package Foo; | |
434 | @ISA = qw( Bar ); | |
435 | ||
436 | %fizzle = ( 'Password' => 'Rumple' ); | |
437 | ||
438 | sub new { | |
c2960299 | 439 | my $type = shift; |
a0d0e21e LW |
440 | my $self = Bar->new; |
441 | $self->{'fizzle'} = \%fizzle; | |
c2960299 | 442 | bless $self, $type; |
a0d0e21e LW |
443 | } |
444 | ||
445 | package main; | |
446 | ||
447 | $a = Bar->new; | |
448 | $b = Foo->new; | |
449 | $a->enter; | |
450 | $b->enter; | |
451 | ||
d1b91892 AD |
452 | =head1 INHERITING A CONSTRUCTOR |
453 | ||
454 | An inheritable constructor should use the second form of bless() which allows | |
455 | blessing directly into a specified class. Notice in this example that the | |
456 | object will be a BAR not a FOO, even though the constructor is in class FOO. | |
457 | ||
458 | package FOO; | |
459 | ||
460 | sub new { | |
461 | my $type = shift; | |
462 | my $self = {}; | |
463 | bless $self, $type; | |
464 | } | |
465 | ||
466 | sub baz { | |
467 | print "in FOO::baz()\n"; | |
468 | } | |
469 | ||
470 | package BAR; | |
471 | @ISA = qw(FOO); | |
472 | ||
473 | sub baz { | |
474 | print "in BAR::baz()\n"; | |
475 | } | |
476 | ||
477 | package main; | |
478 | ||
479 | $a = BAR->new; | |
480 | $a->baz; | |
481 | ||
482 | =head1 DELEGATION | |
483 | ||
484 | Some classes, such as SDBM_File, cannot be effectively subclassed because | |
485 | they create foreign objects. Such a class can be extended with some sort of | |
486 | aggregation technique such as the "using" relationship mentioned earlier or | |
487 | by delegation. | |
488 | ||
489 | The following example demonstrates delegation using an AUTOLOAD() function to | |
490 | perform message-forwarding. This will allow the Mydbm object to behave | |
491 | exactly like an SDBM_File object. The Mydbm class could now extend the | |
492 | behavior by adding custom FETCH() and STORE() methods, if this is desired. | |
493 | ||
494 | package Mydbm; | |
495 | ||
496 | require SDBM_File; | |
497 | require TieHash; | |
498 | @ISA = qw(TieHash); | |
499 | ||
500 | sub TIEHASH { | |
501 | my $type = shift; | |
502 | my $ref = SDBM_File->new(@_); | |
503 | bless {'delegate' => $ref}; | |
504 | } | |
505 | ||
506 | sub AUTOLOAD { | |
507 | my $self = shift; | |
508 | ||
509 | # The Perl interpreter places the name of the | |
510 | # message in a variable called $AUTOLOAD. | |
511 | ||
512 | # DESTROY messages should never be propagated. | |
513 | return if $AUTOLOAD =~ /::DESTROY$/; | |
514 | ||
515 | # Remove the package name. | |
516 | $AUTOLOAD =~ s/^Mydbm:://; | |
517 | ||
518 | # Pass the message to the delegate. | |
519 | $self->{'delegate'}->$AUTOLOAD(@_); | |
520 | } | |
521 | ||
522 | package main; | |
523 | use Fcntl qw( O_RDWR O_CREAT ); | |
524 | ||
525 | tie %foo, Mydbm, "adbm", O_RDWR|O_CREAT, 0640; | |
526 | $foo{'bar'} = 123; | |
527 | print "foo-bar = $foo{'bar'}\n"; |