This is a live mirror of the Perl 5 development currently hosted at https://github.com/perl/perl5
Quote string argument in example -- necessary if using strict subs
[perl5.git] / pod / perlbot.pod
CommitLineData
a0d0e21e
LW
1=head1 NAME
2
c2960299 3perlbot - Bag'o Object Tricks (the BOT)
a0d0e21e 4
cb1a09d0 5=head1 DESCRIPTION
a0d0e21e
LW
6
7The following collection of tricks and hints is intended to whet curious
8appetites about such things as the use of instance variables and the
9mechanics of object and class relationships. The reader is encouraged to
10consult relevant textbooks for discussion of Object Oriented definitions and
c2960299
AD
11methodology. This is not intended as a tutorial for object-oriented
12programming or as a comprehensive guide to Perl's object oriented features,
13nor should it be construed as a style guide.
a0d0e21e
LW
14
15The 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
23Do not attempt to verify the type of $self. That'll break if the class is
24inherited, when the type of $self is valid but its package isn't what you
25expect. See rule 5.
26
27=item 2
28
29If an object-oriented (OO) or indirect-object (IO) syntax was used, then the
30object is probably the correct type and there's no need to become paranoid
31about it. Perl isn't a paranoid language anyway. If people subvert the OO
32or IO syntax then they probably know what they're doing and you should let
33them do it. See rule 1.
34
35=item 3
36
37Use the two-argument form of bless(). Let a subclass use your constructor.
38See L<INHERITING A CONSTRUCTOR>.
39
40=item 4
41
42The subclass is allowed to know things about its immediate superclass, the
43superclass is allowed to know nothing about a subclass.
44
45=item 5
46
47Don't be trigger happy with inheritance. A "using", "containing", or
48"delegation" relationship (some sort of aggregation, at least) is often more
49appropriate. See L<OBJECT RELATIONSHIPS>, L<USING RELATIONSHIP WITH SDBM>,
50and L<"DELEGATION">.
51
52=item 6
53
54The object is the namespace. Make package globals accessible via the
55object. This will remove the guess work about the symbol's home package.
56See L<CLASS CONTEXT AND THE OBJECT>.
57
58=item 7
59
60IO syntax is certainly less noisy, but it is also prone to ambiguities which
61can cause difficult-to-find bugs. Allow people to use the sure-thing OO
62syntax, even if you don't like it.
63
64=item 8
65
66Do not use function-call syntax on a method. You're going to be bitten
67someday. Someone might move that method into a superclass and your code
68will be broken. On top of that you're feeding the paranoia in rule 2.
69
70=item 9
71
72Don't assume you know the home package of a method. You're making it
73difficult for someone to override that method. See L<THINKING OF CODE REUSE>.
74
75=back
76
a0d0e21e
LW
77=head1 INSTANCE VARIABLES
78
79An anonymous array or anonymous hash can be used to hold instance
80variables. 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
117An 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
136This example demonstrates how one might inherit instance variables from a
137superclass for inclusion in the new class. This requires calling the
138superclass's constructor and adding one's own instance variables to the new
139object.
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
170The following demonstrates how one might implement "containing" and "using"
171relationships 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
202The following example demonstrates how to override a superclass method and
203then call the overridden method. The B<SUPER> pseudo-class allows the
204programmer to call an overridden superclass method without actually knowing
205where 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
248This 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 253 require SDBM_File;
af131e48
PP
254 require Tie::Hash;
255 @ISA = qw( Tie::Hash );
c2960299 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 279
c954a603 280 tie %foo, "Mydbm", "Sdbm", O_RDWR|O_CREAT, 0640;
a0d0e21e
LW
281 $foo{'bar'} = 123;
282 print "foo-bar = $foo{'bar'}\n";
283
c954a603 284 tie %bar, "Mydbm", "Sdbm2", O_RDWR|O_CREAT, 0640;
a0d0e21e
LW
285 $bar{'Cathy'} = 456;
286 print "bar-Cathy = $bar{'Cathy'}\n";
287
288=head1 THINKING OF CODE REUSE
289
290One strength of Object-Oriented languages is the ease with which old code
291can use new code. The following examples will demonstrate first how one can
292hinder code reuse and then how one can promote code reuse.
293
294This first example illustrates a class which uses a fully-qualified method
295call to access the "private" method BAZ(). The second example will show
296that 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
320Now we try to override the BAZ() method. We would like FOO::bar() to call
d1b91892 321GOOP::BAZ(), but this cannot happen because FOO::bar() explicitly calls
a0d0e21e
LW
322FOO::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
357To create reusable code we must modify class FOO, flattening class
358FOO::private. The next example shows a reusable class FOO which allows the
359method 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
394Use the object to solve package and class context problems. Everything a
395method needs should be available via the object or should be passed as a
396parameter to the method.
397
398A class will sometimes have static or global data to be used by the
399methods. A subclass may want to override that data and replace it with new
400data. When this happens the superclass may not know how to find the new
401copy of the data.
402
403This problem can be solved by using the object to define the context of the
404method. Let the method look in the object for a reference to the data. The
405alternative is to force the method to go hunting for the data ("Is it in my
406class, or in a subclass? Which subclass?"), and this can be inconvenient
407and will lead to hackery. It is better to just let the object tell the
408method 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
454An inheritable constructor should use the second form of bless() which allows
455blessing directly into a specified class. Notice in this example that the
456object 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
484Some classes, such as SDBM_File, cannot be effectively subclassed because
485they create foreign objects. Such a class can be extended with some sort of
486aggregation technique such as the "using" relationship mentioned earlier or
487by delegation.
488
489The following example demonstrates delegation using an AUTOLOAD() function to
490perform message-forwarding. This will allow the Mydbm object to behave
491exactly like an SDBM_File object. The Mydbm class could now extend the
492behavior by adding custom FETCH() and STORE() methods, if this is desired.
493
494 package Mydbm;
495
496 require SDBM_File;
af131e48
PP
497 require Tie::Hash;
498 @ISA = qw(Tie::Hash);
d1b91892
AD
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
c954a603 525 tie %foo, "Mydbm", "adbm", O_RDWR|O_CREAT, 0640;
d1b91892
AD
526 $foo{'bar'} = 123;
527 print "foo-bar = $foo{'bar'}\n";