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1 | =head1 NAME |
2 | |
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3 | perlbot - Bag'o Object Tricks (the BOT) |
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4 | |
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5 | =head1 DESCRIPTION |
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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 |
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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, |
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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>. |
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15 | |
16 | The Perl motto still holds: There's more than one way to do it. |
17 | |
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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 | |
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61 | IO syntax is certainly less noisy, but it is also prone to ambiguities that |
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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 | |
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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 { |
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86 | my $type = shift; |
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87 | my %params = @_; |
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88 | my $self = {}; |
89 | $self->{'High'} = $params{'High'}; |
90 | $self->{'Low'} = $params{'Low'}; |
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91 | bless $self, $type; |
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92 | } |
93 | |
94 | |
95 | package Bar; |
96 | |
97 | sub new { |
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98 | my $type = shift; |
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99 | my %params = @_; |
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100 | my $self = []; |
101 | $self->[0] = $params{'Left'}; |
102 | $self->[1] = $params{'Right'}; |
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103 | bless $self, $type; |
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104 | } |
105 | |
106 | package main; |
107 | |
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108 | $a = Foo->new( 'High' => 42, 'Low' => 11 ); |
109 | print "High=$a->{'High'}\n"; |
110 | print "Low=$a->{'Low'}\n"; |
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111 | |
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112 | $b = Bar->new( 'Left' => 78, 'Right' => 40 ); |
113 | print "Left=$b->[0]\n"; |
114 | print "Right=$b->[1]\n"; |
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115 | |
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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; |
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124 | my $self; |
125 | $self = shift; |
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126 | bless \$self, $type; |
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127 | } |
128 | |
129 | package main; |
130 | |
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131 | $a = Foo->new( 42 ); |
132 | print "a=$$a\n"; |
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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 { |
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145 | my $type = shift; |
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146 | my $self = {}; |
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147 | $self->{'buz'} = 42; |
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148 | bless $self, $type; |
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149 | } |
150 | |
151 | package Foo; |
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152 | @ISA = qw( Bar ); |
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153 | |
154 | sub new { |
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155 | my $type = shift; |
156 | my $self = Bar->new; |
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157 | $self->{'biz'} = 11; |
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158 | bless $self, $type; |
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159 | } |
160 | |
161 | package main; |
162 | |
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163 | $a = Foo->new; |
164 | print "buz = ", $a->{'buz'}, "\n"; |
165 | print "biz = ", $a->{'biz'}, "\n"; |
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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 { |
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177 | my $type = shift; |
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178 | my $self = {}; |
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179 | $self->{'buz'} = 42; |
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180 | bless $self, $type; |
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181 | } |
182 | |
183 | package Foo; |
184 | |
185 | sub new { |
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186 | my $type = shift; |
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187 | my $self = {}; |
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188 | $self->{'Bar'} = Bar->new; |
189 | $self->{'biz'} = 11; |
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190 | bless $self, $type; |
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191 | } |
192 | |
193 | package main; |
194 | |
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195 | $a = Foo->new; |
196 | print "buz = ", $a->{'Bar'}->{'buz'}, "\n"; |
197 | print "biz = ", $a->{'biz'}, "\n"; |
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198 | |
199 | |
200 | |
201 | =head1 OVERRIDING SUPERCLASS METHODS |
202 | |
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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. |
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207 | |
208 | package Buz; |
209 | sub goo { print "here's the goo\n" } |
210 | |
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211 | package Bar; @ISA = qw( Buz ); |
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212 | sub google { print "google here\n" } |
213 | |
214 | package Baz; |
215 | sub mumble { print "mumbling\n" } |
216 | |
217 | package Foo; |
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218 | @ISA = qw( Bar Baz ); |
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219 | |
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220 | sub new { |
221 | my $type = shift; |
222 | bless [], $type; |
223 | } |
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224 | sub grr { print "grumble\n" } |
225 | sub goo { |
226 | my $self = shift; |
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227 | $self->SUPER::goo(); |
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228 | } |
229 | sub mumble { |
230 | my $self = shift; |
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231 | $self->SUPER::mumble(); |
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232 | } |
233 | sub google { |
234 | my $self = shift; |
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235 | $self->SUPER::google(); |
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236 | } |
237 | |
238 | package main; |
239 | |
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240 | $foo = Foo->new; |
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241 | $foo->mumble; |
242 | $foo->grr; |
243 | $foo->goo; |
244 | $foo->google; |
245 | |
246 | |
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247 | =head1 USING RELATIONSHIP WITH SDBM |
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248 | |
249 | This example demonstrates an interface for the SDBM class. This creates a |
250 | "using" relationship between the SDBM class and the new class Mydbm. |
251 | |
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252 | package Mydbm; |
253 | |
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254 | require SDBM_File; |
255 | require Tie::Hash; |
256 | @ISA = qw( Tie::Hash ); |
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257 | |
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258 | sub TIEHASH { |
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259 | my $type = shift; |
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260 | my $ref = SDBM_File->new(@_); |
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261 | bless {'dbm' => $ref}, $type; |
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262 | } |
263 | sub FETCH { |
264 | my $self = shift; |
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265 | my $ref = $self->{'dbm'}; |
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266 | $ref->FETCH(@_); |
267 | } |
268 | sub STORE { |
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269 | my $self = shift; |
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270 | if (defined $_[0]){ |
271 | my $ref = $self->{'dbm'}; |
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272 | $ref->STORE(@_); |
273 | } else { |
274 | die "Cannot STORE an undefined key in Mydbm\n"; |
275 | } |
276 | } |
277 | |
278 | package main; |
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279 | use Fcntl qw( O_RDWR O_CREAT ); |
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280 | |
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281 | tie %foo, "Mydbm", "Sdbm", O_RDWR|O_CREAT, 0640; |
282 | $foo{'bar'} = 123; |
283 | print "foo-bar = $foo{'bar'}\n"; |
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284 | |
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285 | tie %bar, "Mydbm", "Sdbm2", O_RDWR|O_CREAT, 0640; |
286 | $bar{'Cathy'} = 456; |
287 | print "bar-Cathy = $bar{'Cathy'}\n"; |
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288 | |
289 | =head1 THINKING OF CODE REUSE |
290 | |
291 | One strength of Object-Oriented languages is the ease with which old code |
292 | can use new code. The following examples will demonstrate first how one can |
293 | hinder code reuse and then how one can promote code reuse. |
294 | |
295 | This first example illustrates a class which uses a fully-qualified method |
296 | call to access the "private" method BAZ(). The second example will show |
297 | that it is impossible to override the BAZ() method. |
298 | |
299 | package FOO; |
300 | |
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301 | sub new { |
302 | my $type = shift; |
303 | bless {}, $type; |
304 | } |
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305 | sub bar { |
306 | my $self = shift; |
307 | $self->FOO::private::BAZ; |
308 | } |
309 | |
310 | package FOO::private; |
311 | |
312 | sub BAZ { |
313 | print "in BAZ\n"; |
314 | } |
315 | |
316 | package main; |
317 | |
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318 | $a = FOO->new; |
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319 | $a->bar; |
320 | |
321 | Now we try to override the BAZ() method. We would like FOO::bar() to call |
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322 | GOOP::BAZ(), but this cannot happen because FOO::bar() explicitly calls |
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323 | FOO::private::BAZ(). |
324 | |
325 | package FOO; |
326 | |
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327 | sub new { |
328 | my $type = shift; |
329 | bless {}, $type; |
330 | } |
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331 | sub bar { |
332 | my $self = shift; |
333 | $self->FOO::private::BAZ; |
334 | } |
335 | |
336 | package FOO::private; |
337 | |
338 | sub BAZ { |
339 | print "in BAZ\n"; |
340 | } |
341 | |
342 | package GOOP; |
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343 | @ISA = qw( FOO ); |
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344 | sub new { |
345 | my $type = shift; |
346 | bless {}, $type; |
347 | } |
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348 | |
349 | sub BAZ { |
350 | print "in GOOP::BAZ\n"; |
351 | } |
352 | |
353 | package main; |
354 | |
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355 | $a = GOOP->new; |
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356 | $a->bar; |
357 | |
358 | To create reusable code we must modify class FOO, flattening class |
359 | FOO::private. The next example shows a reusable class FOO which allows the |
360 | method GOOP::BAZ() to be used in place of FOO::BAZ(). |
361 | |
362 | package FOO; |
363 | |
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364 | sub new { |
365 | my $type = shift; |
366 | bless {}, $type; |
367 | } |
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368 | sub bar { |
369 | my $self = shift; |
370 | $self->BAZ; |
371 | } |
372 | |
373 | sub BAZ { |
374 | print "in BAZ\n"; |
375 | } |
376 | |
377 | package GOOP; |
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378 | @ISA = qw( FOO ); |
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379 | |
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380 | sub new { |
381 | my $type = shift; |
382 | bless {}, $type; |
383 | } |
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384 | sub BAZ { |
385 | print "in GOOP::BAZ\n"; |
386 | } |
387 | |
388 | package main; |
389 | |
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390 | $a = GOOP->new; |
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391 | $a->bar; |
392 | |
393 | =head1 CLASS CONTEXT AND THE OBJECT |
394 | |
395 | Use the object to solve package and class context problems. Everything a |
396 | method needs should be available via the object or should be passed as a |
397 | parameter to the method. |
398 | |
399 | A class will sometimes have static or global data to be used by the |
400 | methods. A subclass may want to override that data and replace it with new |
401 | data. When this happens the superclass may not know how to find the new |
402 | copy of the data. |
403 | |
404 | This problem can be solved by using the object to define the context of the |
405 | method. Let the method look in the object for a reference to the data. The |
406 | alternative is to force the method to go hunting for the data ("Is it in my |
407 | class, or in a subclass? Which subclass?"), and this can be inconvenient |
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408 | and will lead to hackery. It is better just to let the object tell the |
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409 | method where that data is located. |
410 | |
411 | package Bar; |
412 | |
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413 | %fizzle = ( 'Password' => 'XYZZY' ); |
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414 | |
415 | sub new { |
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416 | my $type = shift; |
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417 | my $self = {}; |
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418 | $self->{'fizzle'} = \%fizzle; |
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419 | bless $self, $type; |
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420 | } |
421 | |
422 | sub enter { |
423 | my $self = shift; |
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424 | |
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425 | # Don't try to guess if we should use %Bar::fizzle |
426 | # or %Foo::fizzle. The object already knows which |
427 | # we should use, so just ask it. |
428 | # |
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429 | my $fizzle = $self->{'fizzle'}; |
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430 | |
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431 | print "The word is ", $fizzle->{'Password'}, "\n"; |
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432 | } |
433 | |
434 | package Foo; |
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435 | @ISA = qw( Bar ); |
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436 | |
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437 | %fizzle = ( 'Password' => 'Rumple' ); |
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438 | |
439 | sub new { |
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440 | my $type = shift; |
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441 | my $self = Bar->new; |
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442 | $self->{'fizzle'} = \%fizzle; |
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443 | bless $self, $type; |
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444 | } |
445 | |
446 | package main; |
447 | |
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448 | $a = Bar->new; |
449 | $b = Foo->new; |
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450 | $a->enter; |
451 | $b->enter; |
452 | |
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453 | =head1 INHERITING A CONSTRUCTOR |
454 | |
455 | An inheritable constructor should use the second form of bless() which allows |
456 | blessing directly into a specified class. Notice in this example that the |
457 | object will be a BAR not a FOO, even though the constructor is in class FOO. |
458 | |
459 | package FOO; |
460 | |
461 | sub new { |
462 | my $type = shift; |
463 | my $self = {}; |
464 | bless $self, $type; |
465 | } |
466 | |
467 | sub baz { |
468 | print "in FOO::baz()\n"; |
469 | } |
470 | |
471 | package BAR; |
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472 | @ISA = qw(FOO); |
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473 | |
474 | sub baz { |
475 | print "in BAR::baz()\n"; |
476 | } |
477 | |
478 | package main; |
479 | |
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480 | $a = BAR->new; |
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481 | $a->baz; |
482 | |
483 | =head1 DELEGATION |
484 | |
485 | Some classes, such as SDBM_File, cannot be effectively subclassed because |
486 | they create foreign objects. Such a class can be extended with some sort of |
487 | aggregation technique such as the "using" relationship mentioned earlier or |
488 | by delegation. |
489 | |
490 | The following example demonstrates delegation using an AUTOLOAD() function to |
491 | perform message-forwarding. This will allow the Mydbm object to behave |
492 | exactly like an SDBM_File object. The Mydbm class could now extend the |
493 | behavior by adding custom FETCH() and STORE() methods, if this is desired. |
494 | |
495 | package Mydbm; |
496 | |
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497 | require SDBM_File; |
498 | require Tie::Hash; |
499 | @ISA = qw(Tie::Hash); |
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500 | |
501 | sub TIEHASH { |
502 | my $type = shift; |
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503 | my $ref = SDBM_File->new(@_); |
504 | bless {'delegate' => $ref}; |
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505 | } |
506 | |
507 | sub AUTOLOAD { |
508 | my $self = shift; |
509 | |
510 | # The Perl interpreter places the name of the |
511 | # message in a variable called $AUTOLOAD. |
512 | |
513 | # DESTROY messages should never be propagated. |
514 | return if $AUTOLOAD =~ /::DESTROY$/; |
515 | |
516 | # Remove the package name. |
517 | $AUTOLOAD =~ s/^Mydbm:://; |
518 | |
519 | # Pass the message to the delegate. |
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520 | $self->{'delegate'}->$AUTOLOAD(@_); |
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521 | } |
522 | |
523 | package main; |
524 | use Fcntl qw( O_RDWR O_CREAT ); |
525 | |
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526 | tie %foo, "Mydbm", "adbm", O_RDWR|O_CREAT, 0640; |
527 | $foo{'bar'} = 123; |
528 | print "foo-bar = $foo{'bar'}\n"; |
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529 | |
530 | =head1 SEE ALSO |
531 | |
532 | L<perlboot>, L<perltoot>, L<perltooc>. |