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1 | =head1 NAME |
2 | |
3 | perlobj - Perl objects |
4 | |
5 | =head1 DESCRIPTION |
6 | |
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7 | First of all, you need to understand what references are in Perl. |
8 | See L<perlref> for that. Second, if you still find the following |
9 | reference work too complicated, a tutorial on object-oriented programming |
10 | in Perl can be found in L<perltoot>. |
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11 | |
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12 | If you're still with us, then |
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13 | here are three very simple definitions that you should find reassuring. |
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14 | |
15 | =over 4 |
16 | |
17 | =item 1. |
18 | |
19 | An object is simply a reference that happens to know which class it |
20 | belongs to. |
21 | |
22 | =item 2. |
23 | |
24 | A class is simply a package that happens to provide methods to deal |
25 | with object references. |
26 | |
27 | =item 3. |
28 | |
29 | A method is simply a subroutine that expects an object reference (or |
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30 | a package name, for class methods) as the first argument. |
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31 | |
32 | =back |
33 | |
34 | We'll cover these points now in more depth. |
35 | |
36 | =head2 An Object is Simply a Reference |
37 | |
38 | Unlike say C++, Perl doesn't provide any special syntax for |
39 | constructors. A constructor is merely a subroutine that returns a |
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40 | reference to something "blessed" into a class, generally the |
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41 | class that the subroutine is defined in. Here is a typical |
42 | constructor: |
43 | |
44 | package Critter; |
45 | sub new { bless {} } |
46 | |
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47 | That word C<new> isn't special. You could have written |
48 | a construct this way, too: |
49 | |
50 | package Critter; |
51 | sub spawn { bless {} } |
52 | |
53 | In fact, this might even be preferable, because the C++ programmers won't |
54 | be tricked into thinking that C<new> works in Perl as it does in C++. |
55 | It doesn't. We recommend that you name your constructors whatever |
56 | makes sense in the context of the problem you're solving. For example, |
57 | constructors in the Tk extension to Perl are named after the widgets |
58 | they create. |
59 | |
60 | One thing that's different about Perl constructors compared with those in |
61 | C++ is that in Perl, they have to allocate their own memory. (The other |
62 | things is that they don't automatically call overridden base-class |
63 | constructors.) The C<{}> allocates an anonymous hash containing no |
64 | key/value pairs, and returns it The bless() takes that reference and |
65 | tells the object it references that it's now a Critter, and returns |
66 | the reference. This is for convenience, because the referenced object |
67 | itself knows that it has been blessed, and the reference to it could |
68 | have been returned directly, like this: |
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69 | |
70 | sub new { |
71 | my $self = {}; |
72 | bless $self; |
73 | return $self; |
74 | } |
75 | |
76 | In fact, you often see such a thing in more complicated constructors |
77 | that wish to call methods in the class as part of the construction: |
78 | |
79 | sub new { |
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80 | my $self = {}; |
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81 | bless $self; |
82 | $self->initialize(); |
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83 | return $self; |
84 | } |
85 | |
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86 | If you care about inheritance (and you should; see |
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87 | L<perlmod/"Modules: Creation, Use, and Abuse">), |
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88 | then you want to use the two-arg form of bless |
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89 | so that your constructors may be inherited: |
90 | |
91 | sub new { |
92 | my $class = shift; |
93 | my $self = {}; |
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94 | bless $self, $class; |
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95 | $self->initialize(); |
96 | return $self; |
97 | } |
98 | |
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99 | Or if you expect people to call not just C<CLASS-E<gt>new()> but also |
100 | C<$obj-E<gt>new()>, then use something like this. The initialize() |
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101 | method used will be of whatever $class we blessed the |
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102 | object into: |
103 | |
104 | sub new { |
105 | my $this = shift; |
106 | my $class = ref($this) || $this; |
107 | my $self = {}; |
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108 | bless $self, $class; |
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109 | $self->initialize(); |
110 | return $self; |
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111 | } |
112 | |
113 | Within the class package, the methods will typically deal with the |
114 | reference as an ordinary reference. Outside the class package, |
115 | the reference is generally treated as an opaque value that may |
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116 | be accessed only through the class's methods. |
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117 | |
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118 | A constructor may re-bless a referenced object currently belonging to |
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119 | another class, but then the new class is responsible for all cleanup |
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120 | later. The previous blessing is forgotten, as an object may belong |
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121 | to only one class at a time. (Although of course it's free to |
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122 | inherit methods from many classes.) If you find yourself having to |
123 | do this, the parent class is probably misbehaving, though. |
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124 | |
125 | A clarification: Perl objects are blessed. References are not. Objects |
126 | know which package they belong to. References do not. The bless() |
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127 | function uses the reference to find the object. Consider |
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128 | the following example: |
129 | |
130 | $a = {}; |
131 | $b = $a; |
132 | bless $a, BLAH; |
133 | print "\$b is a ", ref($b), "\n"; |
134 | |
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135 | This reports $b as being a BLAH, so obviously bless() |
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136 | operated on the object and not on the reference. |
137 | |
138 | =head2 A Class is Simply a Package |
139 | |
140 | Unlike say C++, Perl doesn't provide any special syntax for class |
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141 | definitions. You use a package as a class by putting method |
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142 | definitions into the class. |
143 | |
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144 | There is a special array within each package called @ISA, which says |
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145 | where else to look for a method if you can't find it in the current |
146 | package. This is how Perl implements inheritance. Each element of the |
147 | @ISA array is just the name of another package that happens to be a |
148 | class package. The classes are searched (depth first) for missing |
149 | methods in the order that they occur in @ISA. The classes accessible |
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150 | through @ISA are known as base classes of the current class. |
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151 | |
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152 | All classes implicitly inherit from class C<UNIVERSAL> as their |
153 | last base class. Several commonly used methods are automatically |
154 | supplied in the UNIVERSAL class; see L<"Default UNIVERSAL methods"> for |
155 | more details. |
156 | |
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157 | If a missing method is found in one of the base classes, it is cached |
158 | in the current class for efficiency. Changing @ISA or defining new |
159 | subroutines invalidates the cache and causes Perl to do the lookup again. |
160 | |
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161 | If neither the current class, its named base classes, nor the UNIVERSAL |
162 | class contains the requested method, these three places are searched |
163 | all over again, this time looking for a method named AUTOLOAD(). If an |
164 | AUTOLOAD is found, this method is called on behalf of the missing method, |
165 | setting the package global $AUTOLOAD to be the fully qualified name of |
166 | the method that was intended to be called. |
167 | |
168 | If none of that works, Perl finally gives up and complains. |
169 | |
170 | Perl classes do method inheritance only. Data inheritance is left up |
171 | to the class itself. By and large, this is not a problem in Perl, |
172 | because most classes model the attributes of their object using an |
173 | anonymous hash, which serves as its own little namespace to be carved up |
174 | by the various classes that might want to do something with the object. |
175 | The only problem with this is that you can't sure that you aren't using |
176 | a piece of the hash that isn't already used. A reasonable workaround |
177 | is to prepend your fieldname in the hash with the package name. |
178 | |
179 | sub bump { |
180 | my $self = shift; |
181 | $self->{ __PACKAGE__ . ".count"}++; |
182 | } |
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183 | |
184 | =head2 A Method is Simply a Subroutine |
185 | |
186 | Unlike say C++, Perl doesn't provide any special syntax for method |
187 | definition. (It does provide a little syntax for method invocation |
188 | though. More on that later.) A method expects its first argument |
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189 | to be the object (reference) or package (string) it is being invoked on. There are just two |
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190 | types of methods, which we'll call class and instance. |
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191 | (Sometimes you'll hear these called static and virtual, in honor of |
192 | the two C++ method types they most closely resemble.) |
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193 | |
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194 | A class method expects a class name as the first argument. It |
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195 | provides functionality for the class as a whole, not for any individual |
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196 | object belonging to the class. Constructors are typically class |
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197 | methods. Many class methods simply ignore their first argument, because |
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198 | they already know what package they're in, and don't care what package |
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199 | they were invoked via. (These aren't necessarily the same, because |
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200 | class methods follow the inheritance tree just like ordinary instance |
201 | methods.) Another typical use for class methods is to look up an |
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202 | object by name: |
203 | |
204 | sub find { |
205 | my ($class, $name) = @_; |
206 | $objtable{$name}; |
207 | } |
208 | |
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209 | An instance method expects an object reference as its first argument. |
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210 | Typically it shifts the first argument into a "self" or "this" variable, |
211 | and then uses that as an ordinary reference. |
212 | |
213 | sub display { |
214 | my $self = shift; |
215 | my @keys = @_ ? @_ : sort keys %$self; |
216 | foreach $key (@keys) { |
217 | print "\t$key => $self->{$key}\n"; |
218 | } |
219 | } |
220 | |
221 | =head2 Method Invocation |
222 | |
223 | There are two ways to invoke a method, one of which you're already |
224 | familiar with, and the other of which will look familiar. Perl 4 |
225 | already had an "indirect object" syntax that you use when you say |
226 | |
227 | print STDERR "help!!!\n"; |
228 | |
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229 | This same syntax can be used to call either class or instance methods. |
230 | We'll use the two methods defined above, the class method to lookup |
231 | an object reference and the instance method to print out its attributes. |
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232 | |
233 | $fred = find Critter "Fred"; |
234 | display $fred 'Height', 'Weight'; |
235 | |
236 | These could be combined into one statement by using a BLOCK in the |
237 | indirect object slot: |
238 | |
239 | display {find Critter "Fred"} 'Height', 'Weight'; |
240 | |
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241 | For C++ fans, there's also a syntax using -E<gt> notation that does exactly |
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242 | the same thing. The parentheses are required if there are any arguments. |
243 | |
244 | $fred = Critter->find("Fred"); |
245 | $fred->display('Height', 'Weight'); |
246 | |
247 | or in one statement, |
248 | |
249 | Critter->find("Fred")->display('Height', 'Weight'); |
250 | |
251 | There are times when one syntax is more readable, and times when the |
252 | other syntax is more readable. The indirect object syntax is less |
253 | cluttered, but it has the same ambiguity as ordinary list operators. |
254 | Indirect object method calls are parsed using the same rule as list |
255 | operators: "If it looks like a function, it is a function". (Presuming |
256 | for the moment that you think two words in a row can look like a |
257 | function name. C++ programmers seem to think so with some regularity, |
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258 | especially when the first word is "new".) Thus, the parentheses of |
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259 | |
260 | new Critter ('Barney', 1.5, 70) |
261 | |
262 | are assumed to surround ALL the arguments of the method call, regardless |
263 | of what comes after. Saying |
264 | |
265 | new Critter ('Bam' x 2), 1.4, 45 |
266 | |
267 | would be equivalent to |
268 | |
269 | Critter->new('Bam' x 2), 1.4, 45 |
270 | |
271 | which is unlikely to do what you want. |
272 | |
273 | There are times when you wish to specify which class's method to use. |
274 | In this case, you can call your method as an ordinary subroutine |
275 | call, being sure to pass the requisite first argument explicitly: |
276 | |
277 | $fred = MyCritter::find("Critter", "Fred"); |
278 | MyCritter::display($fred, 'Height', 'Weight'); |
279 | |
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280 | Note however, that this does not do any inheritance. If you wish |
281 | merely to specify that Perl should I<START> looking for a method in a |
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282 | particular package, use an ordinary method call, but qualify the method |
283 | name with the package like this: |
284 | |
285 | $fred = Critter->MyCritter::find("Fred"); |
286 | $fred->MyCritter::display('Height', 'Weight'); |
287 | |
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288 | If you're trying to control where the method search begins I<and> you're |
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289 | executing in the class itself, then you may use the SUPER pseudo class, |
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290 | which says to start looking in your base class's @ISA list without having |
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291 | to name it explicitly: |
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292 | |
293 | $self->SUPER::display('Height', 'Weight'); |
294 | |
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295 | Please note that the C<SUPER::> construct is meaningful I<only> within the |
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296 | class. |
297 | |
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298 | Sometimes you want to call a method when you don't know the method name |
299 | ahead of time. You can use the arrow form, replacing the method name |
300 | with a simple scalar variable containing the method name: |
301 | |
302 | $method = $fast ? "findfirst" : "findbest"; |
303 | $fred->$method(@args); |
304 | |
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305 | =head2 Default UNIVERSAL methods |
306 | |
307 | The C<UNIVERSAL> package automatically contains the following methods that |
308 | are inherited by all other classes: |
309 | |
310 | =over 4 |
311 | |
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312 | =item isa(CLASS) |
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313 | |
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314 | C<isa> returns I<true> if its object is blessed into a subclass of C<CLASS> |
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315 | |
316 | C<isa> is also exportable and can be called as a sub with two arguments. This |
317 | allows the ability to check what a reference points to. Example |
318 | |
319 | use UNIVERSAL qw(isa); |
320 | |
321 | if(isa($ref, 'ARRAY')) { |
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322 | #... |
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323 | } |
324 | |
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325 | =item can(METHOD) |
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326 | |
327 | C<can> checks to see if its object has a method called C<METHOD>, |
328 | if it does then a reference to the sub is returned, if it does not then |
329 | I<undef> is returned. |
330 | |
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331 | =item VERSION( [NEED] ) |
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332 | |
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333 | C<VERSION> returns the version number of the class (package). If the |
334 | NEED argument is given then it will check that the current version (as |
335 | defined by the $VERSION variable in the given package) not less than |
336 | NEED; it will die if this is not the case. This method is normally |
337 | called as a class method. This method is called automatically by the |
338 | C<VERSION> form of C<use>. |
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339 | |
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340 | use A 1.2 qw(some imported subs); |
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341 | # implies: |
342 | A->VERSION(1.2); |
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343 | |
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344 | =back |
345 | |
346 | B<NOTE:> C<can> directly uses Perl's internal code for method lookup, and |
347 | C<isa> uses a very similar method and cache-ing strategy. This may cause |
348 | strange effects if the Perl code dynamically changes @ISA in any package. |
349 | |
350 | You may add other methods to the UNIVERSAL class via Perl or XS code. |
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351 | You do not need to C<use UNIVERSAL> in order to make these methods |
352 | available to your program. This is necessary only if you wish to |
353 | have C<isa> available as a plain subroutine in the current package. |
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354 | |
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355 | =head2 Destructors |
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356 | |
357 | When the last reference to an object goes away, the object is |
358 | automatically destroyed. (This may even be after you exit, if you've |
359 | stored references in global variables.) If you want to capture control |
360 | just before the object is freed, you may define a DESTROY method in |
361 | your class. It will automatically be called at the appropriate moment, |
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362 | and you can do any extra cleanup you need to do. Perl passes a reference |
363 | to the object under destruction as the first (and only) argument. Beware |
364 | that the reference is a read-only value, and cannot be modified by |
365 | manipulating C<$_[0]> within the destructor. The object itself (i.e. |
366 | the thingy the reference points to, namely C<${$_[0]}>, C<@{$_[0]}>, |
367 | C<%{$_[0]}> etc.) is not similarly constrained. |
368 | |
369 | If you arrange to re-bless the reference before the destructor returns, |
370 | perl will again call the DESTROY method for the re-blessed object after |
371 | the current one returns. This can be used for clean delegation of |
372 | object destruction, or for ensuring that destructors in the base classes |
373 | of your choosing get called. Explicitly calling DESTROY is also possible, |
374 | but is usually never needed. |
375 | |
376 | Do not confuse the foregoing with how objects I<CONTAINED> in the current |
377 | one are destroyed. Such objects will be freed and destroyed automatically |
378 | when the current object is freed, provided no other references to them exist |
379 | elsewhere. |
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380 | |
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381 | =head2 WARNING |
382 | |
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383 | While indirect object syntax may well be appealing to English speakers and |
384 | to C++ programmers, be not seduced! It suffers from two grave problems. |
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385 | |
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386 | The first problem is that an indirect object is limited to a name, |
387 | a scalar variable, or a block, because it would have to do too much |
388 | lookahead otherwise, just like any other postfix dereference in the |
389 | language. (These are the same quirky rules as are used for the filehandle |
390 | slot in functions like C<print> and C<printf>.) This can lead to horribly |
391 | confusing precedence problems, as in these next two lines: |
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392 | |
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393 | move $obj->{FIELD}; # probably wrong! |
394 | move $ary[$i]; # probably wrong! |
395 | |
396 | Those actually parse as the very surprising: |
397 | |
398 | $obj->move->{FIELD}; # Well, lookee here |
399 | $ary->move->[$i]; # Didn't expect this one, eh? |
400 | |
401 | Rather than what you might have expected: |
402 | |
403 | $obj->{FIELD}->move(); # You should be so lucky. |
404 | $ary[$i]->move; # Yeah, sure. |
405 | |
406 | The left side of ``-E<gt>'' is not so limited, because it's an infix operator, |
407 | not a postfix operator. |
408 | |
409 | As if that weren't bad enough, think about this: Perl must guess I<at |
410 | compile time> whether C<name> and C<move> above are functions or methods. |
411 | Usually Perl gets it right, but when it doesn't it, you get a function |
412 | call compiled as a method, or vice versa. This can introduce subtle |
413 | bugs that are hard to unravel. For example, calling a method C<new> |
414 | in indirect notation--as C++ programmers are so wont to do--can |
415 | be miscompiled into a subroutine call if there's already a C<new> |
416 | function in scope. You'd end up calling the current package's C<new> |
417 | as a subroutine, rather than the desired class's method. The compiler |
418 | tries to cheat by remembering bareword C<require>s, but the grief if it |
419 | messes up just isn't worth the years of debugging it would likely take |
420 | you to to track such subtle bugs down. |
421 | |
422 | The infix arrow notation using ``C<-E<gt>>'' doesn't suffer from either |
423 | of these disturbing ambiguities, so we recommend you use it exclusively. |
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424 | |
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425 | =head2 Summary |
426 | |
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427 | That's about all there is to it. Now you need just to go off and buy a |
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428 | book about object-oriented design methodology, and bang your forehead |
429 | with it for the next six months or so. |
430 | |
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431 | =head2 Two-Phased Garbage Collection |
432 | |
433 | For most purposes, Perl uses a fast and simple reference-based |
434 | garbage collection system. For this reason, there's an extra |
435 | dereference going on at some level, so if you haven't built |
436 | your Perl executable using your C compiler's C<-O> flag, performance |
437 | will suffer. If you I<have> built Perl with C<cc -O>, then this |
438 | probably won't matter. |
439 | |
440 | A more serious concern is that unreachable memory with a non-zero |
441 | reference count will not normally get freed. Therefore, this is a bad |
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442 | idea: |
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443 | |
444 | { |
445 | my $a; |
446 | $a = \$a; |
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447 | } |
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448 | |
449 | Even thought $a I<should> go away, it can't. When building recursive data |
450 | structures, you'll have to break the self-reference yourself explicitly |
451 | if you don't care to leak. For example, here's a self-referential |
452 | node such as one might use in a sophisticated tree structure: |
453 | |
454 | sub new_node { |
455 | my $self = shift; |
456 | my $class = ref($self) || $self; |
457 | my $node = {}; |
458 | $node->{LEFT} = $node->{RIGHT} = $node; |
459 | $node->{DATA} = [ @_ ]; |
460 | return bless $node => $class; |
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461 | } |
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462 | |
463 | If you create nodes like that, they (currently) won't go away unless you |
464 | break their self reference yourself. (In other words, this is not to be |
465 | construed as a feature, and you shouldn't depend on it.) |
466 | |
467 | Almost. |
468 | |
469 | When an interpreter thread finally shuts down (usually when your program |
470 | exits), then a rather costly but complete mark-and-sweep style of garbage |
471 | collection is performed, and everything allocated by that thread gets |
472 | destroyed. This is essential to support Perl as an embedded or a |
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473 | multithreadable language. For example, this program demonstrates Perl's |
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474 | two-phased garbage collection: |
475 | |
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476 | #!/usr/bin/perl |
cb1a09d0 |
477 | package Subtle; |
478 | |
479 | sub new { |
480 | my $test; |
481 | $test = \$test; |
482 | warn "CREATING " . \$test; |
483 | return bless \$test; |
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484 | } |
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485 | |
486 | sub DESTROY { |
487 | my $self = shift; |
488 | warn "DESTROYING $self"; |
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489 | } |
cb1a09d0 |
490 | |
491 | package main; |
492 | |
493 | warn "starting program"; |
494 | { |
495 | my $a = Subtle->new; |
496 | my $b = Subtle->new; |
497 | $$a = 0; # break selfref |
498 | warn "leaving block"; |
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499 | } |
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500 | |
501 | warn "just exited block"; |
502 | warn "time to die..."; |
503 | exit; |
504 | |
505 | When run as F</tmp/test>, the following output is produced: |
506 | |
507 | starting program at /tmp/test line 18. |
508 | CREATING SCALAR(0x8e5b8) at /tmp/test line 7. |
509 | CREATING SCALAR(0x8e57c) at /tmp/test line 7. |
510 | leaving block at /tmp/test line 23. |
511 | DESTROYING Subtle=SCALAR(0x8e5b8) at /tmp/test line 13. |
512 | just exited block at /tmp/test line 26. |
513 | time to die... at /tmp/test line 27. |
514 | DESTROYING Subtle=SCALAR(0x8e57c) during global destruction. |
515 | |
516 | Notice that "global destruction" bit there? That's the thread |
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517 | garbage collector reaching the unreachable. |
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518 | |
519 | Objects are always destructed, even when regular refs aren't and in fact |
520 | are destructed in a separate pass before ordinary refs just to try to |
521 | prevent object destructors from using refs that have been themselves |
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522 | destructed. Plain refs are only garbage-collected if the destruct level |
cb1a09d0 |
523 | is greater than 0. You can test the higher levels of global destruction |
524 | by setting the PERL_DESTRUCT_LEVEL environment variable, presuming |
525 | C<-DDEBUGGING> was enabled during perl build time. |
526 | |
527 | A more complete garbage collection strategy will be implemented |
528 | at a future date. |
529 | |
5a964f20 |
530 | In the meantime, the best solution is to create a non-recursive container |
531 | class that holds a pointer to the self-referential data structure. |
532 | Define a DESTROY method for the containing object's class that manually |
533 | breaks the circularities in the self-referential structure. |
534 | |
a0d0e21e |
535 | =head1 SEE ALSO |
536 | |
54310121 |
537 | A kinder, gentler tutorial on object-oriented programming in Perl can |
5f05dabc |
538 | be found in L<perltoot>. |
54310121 |
539 | You should also check out L<perlbot> for other object tricks, traps, and tips, |
f102b883 |
540 | as well as L<perlmodlib> for some style guides on constructing both modules |
cb1a09d0 |
541 | and classes. |