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1 | =head1 NAME |
2 | |
3 | perltoot - Tom's object-oriented tutorial for perl |
4 | |
5 | =head1 DESCRIPTION |
6 | |
7 | Object-oriented programming is a big seller these days. Some managers |
8 | would rather have objects than sliced bread. Why is that? What's so |
9 | special about an object? Just what I<is> an object anyway? |
10 | |
11 | An object is nothing but a way of tucking away complex behaviours into |
12 | a neat little easy-to-use bundle. (This is what professors call |
13 | abstraction.) Smart people who have nothing to do but sit around for |
14 | weeks on end figuring out really hard problems make these nifty |
15 | objects that even regular people can use. (This is what professors call |
16 | software reuse.) Users (well, programmers) can play with this little |
17 | bundle all they want, but they aren't to open it up and mess with the |
18 | insides. Just like an expensive piece of hardware, the contract says |
19 | that you void the warranty if you muck with the cover. So don't do that. |
20 | |
21 | The heart of objects is the class, a protected little private namespace |
22 | full of data and functions. A class is a set of related routines that |
23 | addresses some problem area. You can think of it as a user-defined type. |
24 | The Perl package mechanism, also used for more traditional modules, |
25 | is used for class modules as well. Objects "live" in a class, meaning |
26 | that they belong to some package. |
27 | |
28 | More often than not, the class provides the user with little bundles. |
29 | These bundles are objects. They know whose class they belong to, |
30 | and how to behave. Users ask the class to do something, like "give |
31 | me an object." Or they can ask one of these objects to do something. |
32 | Asking a class to do something for you is calling a I<class method>. |
33 | Asking an object to do something for you is calling an I<object method>. |
34 | Asking either a class (usually) or an object (sometimes) to give you |
35 | back an object is calling a I<constructor>, which is just a |
36 | kind of method. |
37 | |
38 | That's all well and good, but how is an object different from any other |
39 | Perl data type? Just what is an object I<really>; that is, what's its |
40 | fundamental type? The answer to the first question is easy. An object |
41 | is different from any other data type in Perl in one and only one way: |
42 | you may dereference it using not merely string or numeric subscripts |
43 | as with simple arrays and hashes, but with named subroutine calls. |
44 | In a word, with I<methods>. |
45 | |
46 | The answer to the second question is that it's a reference, and not just |
47 | any reference, mind you, but one whose referent has been I<bless>()ed |
48 | into a particular class (read: package). What kind of reference? Well, |
49 | the answer to that one is a bit less concrete. That's because in Perl |
50 | the designer of the class can employ any sort of reference they'd like |
51 | as the underlying intrinsic data type. It could be a scalar, an array, |
52 | or a hash reference. It could even be a code reference. But because |
53 | of its inherent flexibility, an object is usually a hash reference. |
54 | |
55 | =head1 Creating a Class |
56 | |
57 | Before you create a class, you need to decide what to name it. That's |
58 | because the class (package) name governs the name of the file used to |
59 | house it, just as with regular modules. Then, that class (package) |
60 | should provide one or more ways to generate objects. Finally, it should |
61 | provide mechanisms to allow users of its objects to indirectly manipulate |
62 | these objects from a distance. |
63 | |
64 | For example, let's make a simple Person class module. It gets stored in |
65 | the file Person.pm. If it were called a Happy::Person class, it would |
66 | be stored in the file Happy/Person.pm, and its package would become |
67 | Happy::Person instead of just Person. (On a personal computer not |
68 | running Unix or Plan 9, but something like MacOS or VMS, the directory |
69 | separator may be different, but the principle is the same.) Do not assume |
70 | any formal relationship between modules based on their directory names. |
71 | This is merely a grouping convenience, and has no effect on inheritance, |
72 | variable accessibility, or anything else. |
73 | |
74 | For this module we aren't going to use Exporter, because we're |
75 | a well-behaved class module that doesn't export anything at all. |
76 | In order to manufacture objects, a class needs to have a I<constructor |
77 | method>. A constructor gives you back not just a regular data type, |
78 | but a brand-new object in that class. This magic is taken care of by |
79 | the bless() function, whose sole purpose is to enable its referent to |
80 | be used as an object. Remember: being an object really means nothing |
81 | more than that methods may now be called against it. |
82 | |
83 | While a constructor may be named anything you'd like, most Perl |
84 | programmers seem to like to call theirs new(). However, new() is not |
85 | a reserved word, and a class is under no obligation to supply such. |
86 | Some programmers have also been known to use a function with |
87 | the same name as the class as the constructor. |
88 | |
89 | =head2 Object Representation |
90 | |
91 | By far the most common mechanism used in Perl to represent a Pascal |
92 | record, a C struct, or a C++ class an anonymous hash. That's because a |
93 | hash has an arbitrary number of data fields, each conveniently accessed by |
94 | an arbitrary name of your own devising. |
95 | |
96 | If you were just doing a simple |
97 | struct-like emulation, you would likely go about it something like this: |
98 | |
99 | $rec = { |
100 | name => "Jason", |
101 | age => 23, |
102 | peers => [ "Norbert", "Rhys", "Phineas"], |
103 | }; |
104 | |
105 | If you felt like it, you could add a bit of visual distinction |
106 | by up-casing the hash keys: |
107 | |
108 | $rec = { |
109 | NAME => "Jason", |
110 | AGE => 23, |
111 | PEERS => [ "Norbert", "Rhys", "Phineas"], |
112 | }; |
113 | |
114 | And so you could get at C<$rec-E<gt>{NAME}> to find "Jason", or |
115 | C<@{ $rec-E<gt>{PEERS} }> to get at "Norbert", "Rhys", and "Phineas". |
116 | (Have you ever noticed how many 23-year-old programmers seem to |
117 | be named "Jason" these days? :-) |
118 | |
119 | This same model is often used for classes, although it is not considered |
120 | the pinnacle of programming propriety for folks from outside the |
121 | class to come waltzing into an object, brazenly accessing its data |
122 | members directly. Generally speaking, an object should be considered |
123 | an opaque cookie that you use I<object methods> to access. Visually, |
124 | methods look like you're dereffing a reference using a function name |
125 | instead of brackets or braces. |
126 | |
127 | =head2 Class Interface |
128 | |
129 | Some languages provide a formal syntactic interface to a class's methods, |
130 | but Perl does not. It relies on you to read the documentation of each |
131 | class. If you try to call an undefined method on an object, Perl won't |
132 | complain, but the program will trigger an exception while it's running. |
133 | Likewise, if you call a method expecting a prime number as its argument |
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134 | with a non-prime one instead, you can't expect the compiler to catch this. |
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135 | (Well, you can expect it all you like, but it's not going to happen.) |
136 | |
137 | Let's suppose you have a well-educated user of your Person class, |
138 | someone who has read the docs that explain the prescribed |
139 | interface. Here's how they might use the Person class: |
140 | |
141 | use Person; |
142 | |
143 | $him = Person->new(); |
144 | $him->name("Jason"); |
145 | $him->age(23); |
146 | $him->peers( "Norbert", "Rhys", "Phineas" ); |
147 | |
148 | push @All_Recs, $him; # save object in array for later |
149 | |
150 | printf "%s is %d years old.\n", $him->name, $him->age; |
151 | print "His peers are: ", join(", ", $him->peers), "\n"; |
152 | |
153 | printf "Last rec's name is %s\n", $All_Recs[-1]->name; |
154 | |
155 | As you can see, the user of the class doesn't know (or at least, has no |
156 | business paying attention to the fact) that the object has one particular |
157 | implementation or another. The interface to the class and its objects |
158 | is exclusively via methods, and that's all the user of the class should |
159 | ever play with. |
160 | |
161 | =head2 Constructors and Instance Methods |
162 | |
163 | Still, I<someone> has to know what's in the object. And that someone is |
164 | the class. It implements methods that the programmer uses to access |
165 | the object. Here's how to implement the Person class using the standard |
166 | hash-ref-as-an-object idiom. We'll make a class method called new() to |
167 | act as the constructor, and three object methods called name(), age(), and |
168 | peers() to get at per-object data hidden away in our anonymous hash. |
169 | |
170 | package Person; |
171 | use strict; |
172 | |
173 | ################################################## |
174 | ## the object constructor (simplistic version) ## |
175 | ################################################## |
176 | sub new { |
177 | my $self = {}; |
178 | $self->{NAME} = undef; |
179 | $self->{AGE} = undef; |
180 | $self->{PEERS} = []; |
181 | bless($self); # but see below |
182 | return $self; |
183 | } |
184 | |
185 | ############################################## |
186 | ## methods to access per-object data ## |
187 | ## ## |
188 | ## With args, they set the value. Without ## |
189 | ## any, they only retrieve it/them. ## |
190 | ############################################## |
191 | |
192 | sub name { |
193 | my $self = shift; |
194 | if (@_) { $self->{NAME} = shift } |
195 | return $self->{NAME}; |
196 | } |
197 | |
198 | sub age { |
199 | my $self = shift; |
200 | if (@_) { $self->{AGE} = shift } |
201 | return $self->{AGE}; |
202 | } |
203 | |
204 | sub peers { |
205 | my $self = shift; |
206 | if (@_) { @{ $self->{PEERS} } = @_ } |
207 | return @{ $self->{PEERS} }; |
208 | } |
209 | |
210 | 1; # so the require or use succeeds |
211 | |
212 | We've created three methods to access an object's data, name(), age(), |
213 | and peers(). These are all substantially similar. If called with an |
214 | argument, they set the appropriate field; otherwise they return the |
215 | value held by that field, meaning the value of that hash key. |
216 | |
217 | =head2 Planning for the Future: Better Constructors |
218 | |
219 | Even though at this point you may not even know what it means, someday |
220 | you're going to worry about inheritance. (You can safely ignore this |
221 | for now and worry about it later if you'd like.) To ensure that this |
222 | all works out smoothly, you must use the double-argument form of bless(). |
223 | The second argument is the class into which the referent will be blessed. |
224 | By not assuming our own class as the default second argument and instead |
225 | using the class passed into us, we make our constructor inheritable. |
226 | |
227 | While we're at it, let's make our constructor a bit more flexible. |
228 | Rather than being uniquely a class method, we'll set it up so that |
229 | it can be called as either a class method I<or> an object |
230 | method. That way you can say: |
231 | |
232 | $me = Person->new(); |
233 | $him = $me->new(); |
234 | |
235 | To do this, all we have to do is check whether what was passed in |
236 | was a reference or not. If so, we were invoked as an object method, |
237 | and we need to extract the package (class) using the ref() function. |
238 | If not, we just use the string passed in as the package name |
239 | for blessing our referent. |
240 | |
241 | sub new { |
242 | my $proto = shift; |
243 | my $class = ref($proto) || $proto; |
244 | my $self = {}; |
245 | $self->{NAME} = undef; |
246 | $self->{AGE} = undef; |
247 | $self->{PEERS} = []; |
248 | bless ($self, $class); |
249 | return $self; |
250 | } |
251 | |
252 | That's about all there is for constructors. These methods bring objects |
253 | to life, returning neat little opaque bundles to the user to be used in |
254 | subsequent method calls. |
255 | |
256 | =head2 Destructors |
257 | |
258 | Every story has a beginning and an end. The beginning of the object's |
259 | story is its constructor, explicitly called when the object comes into |
260 | existence. But the ending of its story is the I<destructor>, a method |
261 | implicitly called when an object leaves this life. Any per-object |
262 | clean-up code is placed in the destructor, which must (in Perl) be called |
263 | DESTROY. |
264 | |
265 | If constructors can have arbitrary names, then why not destructors? |
266 | Because while a constructor is explicitly called, a destructor is not. |
267 | Destruction happens automatically via Perl's garbage collection (GC) |
268 | system, which is a quick but somewhat lazy reference-based GC system. |
269 | To know what to call, Perl insists that the destructor be named DESTROY. |
270 | |
271 | Why is DESTROY in all caps? Perl on occasion uses purely upper-case |
272 | function names as a convention to indicate that the function will |
273 | be automatically called by Perl in some way. Others that are called |
274 | implicitly include BEGIN, END, AUTOLOAD, plus all methods used by |
275 | tied objects, described in L<perltie>. |
276 | |
277 | In really good object-oriented programming languages, the user doesn't |
278 | care when the destructor is called. It just happens when it's supposed |
279 | to. In low-level languages without any GC at all, there's no way to |
280 | depend on this happening at the right time, so the programmer must |
281 | explicitly call the destructor to clean up memory and state, crossing |
282 | their fingers that it's the right time to do so. Unlike C++, an |
283 | object destructor is nearly never needed in Perl, and even when it is, |
284 | explicit invocation is uncalled for. In the case of our Person class, |
285 | we don't need a destructor because Perl takes care of simple matters |
286 | like memory deallocation. |
287 | |
288 | The only situation where Perl's reference-based GC won't work is |
289 | when there's a circularity in the data structure, such as: |
290 | |
291 | $this->{WHATEVER} = $this; |
292 | |
293 | In that case, you must delete the self-reference manually if you expect |
294 | your program not to leak memory. While admittedly error-prone, this is |
295 | the best we can do right now. Nonetheless, rest assured that when your |
296 | program is finished, its objects' destructors are all duly called. |
297 | So you are guaranteed that an object I<eventually> gets properly |
298 | destroyed, except in the unique case of a program that never exits. |
299 | (If you're running Perl embedded in another application, this full GC |
300 | pass happens a bit more frequently--whenever a thread shuts down.) |
301 | |
302 | =head2 Other Object Methods |
303 | |
304 | The methods we've talked about so far have either been constructors or |
305 | else simple "data methods", interfaces to data stored in the object. |
306 | These are a bit like an object's data members in the C++ world, except |
307 | that strangers don't access them as data. Instead, they should only |
308 | access the object's data indirectly via its methods. This is an |
309 | important rule: in Perl, access to an object's data should I<only> |
310 | be made through methods. |
311 | |
312 | Perl doesn't impose restrictions on who gets to use which methods. |
313 | The public-versus-private distinction is by convention, not syntax. |
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314 | (Well, unless you use the Alias module described below in |
315 | L</"Data Members as Variables">.) Occasionally you'll see method names beginning or ending |
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316 | with an underscore or two. This marking is a convention indicating |
317 | that the methods are private to that class alone and sometimes to its |
318 | closest acquaintances, its immediate subclasses. But this distinction |
319 | is not enforced by Perl itself. It's up to the programmer to behave. |
320 | |
321 | There's no reason to limit methods to those that simply access data. |
322 | Methods can do anything at all. The key point is that they're invoked |
323 | against an object or a class. Let's say we'd like object methods that |
324 | do more than fetch or set one particular field. |
325 | |
326 | sub exclaim { |
327 | my $self = shift; |
328 | return sprintf "Hi, I'm %s, age %d, working with %s", |
329 | $self->{NAME}, $self->{AGE}, join(", ", $self->{PEERS}); |
330 | } |
331 | |
332 | Or maybe even one like this: |
333 | |
334 | sub happy_birthday { |
335 | my $self = shift; |
336 | return ++$self->{AGE}; |
337 | } |
338 | |
339 | Some might argue that one should go at these this way: |
340 | |
341 | sub exclaim { |
342 | my $self = shift; |
343 | return sprintf "Hi, I'm %s, age %d, working with %s", |
344 | $self->name, $self->age, join(", ", $self->peers); |
345 | } |
346 | |
347 | sub happy_birthday { |
348 | my $self = shift; |
349 | return $self->age( $self->age() + 1 ); |
350 | } |
351 | |
352 | But since these methods are all executing in the class itself, this |
353 | may not be critical. There are trade-offs to be made. Using direct |
354 | hash access is faster (about an order of magnitude faster, in fact), and |
355 | it's more convenient when you want to interpolate in strings. But using |
356 | methods (the external interface) internally shields not just the users of |
357 | your class but even you yourself from changes in your data representation. |
358 | |
359 | =head1 Class Data |
360 | |
361 | What about "class data", data items common to each object in a class? |
362 | What would you want that for? Well, in your Person class, you might |
363 | like to keep track of the total people alive. How do you implement that? |
364 | |
365 | You I<could> make it a global variable called $Person::Census. But about |
366 | only reason you'd do that would be if you I<wanted> people to be able to |
367 | get at your class data directly. They could just say $Person::Census |
368 | and play around with it. Maybe this is ok in your design scheme. |
369 | You might even conceivably want to make it an exported variable. To be |
370 | exportable, a variable must be a (package) global. If this were a |
371 | traditional module rather than an object-oriented one, you might do that. |
372 | |
373 | While this approach is expected in most traditional modules, it's |
374 | generally considered rather poor form in most object modules. In an |
375 | object module, you should set up a protective veil to separate interface |
376 | from implementation. So provide a class method to access class data |
377 | just as you provide object methods to access object data. |
378 | |
379 | So, you I<could> still keep $Census as a package global and rely upon |
380 | others to honor the contract of the module and therefore not play around |
381 | with its implementation. You could even be supertricky and make $Census a |
382 | tied object as described in L<perltie>, thereby intercepting all accesses. |
383 | |
384 | But more often than not, you just want to make your class data a |
385 | file-scoped lexical. To do so, simply put this at the top of the file: |
386 | |
387 | my $Census = 0; |
388 | |
389 | Even though the scope of a my() normally expires when the block in which |
390 | it was declared is done (in this case the whole file being required or |
391 | used), Perl's deep binding of lexical variables guarantees that the |
392 | variable will not be deallocated, remaining accessible to functions |
393 | declared within that scope. This doesn't work with global variables |
394 | given temporary values via local(), though. |
395 | |
396 | Irrespective of whether you leave $Census a package global or make |
397 | it instead a file-scoped lexical, you should make these |
398 | changes to your Person::new() constructor: |
399 | |
400 | sub new { |
401 | my $proto = shift; |
402 | my $class = ref($proto) || $proto; |
403 | my $self = {}; |
404 | $Census++; |
405 | $self->{NAME} = undef; |
406 | $self->{AGE} = undef; |
407 | $self->{PEERS} = []; |
408 | bless ($self, $class); |
409 | return $self; |
410 | } |
411 | |
412 | sub population { |
413 | return $Census; |
414 | } |
415 | |
416 | Now that we've done this, we certainly do need a destructor so that |
417 | when Person is destroyed, the $Census goes down. Here's how |
418 | this could be done: |
419 | |
420 | sub DESTROY { --$Census } |
421 | |
422 | Notice how there's no memory to deallocate in the destructor? That's |
423 | something that Perl takes care of for you all by itself. |
424 | |
425 | =head2 Accessing Class Data |
426 | |
427 | It turns out that this is not really a good way to go about handling |
428 | class data. A good scalable rule is that I<you must never reference class |
429 | data directly from an object method>. Otherwise you aren't building a |
430 | scalable, inheritable class. The object must be the rendezvous point |
431 | for all operations, especially from an object method. The globals |
432 | (class data) would in some sense be in the "wrong" package in your |
433 | derived classes. In Perl, methods execute in the context of the class |
434 | they were defined in, I<not> that of the object that triggered them. |
435 | Therefore, namespace visibility of package globals in methods is unrelated |
436 | to inheritance. |
437 | |
438 | Got that? Maybe not. Ok, let's say that some other class "borrowed" |
439 | (well, inherited) the DESTROY method as it was defined above. When those |
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440 | objects are destroyed, the original $Census variable will be altered, |
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441 | not the one in the new class's package namespace. Perhaps this is what |
442 | you want, but probably it isn't. |
443 | |
444 | Here's how to fix this. We'll store a reference to the data in the |
445 | value accessed by the hash key "_CENSUS". Why the underscore? Well, |
446 | mostly because an initial underscore already conveys strong feelings |
447 | of magicalness to a C programmer. It's really just a mnemonic device |
448 | to remind ourselves that this field is special and not to be used as |
449 | a public data member in the same way that NAME, AGE, and PEERS are. |
450 | (Because we've been developing this code under the strict pragma, prior |
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451 | to perl version 5.004 we'll have to quote the field name.) |
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452 | |
453 | sub new { |
454 | my $proto = shift; |
455 | my $class = ref($proto) || $proto; |
456 | my $self = {}; |
457 | $self->{NAME} = undef; |
458 | $self->{AGE} = undef; |
459 | $self->{PEERS} = []; |
460 | # "private" data |
461 | $self->{"_CENSUS"} = \$Census; |
462 | bless ($self, $class); |
463 | ++ ${ $self->{"_CENSUS"} }; |
464 | return $self; |
465 | } |
466 | |
467 | sub population { |
468 | my $self = shift; |
469 | if (ref $self) { |
470 | return ${ $self->{"_CENSUS"} }; |
471 | } else { |
472 | return $Census; |
473 | } |
474 | } |
475 | |
476 | sub DESTROY { |
477 | my $self = shift; |
478 | -- ${ $self->{"_CENSUS"} }; |
479 | } |
480 | |
481 | =head2 Debugging Methods |
482 | |
483 | It's common for a class to have a debugging mechanism. For example, |
484 | you might want to see when objects are created or destroyed. To do that, |
485 | add a debugging variable as a file-scoped lexical. For this, we'll pull |
486 | in the standard Carp module to emit our warnings and fatal messages. |
487 | That way messages will come out with the caller's filename and |
488 | line number instead of our own; if we wanted them to be from our own |
489 | perspective, we'd just use die() and warn() directly instead of croak() |
490 | and carp() respectively. |
491 | |
492 | use Carp; |
493 | my $Debugging = 0; |
494 | |
495 | Now add a new class method to access the variable. |
496 | |
497 | sub debug { |
498 | my $class = shift; |
499 | if (ref $class) { confess "Class method called as object method" } |
500 | unless (@_ == 1) { confess "usage: CLASSNAME->debug(level)" } |
501 | $Debugging = shift; |
502 | } |
503 | |
504 | Now fix up DESTROY to murmur a bit as the moribund object expires: |
505 | |
506 | sub DESTROY { |
507 | my $self = shift; |
508 | if ($Debugging) { carp "Destroying $self " . $self->name } |
509 | -- ${ $self->{"_CENSUS"} }; |
510 | } |
511 | |
512 | One could conceivably make a per-object debug state. That |
513 | way you could call both of these: |
514 | |
515 | Person->debug(1); # entire class |
516 | $him->debug(1); # just this object |
517 | |
518 | To do so, we need our debugging method to be a "bimodal" one, one that |
519 | works on both classes I<and> objects. Therefore, adjust the debug() |
520 | and DESTROY methods as follows: |
521 | |
522 | sub debug { |
523 | my $self = shift; |
524 | confess "usage: thing->debug(level)" unless @_ == 1; |
525 | my $level = shift; |
526 | if (ref($self)) { |
527 | $self->{"_DEBUG"} = $level; # just myself |
528 | } else { |
529 | $Debugging = $level; # whole class |
530 | } |
531 | } |
532 | |
533 | sub DESTROY { |
534 | my $self = shift; |
535 | if ($Debugging || $self->{"_DEBUG"}) { |
536 | carp "Destroying $self " . $self->name; |
537 | } |
538 | -- ${ $self->{"_CENSUS"} }; |
539 | } |
540 | |
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541 | What happens if a derived class (which we'll call Employee) inherits |
542 | methods from this Person base class? Then C<Employee-E<gt>debug()>, when called |
543 | as a class method, manipulates $Person::Debugging not $Employee::Debugging. |
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544 | |
545 | =head2 Class Destructors |
546 | |
547 | The object destructor handles the death of each distinct object. But sometimes |
548 | you want a bit of cleanup when the entire class is shut down, which |
549 | currently only happens when the program exits. To make such a |
550 | I<class destructor>, create a function in that class's package named |
551 | END. This works just like the END function in traditional modules, |
552 | meaning that it gets called whenever your program exits unless it execs |
553 | or dies of an uncaught signal. For example, |
554 | |
555 | sub END { |
556 | if ($Debugging) { |
557 | print "All persons are going away now.\n"; |
558 | } |
559 | } |
560 | |
561 | When the program exits, all the class destructors (END functions) are |
562 | be called in the opposite order that they were loaded in (LIFO order). |
563 | |
564 | =head2 Documenting the Interface |
565 | |
566 | And there you have it: we've just shown you the I<implementation> of this |
567 | Person class. Its I<interface> would be its documentation. Usually this |
568 | means putting it in pod ("plain old documentation") format right there |
569 | in the same file. In our Person example, we would place the following |
570 | docs anywhere in the Person.pm file. Even though it looks mostly like |
571 | code, it's not. It's embedded documentation such as would be used by |
572 | the pod2man, pod2html, or pod2text programs. The Perl compiler ignores |
573 | pods entirely, just as the translators ignore code. Here's an example of |
574 | some pods describing the informal interface: |
575 | |
576 | =head1 NAME |
577 | |
578 | Person - class to implement people |
579 | |
580 | =head1 SYNOPSIS |
581 | |
582 | use Person; |
583 | |
584 | ################# |
585 | # class methods # |
586 | ################# |
587 | $ob = Person->new; |
588 | $count = Person->population; |
589 | |
590 | ####################### |
591 | # object data methods # |
592 | ####################### |
593 | |
594 | ### get versions ### |
595 | $who = $ob->name; |
596 | $years = $ob->age; |
597 | @pals = $ob->peers; |
598 | |
599 | ### set versions ### |
600 | $ob->name("Jason"); |
601 | $ob->age(23); |
602 | $ob->peers( "Norbert", "Rhys", "Phineas" ); |
603 | |
604 | ######################## |
605 | # other object methods # |
606 | ######################## |
607 | |
608 | $phrase = $ob->exclaim; |
609 | $ob->happy_birthday; |
610 | |
611 | =head1 DESCRIPTION |
612 | |
613 | The Person class implements dah dee dah dee dah.... |
614 | |
615 | That's all there is to the matter of interface versus implementation. |
616 | A programmer who opens up the module and plays around with all the private |
617 | little shiny bits that were safely locked up behind the interface contract |
618 | has voided the warranty, and you shouldn't worry about their fate. |
619 | |
620 | =head1 Aggregation |
621 | |
622 | Suppose you later want to change the class to implement better names. |
623 | Perhaps you'd like to support both given names (called Christian names, |
624 | irrespective of one's religion) and family names (called surnames), plus |
625 | nicknames and titles. If users of your Person class have been properly |
626 | accessing it through its documented interface, then you can easily change |
627 | the underlying implementation. If they haven't, then they lose and |
628 | it's their fault for breaking the contract and voiding their warranty. |
629 | |
630 | To do this, we'll make another class, this one called Fullname. What's |
631 | the Fullname class look like? To answer that question, you have to |
632 | first figure out how you want to use it. How about we use it this way: |
633 | |
634 | $him = Person->new(); |
635 | $him->fullname->title("St"); |
636 | $him->fullname->christian("Thomas"); |
637 | $him->fullname->surname("Aquinas"); |
638 | $him->fullname->nickname("Tommy"); |
639 | printf "His normal name is %s\n", $him->name; |
640 | printf "But his real name is %s\n", $him->fullname->as_string; |
641 | |
642 | Ok. To do this, we'll change Person::new() so that it supports |
643 | a full name field this way: |
644 | |
645 | sub new { |
646 | my $proto = shift; |
647 | my $class = ref($proto) || $proto; |
648 | my $self = {}; |
649 | $self->{FULLNAME} = Fullname->new(); |
650 | $self->{AGE} = undef; |
651 | $self->{PEERS} = []; |
652 | $self->{"_CENSUS"} = \$Census; |
653 | bless ($self, $class); |
654 | ++ ${ $self->{"_CENSUS"} }; |
655 | return $self; |
656 | } |
657 | |
658 | sub fullname { |
659 | my $self = shift; |
660 | return $self->{FULLNAME}; |
661 | } |
662 | |
663 | Then to support old code, define Person::name() this way: |
664 | |
665 | sub name { |
666 | my $self = shift; |
667 | return $self->{FULLNAME}->nickname(@_) |
668 | || $self->{FULLNAME}->christian(@_); |
669 | } |
670 | |
671 | Here's the Fullname class. We'll use the same technique |
672 | of using a hash reference to hold data fields, and methods |
673 | by the appropriate name to access them: |
674 | |
675 | package Fullname; |
676 | use strict; |
677 | |
678 | sub new { |
679 | my $proto = shift; |
680 | my $class = ref($proto) || $proto; |
681 | my $self = { |
682 | TITLE => undef, |
683 | CHRISTIAN => undef, |
684 | SURNAME => undef, |
685 | NICK => undef, |
686 | }; |
687 | bless ($self, $class); |
688 | return $self; |
689 | } |
690 | |
691 | sub christian { |
692 | my $self = shift; |
693 | if (@_) { $self->{CHRISTIAN} = shift } |
694 | return $self->{CHRISTIAN}; |
695 | } |
696 | |
697 | sub surname { |
698 | my $self = shift; |
699 | if (@_) { $self->{SURNAME} = shift } |
700 | return $self->{SURNAME}; |
701 | } |
702 | |
703 | sub nickname { |
704 | my $self = shift; |
705 | if (@_) { $self->{NICK} = shift } |
706 | return $self->{NICK}; |
707 | } |
708 | |
709 | sub title { |
710 | my $self = shift; |
711 | if (@_) { $self->{TITLE} = shift } |
712 | return $self->{TITLE}; |
713 | } |
714 | |
715 | sub as_string { |
716 | my $self = shift; |
717 | my $name = join(" ", @$self{'CHRISTIAN', 'SURNAME'}); |
718 | if ($self->{TITLE}) { |
719 | $name = $self->{TITLE} . " " . $name; |
720 | } |
721 | return $name; |
722 | } |
723 | |
724 | 1; |
725 | |
726 | Finally, here's the test program: |
727 | |
728 | #!/usr/bin/perl -w |
729 | use strict; |
730 | use Person; |
731 | sub END { show_census() } |
732 | |
733 | sub show_census () { |
734 | printf "Current population: %d\n", Person->population; |
735 | } |
736 | |
737 | Person->debug(1); |
738 | |
739 | show_census(); |
740 | |
741 | my $him = Person->new(); |
742 | |
743 | $him->fullname->christian("Thomas"); |
744 | $him->fullname->surname("Aquinas"); |
745 | $him->fullname->nickname("Tommy"); |
746 | $him->fullname->title("St"); |
747 | $him->age(1); |
748 | |
749 | printf "%s is really %s.\n", $him->name, $him->fullname; |
750 | printf "%s's age: %d.\n", $him->name, $him->age; |
751 | $him->happy_birthday; |
752 | printf "%s's age: %d.\n", $him->name, $him->age; |
753 | |
754 | show_census(); |
755 | |
756 | =head1 Inheritance |
757 | |
758 | Object-oriented programming systems all support some notion of |
759 | inheritance. Inheritance means allowing one class to piggy-back on |
760 | top of another one so you don't have to write the same code again and |
761 | again. It's about software reuse, and therefore related to Laziness, |
762 | the principal virtue of a programmer. (The import/export mechanisms in |
763 | traditional modules are also a form of code reuse, but a simpler one than |
764 | the true inheritance that you find in object modules.) |
765 | |
766 | Sometimes the syntax of inheritance is built into the core of the |
767 | language, and sometimes it's not. Perl has no special syntax for |
768 | specifying the class (or classes) to inherit from. Instead, it's all |
769 | strictly in the semantics. Each package can have a variable called @ISA, |
770 | which governs (method) inheritance. If you try to call a method on an |
771 | object or class, and that method is not found in that object's package, |
772 | Perl then looks to @ISA for other packages to go looking through in |
773 | search of the missing method. |
774 | |
775 | Like the special per-package variables recognized by Exporter (such as |
776 | @EXPORT, @EXPORT_OK, @EXPORT_FAIL, %EXPORT_TAGS, and $VERSION), the @ISA |
777 | array I<must> be a package-scoped global and not a file-scoped lexical |
778 | created via my(). Most classes have just one item in their @ISA array. |
779 | In this case, we have what's called "single inheritance", or SI for short. |
780 | |
781 | Consider this class: |
782 | |
783 | package Employee; |
784 | use Person; |
785 | @ISA = ("Person"); |
786 | 1; |
787 | |
788 | Not a lot to it, eh? All it's doing so far is loading in another |
789 | class and stating that this one will inherit methods from that |
790 | other class if need be. We have given it none of its own methods. |
791 | We rely upon an Employee to behave just like a Person. |
792 | |
793 | Setting up an empty class like this is called the "empty subclass test"; |
794 | that is, making a derived class that does nothing but inherit from a |
795 | base class. If the original base class has been designed properly, |
796 | then the new derived class can be used as a drop-in replacement for the |
797 | old one. This means you should be able to write a program like this: |
798 | |
799 | use Employee |
800 | my $empl = Employee->new(); |
801 | $empl->name("Jason"); |
802 | $empl->age(23); |
803 | printf "%s is age %d.\n", $empl->name, $empl->age; |
804 | |
805 | By proper design, we mean always using the two-argument form of bless(), |
806 | avoiding direct access of global data, and not exporting anything. If you |
807 | look back at the Person::new() function we defined above, we were careful |
808 | to do that. There's a bit of package data used in the constructor, |
809 | but the reference to this is stored on the object itself and all other |
810 | methods access package data via that reference, so we should be ok. |
811 | |
812 | What do we mean by the Person::new() function -- isn't that actually |
813 | a method? Well, in principle, yes. A method is just a function that |
814 | expects as its first argument a class name (package) or object |
815 | (blessed reference). Person::new() is the function that both the |
816 | C<Person-E<gt>new()> method and the C<Employee-E<gt>new()> method end |
817 | up calling. Understand that while a method call looks a lot like a |
818 | function call, they aren't really quite the same, and if you treat them |
819 | as the same, you'll very soon be left with nothing but broken programs. |
820 | First, the actual underlying calling conventions are different: method |
821 | calls get an extra argument. Second, function calls don't do inheritance, |
822 | but methods do. |
823 | |
824 | Method Call Resulting Function Call |
825 | ----------- ------------------------ |
826 | Person->new() Person::new("Person") |
827 | Employee->new() Person::new("Employee") |
828 | |
829 | So don't use function calls when you mean to call a method. |
830 | |
831 | If an employee is just a Person, that's not all too very interesting. |
832 | So let's add some other methods. We'll give our employee |
833 | data fields to access their salary, their employee ID, and their |
834 | start date. |
835 | |
836 | If you're getting a little tired of creating all these nearly identical |
837 | methods just to get at the object's data, do not despair. Later, |
838 | we'll describe several different convenience mechanisms for shortening |
839 | this up. Meanwhile, here's the straight-forward way: |
840 | |
841 | sub salary { |
842 | my $self = shift; |
843 | if (@_) { $self->{SALARY} = shift } |
844 | return $self->{SALARY}; |
845 | } |
846 | |
847 | sub id_number { |
848 | my $self = shift; |
849 | if (@_) { $self->{ID} = shift } |
850 | return $self->{ID}; |
851 | } |
852 | |
853 | sub start_date { |
854 | my $self = shift; |
855 | if (@_) { $self->{START_DATE} = shift } |
856 | return $self->{START_DATE}; |
857 | } |
858 | |
859 | =head2 Overridden Methods |
860 | |
861 | What happens when both a derived class and its base class have the same |
862 | method defined? Well, then you get the derived class's version of that |
863 | method. For example, let's say that we want the peers() method called on |
864 | an employee to act a bit differently. Instead of just returning the list |
865 | of peer names, let's return slightly different strings. So doing this: |
866 | |
867 | $empl->peers("Peter", "Paul", "Mary"); |
868 | printf "His peers are: %s\n", join(", ", $empl->peers); |
869 | |
870 | will produce: |
871 | |
872 | His peers are: PEON=PETER, PEON=PAUL, PEON=MARY |
873 | |
874 | To do this, merely add this definition into the Employee.pm file: |
875 | |
876 | sub peers { |
877 | my $self = shift; |
878 | if (@_) { @{ $self->{PEERS} } = @_ } |
879 | return map { "PEON=\U$_" } @{ $self->{PEERS} }; |
880 | } |
881 | |
882 | There, we've just demonstrated the high-falutin' concept known in certain |
883 | circles as I<polymorphism>. We've taken on the form and behaviour of |
884 | an existing object, and then we've altered it to suit our own purposes. |
885 | This is a form of Laziness. (Getting polymorphed is also what happens |
886 | when the wizard decides you'd look better as a frog.) |
887 | |
888 | Every now and then you'll want to have a method call trigger both its |
889 | derived class (also know as "subclass") version as well as its base class |
890 | (also known as "superclass") version. In practice, constructors and |
891 | destructors are likely to want to do this, and it probably also makes |
892 | sense in the debug() method we showed previously. |
893 | |
894 | To do this, add this to Employee.pm: |
895 | |
896 | use Carp; |
897 | my $Debugging = 0; |
898 | |
899 | sub debug { |
900 | my $self = shift; |
901 | confess "usage: thing->debug(level)" unless @_ == 1; |
902 | my $level = shift; |
903 | if (ref($self)) { |
904 | $self->{"_DEBUG"} = $level; |
905 | } else { |
906 | $Debugging = $level; # whole class |
907 | } |
908 | Person::debug($self, $Debugging); # don't really do this |
909 | } |
910 | |
911 | As you see, we turn around and call the Person package's debug() function. |
912 | But this is far too fragile for good design. What if Person doesn't |
913 | have a debug() function, but is inheriting I<its> debug() method |
914 | from elsewhere? It would have been slightly better to say |
915 | |
916 | Person->debug($Debugging); |
917 | |
918 | But even that's got too much hard-coded. It's somewhat better to say |
919 | |
920 | $self->Person::debug($Debugging); |
921 | |
922 | Which is a funny way to say to start looking for a debug() method up |
923 | in Person. This strategy is more often seen on overridden object methods |
924 | than on overridden class methods. |
925 | |
926 | There is still something a bit off here. We've hard-coded our |
927 | superclass's name. This in particular is bad if you change which classes |
928 | you inherit from, or add others. Fortunately, the pseudoclass SUPER |
929 | comes to the rescue here. |
930 | |
71be2cbc |
931 | $self->SUPER::debug($Debugging); |
5f05dabc |
932 | |
933 | This way it starts looking in my class's @ISA. This only makes sense |
934 | from I<within> a method call, though. Don't try to access anything |
935 | in SUPER:: from anywhere else, because it doesn't exist outside |
936 | an overridden method call. |
937 | |
938 | Things are getting a bit complicated here. Have we done anything |
939 | we shouldn't? As before, one way to test whether we're designing |
940 | a decent class is via the empty subclass test. Since we already have |
941 | an Employee class that we're trying to check, we'd better get a new |
942 | empty subclass that can derive from Employee. Here's one: |
943 | |
944 | package Boss; |
945 | use Employee; # :-) |
946 | @ISA = qw(Employee); |
947 | |
948 | And here's the test program: |
949 | |
950 | #!/usr/bin/perl -w |
951 | use strict; |
952 | use Boss; |
953 | Boss->debug(1); |
954 | |
955 | my $boss = Boss->new(); |
956 | |
957 | $boss->fullname->title("Don"); |
958 | $boss->fullname->surname("Pichon Alvarez"); |
959 | $boss->fullname->christian("Federico Jesus"); |
960 | $boss->fullname->nickname("Fred"); |
961 | |
962 | $boss->age(47); |
963 | $boss->peers("Frank", "Felipe", "Faust"); |
964 | |
965 | printf "%s is age %d.\n", $boss->fullname, $boss->age; |
966 | printf "His peers are: %s\n", join(", ", $boss->peers); |
967 | |
968 | Running it, we see that we're still ok. If you'd like to dump out your |
969 | object in a nice format, somewhat like the way the 'x' command works in |
970 | the debugger, you could use the Data::Dumper module from CPAN this way: |
971 | |
972 | use Data::Dumper; |
973 | print "Here's the boss:\n"; |
974 | print Dumper($boss); |
975 | |
976 | Which shows us something like this: |
977 | |
978 | Here's the boss: |
979 | $VAR1 = bless( { |
980 | _CENSUS => \1, |
981 | FULLNAME => bless( { |
982 | TITLE => 'Don', |
983 | SURNAME => 'Pichon Alvarez', |
984 | NICK => 'Fred', |
985 | CHRISTIAN => 'Federico Jesus' |
986 | }, 'Fullname' ), |
987 | AGE => 47, |
988 | PEERS => [ |
989 | 'Frank', |
990 | 'Felipe', |
991 | 'Faust' |
992 | ] |
993 | }, 'Boss' ); |
994 | |
995 | Hm.... something's missing there. What about the salary, start date, |
996 | and ID fields? Well, we never set them to anything, even undef, so they |
997 | don't show up in the hash's keys. The Employee class has no new() method |
998 | of its own, and the new() method in Person doesn't know about Employees. |
999 | (Nor should it: proper OO design dictates that a subclass be allowed to |
1000 | know about its immediate superclass, but never vice-versa.) So let's |
1001 | fix up Employee::new() this way: |
1002 | |
1003 | sub new { |
1004 | my $proto = shift; |
1005 | my $class = ref($proto) || $proto; |
1006 | my $self = $class->SUPER::new(); |
1007 | $self->{SALARY} = undef; |
1008 | $self->{ID} = undef; |
1009 | $self->{START_DATE} = undef; |
1010 | bless ($self, $class); # reconsecrate |
1011 | return $self; |
1012 | } |
1013 | |
1014 | Now if you dump out an Employee or Boss object, you'll find |
1015 | that new fields show up there now. |
1016 | |
1017 | =head2 Multiple Inheritance |
1018 | |
1019 | Ok, at the risk of confusing beginners and annoying OO gurus, it's |
1020 | time to confess that Perl's object system includes that controversial |
1021 | notion known as multiple inheritance, or MI for short. All this means |
1022 | is that rather than having just one parent class who in turn might |
1023 | itself have a parent class, etc., that you can directly inherit from |
1024 | two or more parents. It's true that some uses of MI can get you into |
1025 | trouble, although hopefully not quite so much trouble with Perl as with |
1026 | dubiously-OO languages like C++. |
1027 | |
1028 | The way it works is actually pretty simple: just put more than one package |
1029 | name in your @ISA array. When it comes time for Perl to go finding |
1030 | methods for your object, it looks at each of these packages in order. |
1031 | Well, kinda. It's actually a fully recursive, depth-first order. |
1032 | Consider a bunch of @ISA arrays like this: |
1033 | |
1034 | @First::ISA = qw( Alpha ); |
1035 | @Second::ISA = qw( Beta ); |
1036 | @Third::ISA = qw( First Second ); |
1037 | |
1038 | If you have an object of class Third: |
1039 | |
1040 | my $ob = Third->new(); |
1041 | $ob->spin(); |
1042 | |
1043 | How do we find a spin() method (or a new() method for that matter)? |
1044 | Because the search is depth-first, classes will be looked up |
1045 | in the following order: Third, First, Alpha, Second, and Beta. |
1046 | |
1047 | In practice, few class modules have been seen that actually |
1048 | make use of MI. One nearly always chooses simple containership of |
1049 | one class within another over MI. That's why our Person |
1050 | object I<contained> a Fullname object. That doesn't mean |
1051 | it I<was> one. |
1052 | |
1053 | However, there is one particular area where MI in Perl is rampant: |
1054 | borrowing another class's class methods. This is rather common, |
1055 | especially with some bundled "objectless" classes, |
1056 | like Exporter, DynaLoader, AutoLoader, and SelfLoader. These classes |
1057 | do not provide constructors; they exist only so you may inherit their |
1058 | class methods. (It's not entirely clear why inheritance was done |
1059 | here rather than traditional module importation.) |
1060 | |
1061 | For example, here is the POSIX module's @ISA: |
1062 | |
1063 | package POSIX; |
1064 | @ISA = qw(Exporter DynaLoader); |
1065 | |
1066 | The POSIX module isn't really an object module, but then, |
1067 | neither are Exporter or DynaLoader. They're just lending their |
1068 | classes' behaviours to POSIX. |
1069 | |
1070 | Why don't people use MI for object methods much? One reason is that |
1071 | it can have complicated side-effects. For one thing, your inheritance |
1072 | graph (no longer a tree) might converge back to the same base class. |
1073 | Although Perl guards against recursive inheritance, merely having parents |
1074 | who are related to each other via a common ancestor, incestuous though |
1075 | it sounds, is not forbidden. What if in our Third class shown above we |
1076 | wanted its new() method to also call both overridden constructors in its |
1077 | two parent classes? The SUPER notation would only find the first one. |
1078 | Also, what about if the Alpha and Beta classes both had a common ancestor, |
1079 | like Nought? If you kept climbing up the inheritance tree calling |
1080 | overridden methods, you'd end up calling Nought::new() twice, |
1081 | which might well be a bad idea. |
1082 | |
1083 | =head2 UNIVERSAL: The Root of All Objects |
1084 | |
1085 | Wouldn't it be convenient if all objects were rooted at some ultimate |
1086 | base class? That way you could give every object common methods without |
1087 | having to go and add it to each and every @ISA. Well, it turns out that |
1088 | you can. You don't see it, but Perl tacitly and irrevocably assumes |
1089 | that there's an extra element at the end of @ISA: the class UNIVERSAL. |
a6006777 |
1090 | In version 5.003, there were no predefined methods there, but you could put |
5f05dabc |
1091 | whatever you felt like into it. |
1092 | |
a6006777 |
1093 | However, as of version 5.004 (or some subversive releases, like 5.003_08), |
5f05dabc |
1094 | UNIVERSAL has some methods in it already. These are built-in to your Perl |
1095 | binary, so they don't take any extra time to load. Predefined methods |
1096 | include isa(), can(), and VERSION(). isa() tells you whether an object or |
1097 | class "is" another one without having to traverse the hierarchy yourself: |
1098 | |
1099 | $has_io = $fd->isa("IO::Handle"); |
1100 | $itza_handle = IO::Socket->isa("IO::Handle"); |
1101 | |
1102 | The can() method, called against that object or class, reports back |
1103 | whether its string argument is a callable method name in that class. |
1104 | In fact, it gives you back a function reference to that method: |
1105 | |
1106 | $his_print_method = $obj->can('as_string'); |
1107 | |
1108 | Finally, the VERSION method checks whether the class (or the object's |
1109 | class) has a package global called $VERSION that's high enough, as in: |
1110 | |
1111 | Some_Module->VERSION(3.0); |
1112 | $his_vers = $ob->VERSION(); |
1113 | |
1114 | However, we don't usually call VERSION ourselves. (Remember that an all |
1115 | upper-case function name is a Perl convention that indicates that the |
1116 | function will be automatically used by Perl in some way.) In this case, |
1117 | it happens when you say |
1118 | |
1119 | use Some_Module 3.0; |
1120 | |
8d9aa5e0 |
1121 | If you wanted to add version checking to your Person class explained |
5f05dabc |
1122 | above, just add this to Person.pm: |
1123 | |
1124 | use vars qw($VERSION); |
1125 | $VERSION = '1.1'; |
1126 | |
1127 | and then in Employee.pm could you can say |
1128 | |
1129 | use Employee 1.1; |
1130 | |
1131 | And it would make sure that you have at least that version number or |
1132 | higher available. This is not the same as loading in that exact version |
1133 | number. No mechanism currently exists for concurrent installation of |
1134 | multiple versions of a module. Lamentably. |
1135 | |
1136 | =head1 Alternate Object Representations |
1137 | |
1138 | Nothing requires objects to be implemented as hash references. An object |
1139 | can be any sort of reference so long as its referent has been suitably |
1140 | blessed. That means scalar, array, and code references are also fair |
1141 | game. |
1142 | |
1143 | A scalar would work if the object has only one datum to hold. An array |
1144 | would work for most cases, but makes inheritance a bit dodgy because |
1145 | you have to invent new indices for the derived classes. |
1146 | |
1147 | =head2 Arrays as Objects |
1148 | |
1149 | If the user of your class honors the contract and sticks to the advertised |
1150 | interface, then you can change its underlying interface if you feel |
1151 | like it. Here's another implementation that conforms to the same |
1152 | interface specification. This time we'll use an array reference |
1153 | instead of a hash reference to represent the object. |
1154 | |
1155 | package Person; |
1156 | use strict; |
1157 | |
1158 | my($NAME, $AGE, $PEERS) = ( 0 .. 2 ); |
1159 | |
1160 | ############################################ |
1161 | ## the object constructor (array version) ## |
1162 | ############################################ |
1163 | sub new { |
1164 | my $self = []; |
1165 | $self->[$NAME] = undef; # this is unnecessary |
1166 | $self->[$AGE] = undef; # as it this |
1167 | $self->[$PEERS] = []; # but this isn't, really |
1168 | bless($self); |
1169 | return $self; |
1170 | } |
1171 | |
1172 | sub name { |
1173 | my $self = shift; |
1174 | if (@_) { $self->[$NAME] = shift } |
1175 | return $self->[$NAME]; |
1176 | } |
1177 | |
1178 | sub age { |
1179 | my $self = shift; |
1180 | if (@_) { $self->[$AGE] = shift } |
1181 | return $self->[$AGE]; |
1182 | } |
1183 | |
1184 | sub peers { |
1185 | my $self = shift; |
1186 | if (@_) { @{ $self->[$PEERS] } = @_ } |
1187 | return @{ $self->[$PEERS] }; |
1188 | } |
1189 | |
1190 | 1; # so the require or use succeeds |
1191 | |
8d9aa5e0 |
1192 | You might guess that the array access would be a lot faster than the |
1193 | hash access, but they're actually comparable. The array is a I<little> |
5f05dabc |
1194 | bit faster, but not more than ten or fifteen percent, even when you |
1195 | replace the variables above like $AGE with literal numbers, like 1. |
1196 | A bigger difference between the two approaches can be found in memory use. |
1197 | A hash representation takes up more memory than an array representation |
8d9aa5e0 |
1198 | because you have to allocate memory for the keys as well as for the values. |
a6006777 |
1199 | However, it really isn't that bad, especially since as of version 5.004, |
5f05dabc |
1200 | memory is only allocated once for a given hash key, no matter how many |
1201 | hashes have that key. It's expected that sometime in the future, even |
1202 | these differences will fade into obscurity as more efficient underlying |
1203 | representations are devised. |
1204 | |
1205 | Still, the tiny edge in speed (and somewhat larger one in memory) |
1206 | is enough to make some programmers choose an array representation |
1207 | for simple classes. There's still a little problem with |
1208 | scalability, though, because later in life when you feel |
1209 | like creating subclasses, you'll find that hashes just work |
1210 | out better. |
1211 | |
1212 | =head2 Closures as Objects |
1213 | |
1214 | Using a code reference to represent an object offers some fascinating |
1215 | possibilities. We can create a new anonymous function (closure) who |
1216 | alone in all the world can see the object's data. This is because we |
1217 | put the data into an anonymous hash that's lexically visible only to |
1218 | the closure we create, bless, and return as the object. This object's |
1219 | methods turn around and call the closure as a regular subroutine call, |
1220 | passing it the field we want to affect. (Yes, |
1221 | the double-function call is slow, but if you wanted fast, you wouldn't |
1222 | be using objects at all, eh? :-) |
1223 | |
1224 | Use would be similar to before: |
1225 | |
1226 | use Person; |
1227 | $him = Person->new(); |
1228 | $him->name("Jason"); |
1229 | $him->age(23); |
1230 | $him->peers( [ "Norbert", "Rhys", "Phineas" ] ); |
1231 | printf "%s is %d years old.\n", $him->name, $him->age; |
1232 | print "His peers are: ", join(", ", @{$him->peers}), "\n"; |
1233 | |
1234 | but the implementation would be radically, perhaps even sublimely |
1235 | different: |
1236 | |
1237 | package Person; |
1238 | |
1239 | sub new { |
1240 | my $that = shift; |
1241 | my $class = ref($that) || $that; |
1242 | my $self = { |
1243 | NAME => undef, |
1244 | AGE => undef, |
1245 | PEERS => [], |
1246 | }; |
1247 | my $closure = sub { |
1248 | my $field = shift; |
1249 | if (@_) { $self->{$field} = shift } |
1250 | return $self->{$field}; |
1251 | }; |
1252 | bless($closure, $class); |
1253 | return $closure; |
1254 | } |
1255 | |
1256 | sub name { &{ $_[0] }("NAME", @_[ 1 .. $#_ ] ) } |
1257 | sub age { &{ $_[0] }("AGE", @_[ 1 .. $#_ ] ) } |
1258 | sub peers { &{ $_[0] }("PEERS", @_[ 1 .. $#_ ] ) } |
1259 | |
1260 | 1; |
1261 | |
1262 | Because this object is hidden behind a code reference, it's probably a bit |
1263 | mysterious to those whose background is more firmly rooted in standard |
1264 | procedural or object-based programming languages than in functional |
1265 | programming languages whence closures derive. The object |
1266 | created and returned by the new() method is itself not a data reference |
1267 | as we've seen before. It's an anonymous code reference that has within |
1268 | it access to a specific version (lexical binding and instantiation) |
1269 | of the object's data, which are stored in the private variable $self. |
1270 | Although this is the same function each time, it contains a different |
1271 | version of $self. |
1272 | |
1273 | When a method like C<$him-E<gt>name("Jason")> is called, its implicit |
8d9aa5e0 |
1274 | zeroth argument is the invoking object--just as it is with all method |
5f05dabc |
1275 | calls. But in this case, it's our code reference (something like a |
1276 | function pointer in C++, but with deep binding of lexical variables). |
1277 | There's not a lot to be done with a code reference beyond calling it, so |
1278 | that's just what we do when we say C<&{$_[0]}>. This is just a regular |
1279 | function call, not a method call. The initial argument is the string |
1280 | "NAME", and any remaining arguments are whatever had been passed to the |
1281 | method itself. |
1282 | |
1283 | Once we're executing inside the closure that had been created in new(), |
1284 | the $self hash reference suddenly becomes visible. The closure grabs |
1285 | its first argument ("NAME" in this case because that's what the name() |
1286 | method passed it), and uses that string to subscript into the private |
1287 | hash hidden in its unique version of $self. |
1288 | |
1289 | Nothing under the sun will allow anyone outside the executing method to |
1290 | be able to get at this hidden data. Well, nearly nothing. You I<could> |
1291 | single step through the program using the debugger and find out the |
1292 | pieces while you're in the method, but everyone else is out of luck. |
1293 | |
1294 | There, if that doesn't excite the Scheme folks, then I just don't know |
1295 | what will. Translation of this technique into C++, Java, or any other |
1296 | braindead-static language is left as a futile exercise for aficionados |
1297 | of those camps. |
1298 | |
1299 | You could even add a bit of nosiness via the caller() function and |
1300 | make the closure refuse to operate unless called via its own package. |
1301 | This would no doubt satisfy certain fastidious concerns of programming |
1302 | police and related puritans. |
1303 | |
1304 | If you were wondering when Hubris, the third principle virtue of a |
1305 | programmer, would come into play, here you have it. (More seriously, |
1306 | Hubris is just the pride in craftsmanship that comes from having written |
1307 | a sound bit of well-designed code.) |
1308 | |
1309 | =head1 AUTOLOAD: Proxy Methods |
1310 | |
1311 | Autoloading is a way to intercept calls to undefined methods. An autoload |
1312 | routine may choose to create a new function on the fly, either loaded |
1313 | from disk or perhaps just eval()ed right there. This define-on-the-fly |
1314 | strategy is why it's called autoloading. |
1315 | |
1316 | But that's only one possible approach. Another one is to just |
1317 | have the autoloaded method itself directly provide the |
1318 | requested service. When used in this way, you may think |
1319 | of autoloaded methods as "proxy" methods. |
1320 | |
1321 | When Perl tries to call an undefined function in a particular package |
1322 | and that function is not defined, it looks for a function in |
1323 | that same package called AUTOLOAD. If one exists, it's called |
1324 | with the same arguments as the original function would have had. |
1325 | The fully-qualified name of the function is stored in that package's |
1326 | global variable $AUTOLOAD. Once called, the function can do anything |
1327 | it would like, including defining a new function by the right name, and |
1328 | then doing a really fancy kind of C<goto> right to it, erasing itself |
1329 | from the call stack. |
1330 | |
1331 | What does this have to do with objects? After all, we keep talking about |
1332 | functions, not methods. Well, since a method is just a function with |
1333 | an extra argument and some fancier semantics about where it's found, |
1334 | we can use autoloading for methods, too. Perl doesn't start looking |
1335 | for an AUTOLOAD method until it has exhausted the recursive hunt up |
1336 | through @ISA, though. Some programmers have even been known to define |
1337 | a UNIVERSAL::AUTOLOAD method to trap unresolved method calls to any |
1338 | kind of object. |
1339 | |
1340 | =head2 Autoloaded Data Methods |
1341 | |
1342 | You probably began to get a little suspicious about the duplicated |
1343 | code way back earlier when we first showed you the Person class, and |
1344 | then later the Employee class. Each method used to access the |
1345 | hash fields looked virtually identical. This should have tickled |
1346 | that great programming virtue, Impatience, but for the time, |
1347 | we let Laziness win out, and so did nothing. Proxy methods can cure |
1348 | this. |
1349 | |
1350 | Instead of writing a new function every time we want a new data field, |
1351 | we'll use the autoload mechanism to generate (actually, mimic) methods on |
1352 | the fly. To verify that we're accessing a valid member, we will check |
1353 | against an C<_permitted> (pronounced "under-permitted") field, which |
1354 | is a reference to a file-scoped lexical (like a C file static) hash of permitted fields in this record |
1355 | called %fields. Why the underscore? For the same reason as the _CENSUS |
1356 | field we once used: as a marker that means "for internal use only". |
1357 | |
1358 | Here's what the module initialization code and class |
1359 | constructor will look like when taking this approach: |
1360 | |
1361 | package Person; |
1362 | use Carp; |
1363 | use vars qw($AUTOLOAD); # it's a package global |
1364 | |
1365 | my %fields = ( |
1366 | name => undef, |
1367 | age => undef, |
1368 | peers => undef, |
1369 | ); |
1370 | |
1371 | sub new { |
1372 | my $that = shift; |
1373 | my $class = ref($that) || $that; |
1374 | my $self = { |
1375 | _permitted => \%fields, |
1376 | %fields, |
1377 | }; |
1378 | bless $self, $class; |
1379 | return $self; |
1380 | } |
1381 | |
1382 | If we wanted our record to have default values, we could fill those in |
1383 | where current we have C<undef> in the %fields hash. |
1384 | |
1385 | Notice how we saved a reference to our class data on the object itself? |
1386 | Remember that it's important to access class data through the object |
1387 | itself instead of having any method reference %fields directly, or else |
1388 | you won't have a decent inheritance. |
1389 | |
1390 | The real magic, though, is going to reside in our proxy method, which |
1391 | will handle all calls to undefined methods for objects of class Person |
1392 | (or subclasses of Person). It has to be called AUTOLOAD. Again, it's |
1393 | all caps because it's called for us implicitly by Perl itself, not by |
1394 | a user directly. |
1395 | |
1396 | sub AUTOLOAD { |
1397 | my $self = shift; |
1398 | my $type = ref($self) |
1399 | or croak "$self is not an object"; |
1400 | |
1401 | my $name = $AUTOLOAD; |
1402 | $name =~ s/.*://; # strip fully-qualified portion |
1403 | |
1404 | unless (exists $self->{_permitted}->{$name} ) { |
1405 | croak "Can't access `$name' field in class $type"; |
1406 | } |
1407 | |
1408 | if (@_) { |
1409 | return $self->{$name} = shift; |
1410 | } else { |
1411 | return $self->{$name}; |
1412 | } |
1413 | } |
1414 | |
1415 | Pretty nifty, eh? All we have to do to add new data fields |
1416 | is modify %fields. No new functions need be written. |
1417 | |
1418 | I could have avoided the C<_permitted> field entirely, but I |
1419 | wanted to demonstrate how to store a reference to class data on the |
1420 | object so you wouldn't have to access that class data |
1421 | directly from an object method. |
1422 | |
1423 | =head2 Inherited Autoloaded Data Methods |
1424 | |
1425 | But what about inheritance? Can we define our Employee |
1426 | class similarly? Yes, so long as we're careful enough. |
1427 | |
1428 | Here's how to be careful: |
1429 | |
1430 | package Employee; |
1431 | use Person; |
1432 | use strict; |
1433 | use vars qw(@ISA); |
1434 | @ISA = qw(Person); |
1435 | |
1436 | my %fields = ( |
1437 | id => undef, |
1438 | salary => undef, |
1439 | ); |
1440 | |
1441 | sub new { |
1442 | my $that = shift; |
1443 | my $class = ref($that) || $that; |
1444 | my $self = bless $that->SUPER::new(), $class; |
1445 | my($element); |
1446 | foreach $element (keys %fields) { |
1447 | $self->{_permitted}->{$element} = $fields{$element}; |
1448 | } |
1449 | @{$self}{keys %fields} = values %fields; |
1450 | return $self; |
1451 | } |
1452 | |
1453 | Once we've done this, we don't even need to have an |
1454 | AUTOLOAD function in the Employee package, because |
1455 | we'll grab Person's version of that via inheritance, |
1456 | and it will all work out just fine. |
1457 | |
1458 | =head1 Metaclassical Tools |
1459 | |
1460 | Even though proxy methods can provide a more convenient approach to making |
1461 | more struct-like classes than tediously coding up data methods as |
1462 | functions, it still leaves a bit to be desired. For one thing, it means |
1463 | you have to handle bogus calls that you don't mean to trap via your proxy. |
1464 | It also means you have to be quite careful when dealing with inheritance, |
1465 | as detailed above. |
1466 | |
1467 | Perl programmers have responded to this by creating several different |
1468 | class construction classes. These metaclasses are classes |
1469 | that create other classes. A couple worth looking at are |
1470 | Class::Template and Alias. These and other related metaclasses can be |
1471 | found in the modules directory on CPAN. |
1472 | |
1473 | =head2 Class::Template |
1474 | |
1475 | One of the older ones is Class::Template. In fact, its syntax and |
1476 | interface were sketched out long before perl5 even solidified into a |
8d9aa5e0 |
1477 | real thing. What it does is provide you a way to "declare" a class |
1478 | as having objects whose fields are of a specific type. The function |
1479 | that does this is called, not surprisingly enough, struct(). Because |
1480 | structures or records are not base types in Perl, each time you want to |
1481 | create a class to provide a record-like data object, you yourself have |
1482 | to define a new() method, plus separate data-access methods for each of |
1483 | that record's fields. You'll quickly become bored with this process. |
1484 | The Class::Template::struct() function alleviates this tedium. |
5f05dabc |
1485 | |
1486 | Here's a simple example of using it: |
1487 | |
1488 | use Class::Template qw(struct); |
1489 | use Jobbie; # user-defined; see below |
1490 | |
1491 | struct 'Fred' => { |
1492 | one => '$', |
1493 | many => '@', |
1494 | profession => Jobbie, # calls Jobbie->new() |
1495 | }; |
1496 | |
1497 | $ob = Fred->new; |
1498 | $ob->one("hmmmm"); |
1499 | |
1500 | $ob->many(0, "here"); |
1501 | $ob->many(1, "you"); |
1502 | $ob->many(2, "go"); |
1503 | print "Just set: ", $ob->many(2), "\n"; |
1504 | |
1505 | $ob->profession->salary(10_000); |
1506 | |
1507 | You can declare types in the struct to be basic Perl types, or |
1508 | user-defined types (classes). User types will be initialized by calling |
1509 | that class's new() method. |
1510 | |
1511 | Here's a real-world example of using struct generation. Let's say you |
1512 | wanted to override Perl's idea of gethostbyname() and gethostbyaddr() so |
1513 | that they would return objects that acted like C structures. We don't |
1514 | care about high-falutin' OO gunk. All we want is for these objects to |
1515 | act like structs in the C sense. |
1516 | |
1517 | use Socket; |
1518 | use Net::hostent; |
1519 | $h = gethostbyname("perl.com"); # object return |
1520 | printf "perl.com's real name is %s, address %s\n", |
1521 | $h->name, inet_ntoa($h->addr); |
1522 | |
1523 | Here's how to do this using the Class::Template module. |
1524 | The crux is going to be this call: |
1525 | |
1526 | struct 'Net::hostent' => [ # note bracket |
1527 | name => '$', |
1528 | aliases => '@', |
1529 | addrtype => '$', |
1530 | 'length' => '$', |
1531 | addr_list => '@', |
1532 | ]; |
1533 | |
1534 | Which creates object methods of those names and types. |
1535 | It even creates a new() method for us. |
1536 | |
1537 | We could also have implemented our object this way: |
1538 | |
1539 | struct 'Net::hostent' => { # note brace |
1540 | name => '$', |
1541 | aliases => '@', |
1542 | addrtype => '$', |
1543 | 'length' => '$', |
1544 | addr_list => '@', |
1545 | }; |
1546 | |
1547 | and then Class::Template would have used an anonymous hash as the object |
1548 | type, instead of an anonymous array. The array is faster and smaller, |
1549 | but the hash works out better if you eventually want to do inheritance. |
1550 | Since for this struct-like object we aren't planning on inheritance, |
1551 | this time we'll opt for better speed and size over better flexibility. |
1552 | |
1553 | Here's the whole implementation: |
1554 | |
1555 | package Net::hostent; |
1556 | use strict; |
1557 | |
1558 | BEGIN { |
1559 | use Exporter (); |
1560 | use vars qw(@ISA @EXPORT @EXPORT_OK %EXPORT_TAGS); |
1561 | @ISA = qw(Exporter); |
1562 | @EXPORT = qw(gethostbyname gethostbyaddr gethost); |
1563 | @EXPORT_OK = qw( |
1564 | $h_name @h_aliases |
1565 | $h_addrtype $h_length |
1566 | @h_addr_list $h_addr |
1567 | ); |
1568 | %EXPORT_TAGS = ( FIELDS => [ @EXPORT_OK, @EXPORT ] ); |
1569 | } |
1570 | use vars @EXPORT_OK; |
1571 | |
1572 | use Class::Template qw(struct); |
1573 | struct 'Net::hostent' => [ |
1574 | name => '$', |
1575 | aliases => '@', |
1576 | addrtype => '$', |
1577 | 'length' => '$', |
1578 | addr_list => '@', |
1579 | ]; |
1580 | |
1581 | sub addr { shift->addr_list->[0] } |
1582 | |
1583 | sub populate (@) { |
1584 | return unless @_; |
1585 | my $hob = new(); # Class::Template made this! |
1586 | $h_name = $hob->[0] = $_[0]; |
1587 | @h_aliases = @{ $hob->[1] } = split ' ', $_[1]; |
1588 | $h_addrtype = $hob->[2] = $_[2]; |
1589 | $h_length = $hob->[3] = $_[3]; |
1590 | $h_addr = $_[4]; |
1591 | @h_addr_list = @{ $hob->[4] } = @_[ (4 .. $#_) ]; |
1592 | return $hob; |
1593 | } |
1594 | |
1595 | sub gethostbyname ($) { populate(CORE::gethostbyname(shift)) } |
1596 | |
1597 | sub gethostbyaddr ($;$) { |
1598 | my ($addr, $addrtype); |
1599 | $addr = shift; |
1600 | require Socket unless @_; |
1601 | $addrtype = @_ ? shift : Socket::AF_INET(); |
1602 | populate(CORE::gethostbyaddr($addr, $addrtype)) |
1603 | } |
1604 | |
1605 | sub gethost($) { |
1606 | if ($_[0] =~ /^\d+(?:\.\d+(?:\.\d+(?:\.\d+)?)?)?$/) { |
1607 | require Socket; |
1608 | &gethostbyaddr(Socket::inet_aton(shift)); |
1609 | } else { |
1610 | &gethostbyname; |
1611 | } |
1612 | } |
1613 | |
1614 | 1; |
1615 | |
1616 | We've snuck in quite a fair bit of other concepts besides just dynamic |
1617 | class creation, like overriding core functions, import/export bits, |
1618 | function prototyping, and short-cut function call via C<&whatever>. |
1619 | These all mostly make sense from the perspective of a traditional module, |
1620 | but as you can see, we can also use them in an object module. |
1621 | |
1622 | You can look at other object-based, struct-like overrides of core |
1623 | functions in the 5.004 release of Perl in File::stat, Net::hostent, |
1624 | Net::netent, Net::protoent, Net::servent, Time::gmtime, Time::localtime, |
1625 | User::grent, and User::pwent. These modules have a final component |
1626 | that's all lower-case, by convention reserved for compiler pragmas, |
1627 | because they affect the compilation and change a built-in function. |
1628 | They also have the type names that a C programmer would most expect. |
1629 | |
1630 | =head2 Data Members as Variables |
1631 | |
1632 | If you're used to C++ objects, then you're accustomed to being able to |
1633 | get at an object's data members as simple variables from within a method. |
1634 | The Alias module provides for this, as well as a good bit more, such |
1635 | as the possibility of private methods that the object can call but folks |
1636 | outside the class cannot. |
1637 | |
1638 | Here's an example of creating a Person using the Alias module. |
1639 | When you update these magical instance variables, you automatically |
1640 | update value fields in the hash. Convenient, eh? |
1641 | |
1642 | package Person; |
1643 | |
1644 | # this is the same as before... |
1645 | sub new { |
1646 | my $that = shift; |
1647 | my $class = ref($that) || $that; |
1648 | my $self = { |
1649 | NAME => undef, |
1650 | AGE => undef, |
1651 | PEERS => [], |
1652 | }; |
1653 | bless($self, $class); |
1654 | return $self; |
1655 | } |
1656 | |
1657 | use Alias qw(attr); |
1658 | use vars qw($NAME $AGE $PEERS); |
1659 | |
1660 | sub name { |
1661 | my $self = attr shift; |
1662 | if (@_) { $NAME = shift; } |
1663 | return $NAME; |
1664 | } |
1665 | |
1666 | sub age { |
1667 | my $self = attr shift; |
1668 | if (@_) { $AGE = shift; } |
1669 | return $AGE; |
1670 | } |
1671 | |
1672 | sub peers { |
1673 | my $self = attr shift; |
1674 | if (@_) { @PEERS = @_; } |
1675 | return @PEERS; |
1676 | } |
1677 | |
1678 | sub exclaim { |
1679 | my $self = attr shift; |
1680 | return sprintf "Hi, I'm %s, age %d, working with %s", |
1681 | $NAME, $AGE, join(", ", @PEERS); |
1682 | } |
1683 | |
1684 | sub happy_birthday { |
1685 | my $self = attr shift; |
1686 | return ++$AGE; |
1687 | } |
1688 | |
1689 | The need for the C<use vars> declaration is because what Alias does |
1690 | is play with package globals with the same name as the fields. To use |
1691 | globals while C<use strict> is in effect, you have to pre-declare them. |
1692 | These package variables are localized to the block enclosing the attr() |
1693 | call just as if you'd used a local() on them. However, that means that |
1694 | they're still considered global variables with temporary values, just |
1695 | as with any other local(). |
1696 | |
1697 | It would be nice to combine Alias with |
1698 | something like Class::Template or Class::MethodMaker. |
1699 | |
1700 | =head2 NOTES |
1701 | |
1702 | =head2 Object Terminology |
1703 | |
1704 | In the various OO literature, it seems that a lot of different words |
1705 | are used to describe only a few different concepts. If you're not |
1706 | already an object programmer, then you don't need to worry about all |
1707 | these fancy words. But if you are, then you might like to know how to |
1708 | get at the same concepts in Perl. |
1709 | |
1710 | For example, it's common to call an object an I<instance> of a class |
1711 | and to call those objects' methods I<instance methods>. Data fields |
1712 | peculiar to each object are often called I<instance data> or I<object |
1713 | attributes>, and data fields common to all members of that class are |
1714 | I<class data>, I<class attributes>, or I<static data members>. |
1715 | |
1716 | Also, I<base class>, I<generic class>, and I<superclass> all describe |
1717 | the same notion, whereas I<derived class>, I<specific class>, and |
1718 | I<subclass> describe the other related one. |
1719 | |
1720 | C++ programmers have I<static methods> and I<virtual methods>, |
1721 | but Perl only has I<class methods> and I<object methods>. |
1722 | Actually, Perl only has methods. Whether a method gets used |
1723 | as a class or object method is by usage only. You could accidentally |
1724 | call a class method (one expecting a string argument) on an |
1725 | object (one expecting a reference), or vice versa. |
1726 | |
8d9aa5e0 |
1727 | Z<>From the C++ perspective, all methods in Perl are virtual. |
5f05dabc |
1728 | This, by the way, is why they are never checked for function |
1729 | prototypes in the argument list as regular built-in and user-defined |
1730 | functions can be. |
1731 | |
1732 | Because a class is itself something of an object, Perl's classes can be |
1733 | taken as describing both a "class as meta-object" (also called I<object |
1734 | factory>) philosophy and the "class as type definition" (I<declaring> |
1735 | behaviour, not I<defining> mechanism) idea. C++ supports the latter |
1736 | notion, but not the former. |
1737 | |
1738 | =head1 SEE ALSO |
1739 | |
1740 | The following man pages will doubtless provide more |
1741 | background for this one: |
1742 | L<perlmod>, |
1743 | L<perlref>, |
1744 | L<perlobj>, |
1745 | L<perlbot>, |
1746 | L<perltie>, |
1747 | and |
1748 | L<overload>. |
1749 | |
1750 | =head1 COPYRIGHT |
1751 | |
1752 | I I<really> hate to have to say this, but recent unpleasant |
1753 | experiences have mandated its inclusion: |
1754 | |
1755 | Copyright 1996 Tom Christiansen. All Rights Reserved. |
1756 | |
1757 | This work derives in part from the second edition of I<Programming Perl>. |
1758 | Although destined for release as a man page with the standard Perl |
1759 | distribution, it is not public domain (nor is any of Perl and its docset: |
1760 | publishers beware). It's expected to someday make its way into a revision |
1761 | of the Camel Book. While it is copyright by me with all rights reserved, |
1762 | permission is granted to freely distribute verbatim copies of this |
1763 | document provided that no modifications outside of formatting be made, |
1764 | and that this notice remain intact. You are permitted and encouraged to |
1765 | use its code and derivatives thereof in your own source code for fun or |
1766 | for profit as you see fit. But so help me, if in six months I find some |
1767 | book out there with a hacked-up version of this material in it claiming to |
1768 | be written by someone else, I'll tell all the world that you're a jerk. |
1769 | Furthermore, your lawyer will meet my lawyer (or O'Reilly's) over lunch |
1770 | to arrange for you to receive your just deserts. Count on it. |
1771 | |
1772 | =head2 Acknowledgments |
1773 | |
1774 | Thanks to |
1775 | Larry Wall, |
1776 | Roderick Schertler, |
1777 | Gurusamy Sarathy, |
1778 | Dean Roehrich, |
1779 | Raphael Manfredi, |
1780 | Brent Halsey, |
1781 | Greg Bacon, |
1782 | Brad Appleton, |
1783 | and many others for their helpful comments. |