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5f05dabc | 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 | |
8d9aa5e0 | 134 | with a non-prime one instead, you can't expect the compiler to catch this. |
5f05dabc | 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. | |
8d9aa5e0 TC |
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 | |
5f05dabc | 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 | |
8d9aa5e0 | 440 | objects are destroyed, the original $Census variable will be altered, |
5f05dabc | 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 | |
a6006777 | 451 | to perl version 5.004 we'll have to quote the field name.) |
5f05dabc | 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 | ||
8d9aa5e0 TC |
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 $Employee::Debugging. | |
5f05dabc | 544 | |
545 | =head2 Class Destructors | |
546 | ||
547 | The object destructor handles the death of each distinct object. But sometimes | |
548 | you want a bit of cleanup when the entire class is shut down, which | |
549 | currently only happens when the program exits. To make such a | |
550 | I<class destructor>, create a function in that class's package named | |
551 | END. This works just like the END function in traditional modules, | |
552 | meaning that it gets called whenever your program exits unless it execs | |
553 | or dies of an uncaught signal. For example, | |
554 | ||
555 | sub END { | |
556 | if ($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 TC |
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 TC |
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. |