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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 |
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68 | running Unix or Plan 9, but something like Mac OS or VMS, the directory |
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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 |
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92 | record, a C struct, or a C++ class is an anonymous hash. That's because a |
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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 | |
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114 | And so you could get at C<< $rec->{NAME} >> to find "Jason", or |
115 | C<< @{ $rec->{PEERS} } >> to get at "Norbert", "Rhys", and "Phineas". |
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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 | |
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227 | sub new { |
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228 | my $class = shift; |
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229 | my $self = {}; |
230 | $self->{NAME} = undef; |
231 | $self->{AGE} = undef; |
232 | $self->{PEERS} = []; |
233 | bless ($self, $class); |
234 | return $self; |
235 | } |
236 | |
237 | That's about all there is for constructors. These methods bring objects |
238 | to life, returning neat little opaque bundles to the user to be used in |
239 | subsequent method calls. |
240 | |
241 | =head2 Destructors |
242 | |
243 | Every story has a beginning and an end. The beginning of the object's |
244 | story is its constructor, explicitly called when the object comes into |
245 | existence. But the ending of its story is the I<destructor>, a method |
246 | implicitly called when an object leaves this life. Any per-object |
247 | clean-up code is placed in the destructor, which must (in Perl) be called |
248 | DESTROY. |
249 | |
250 | If constructors can have arbitrary names, then why not destructors? |
251 | Because while a constructor is explicitly called, a destructor is not. |
252 | Destruction happens automatically via Perl's garbage collection (GC) |
253 | system, which is a quick but somewhat lazy reference-based GC system. |
254 | To know what to call, Perl insists that the destructor be named DESTROY. |
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255 | Perl's notion of the right time to call a destructor is not well-defined |
256 | currently, which is why your destructors should not rely on when they are |
257 | called. |
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258 | |
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259 | Why is DESTROY in all caps? Perl on occasion uses purely uppercase |
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260 | function names as a convention to indicate that the function will |
261 | be automatically called by Perl in some way. Others that are called |
262 | implicitly include BEGIN, END, AUTOLOAD, plus all methods used by |
263 | tied objects, described in L<perltie>. |
264 | |
265 | In really good object-oriented programming languages, the user doesn't |
266 | care when the destructor is called. It just happens when it's supposed |
267 | to. In low-level languages without any GC at all, there's no way to |
268 | depend on this happening at the right time, so the programmer must |
269 | explicitly call the destructor to clean up memory and state, crossing |
270 | their fingers that it's the right time to do so. Unlike C++, an |
271 | object destructor is nearly never needed in Perl, and even when it is, |
272 | explicit invocation is uncalled for. In the case of our Person class, |
273 | we don't need a destructor because Perl takes care of simple matters |
274 | like memory deallocation. |
275 | |
276 | The only situation where Perl's reference-based GC won't work is |
277 | when there's a circularity in the data structure, such as: |
278 | |
279 | $this->{WHATEVER} = $this; |
280 | |
281 | In that case, you must delete the self-reference manually if you expect |
282 | your program not to leak memory. While admittedly error-prone, this is |
283 | the best we can do right now. Nonetheless, rest assured that when your |
284 | program is finished, its objects' destructors are all duly called. |
285 | So you are guaranteed that an object I<eventually> gets properly |
286 | destroyed, except in the unique case of a program that never exits. |
287 | (If you're running Perl embedded in another application, this full GC |
288 | pass happens a bit more frequently--whenever a thread shuts down.) |
289 | |
290 | =head2 Other Object Methods |
291 | |
292 | The methods we've talked about so far have either been constructors or |
293 | else simple "data methods", interfaces to data stored in the object. |
294 | These are a bit like an object's data members in the C++ world, except |
295 | that strangers don't access them as data. Instead, they should only |
296 | access the object's data indirectly via its methods. This is an |
297 | important rule: in Perl, access to an object's data should I<only> |
298 | be made through methods. |
299 | |
300 | Perl doesn't impose restrictions on who gets to use which methods. |
301 | The public-versus-private distinction is by convention, not syntax. |
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302 | (Well, unless you use the Alias module described below in |
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303 | L<Data Members as Variables>.) Occasionally you'll see method names beginning or ending |
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304 | with an underscore or two. This marking is a convention indicating |
305 | that the methods are private to that class alone and sometimes to its |
306 | closest acquaintances, its immediate subclasses. But this distinction |
307 | is not enforced by Perl itself. It's up to the programmer to behave. |
308 | |
309 | There's no reason to limit methods to those that simply access data. |
310 | Methods can do anything at all. The key point is that they're invoked |
311 | against an object or a class. Let's say we'd like object methods that |
312 | do more than fetch or set one particular field. |
313 | |
314 | sub exclaim { |
315 | my $self = shift; |
316 | return sprintf "Hi, I'm %s, age %d, working with %s", |
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317 | $self->{NAME}, $self->{AGE}, join(", ", @{$self->{PEERS}}); |
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318 | } |
319 | |
320 | Or maybe even one like this: |
321 | |
322 | sub happy_birthday { |
323 | my $self = shift; |
324 | return ++$self->{AGE}; |
325 | } |
326 | |
327 | Some might argue that one should go at these this way: |
328 | |
329 | sub exclaim { |
330 | my $self = shift; |
331 | return sprintf "Hi, I'm %s, age %d, working with %s", |
332 | $self->name, $self->age, join(", ", $self->peers); |
333 | } |
334 | |
335 | sub happy_birthday { |
336 | my $self = shift; |
337 | return $self->age( $self->age() + 1 ); |
338 | } |
339 | |
340 | But since these methods are all executing in the class itself, this |
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341 | may not be critical. There are tradeoffs to be made. Using direct |
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342 | hash access is faster (about an order of magnitude faster, in fact), and |
343 | it's more convenient when you want to interpolate in strings. But using |
344 | methods (the external interface) internally shields not just the users of |
345 | your class but even you yourself from changes in your data representation. |
346 | |
347 | =head1 Class Data |
348 | |
349 | What about "class data", data items common to each object in a class? |
350 | What would you want that for? Well, in your Person class, you might |
351 | like to keep track of the total people alive. How do you implement that? |
352 | |
353 | You I<could> make it a global variable called $Person::Census. But about |
354 | only reason you'd do that would be if you I<wanted> people to be able to |
355 | get at your class data directly. They could just say $Person::Census |
356 | and play around with it. Maybe this is ok in your design scheme. |
357 | You might even conceivably want to make it an exported variable. To be |
358 | exportable, a variable must be a (package) global. If this were a |
359 | traditional module rather than an object-oriented one, you might do that. |
360 | |
361 | While this approach is expected in most traditional modules, it's |
362 | generally considered rather poor form in most object modules. In an |
363 | object module, you should set up a protective veil to separate interface |
364 | from implementation. So provide a class method to access class data |
365 | just as you provide object methods to access object data. |
366 | |
367 | So, you I<could> still keep $Census as a package global and rely upon |
368 | others to honor the contract of the module and therefore not play around |
369 | with its implementation. You could even be supertricky and make $Census a |
370 | tied object as described in L<perltie>, thereby intercepting all accesses. |
371 | |
372 | But more often than not, you just want to make your class data a |
373 | file-scoped lexical. To do so, simply put this at the top of the file: |
374 | |
375 | my $Census = 0; |
376 | |
377 | Even though the scope of a my() normally expires when the block in which |
378 | it was declared is done (in this case the whole file being required or |
379 | used), Perl's deep binding of lexical variables guarantees that the |
380 | variable will not be deallocated, remaining accessible to functions |
381 | declared within that scope. This doesn't work with global variables |
382 | given temporary values via local(), though. |
383 | |
384 | Irrespective of whether you leave $Census a package global or make |
385 | it instead a file-scoped lexical, you should make these |
386 | changes to your Person::new() constructor: |
387 | |
388 | sub new { |
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389 | my $class = shift; |
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390 | my $self = {}; |
391 | $Census++; |
392 | $self->{NAME} = undef; |
393 | $self->{AGE} = undef; |
394 | $self->{PEERS} = []; |
395 | bless ($self, $class); |
396 | return $self; |
397 | } |
398 | |
399 | sub population { |
400 | return $Census; |
401 | } |
402 | |
403 | Now that we've done this, we certainly do need a destructor so that |
404 | when Person is destroyed, the $Census goes down. Here's how |
405 | this could be done: |
406 | |
407 | sub DESTROY { --$Census } |
408 | |
409 | Notice how there's no memory to deallocate in the destructor? That's |
410 | something that Perl takes care of for you all by itself. |
411 | |
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412 | Alternatively, you could use the Class::Data::Inheritable module from |
413 | CPAN. |
414 | |
415 | |
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416 | =head2 Accessing Class Data |
417 | |
418 | It turns out that this is not really a good way to go about handling |
419 | class data. A good scalable rule is that I<you must never reference class |
420 | data directly from an object method>. Otherwise you aren't building a |
421 | scalable, inheritable class. The object must be the rendezvous point |
422 | for all operations, especially from an object method. The globals |
423 | (class data) would in some sense be in the "wrong" package in your |
424 | derived classes. In Perl, methods execute in the context of the class |
425 | they were defined in, I<not> that of the object that triggered them. |
426 | Therefore, namespace visibility of package globals in methods is unrelated |
427 | to inheritance. |
428 | |
429 | Got that? Maybe not. Ok, let's say that some other class "borrowed" |
430 | (well, inherited) the DESTROY method as it was defined above. When those |
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431 | objects are destroyed, the original $Census variable will be altered, |
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432 | not the one in the new class's package namespace. Perhaps this is what |
433 | you want, but probably it isn't. |
434 | |
435 | Here's how to fix this. We'll store a reference to the data in the |
436 | value accessed by the hash key "_CENSUS". Why the underscore? Well, |
437 | mostly because an initial underscore already conveys strong feelings |
438 | of magicalness to a C programmer. It's really just a mnemonic device |
439 | to remind ourselves that this field is special and not to be used as |
440 | a public data member in the same way that NAME, AGE, and PEERS are. |
441 | (Because we've been developing this code under the strict pragma, prior |
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442 | to perl version 5.004 we'll have to quote the field name.) |
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443 | |
444 | sub new { |
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445 | my $class = shift; |
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446 | my $self = {}; |
447 | $self->{NAME} = undef; |
448 | $self->{AGE} = undef; |
449 | $self->{PEERS} = []; |
450 | # "private" data |
451 | $self->{"_CENSUS"} = \$Census; |
452 | bless ($self, $class); |
453 | ++ ${ $self->{"_CENSUS"} }; |
454 | return $self; |
455 | } |
456 | |
457 | sub population { |
458 | my $self = shift; |
459 | if (ref $self) { |
460 | return ${ $self->{"_CENSUS"} }; |
461 | } else { |
462 | return $Census; |
463 | } |
464 | } |
465 | |
466 | sub DESTROY { |
467 | my $self = shift; |
468 | -- ${ $self->{"_CENSUS"} }; |
469 | } |
470 | |
471 | =head2 Debugging Methods |
472 | |
473 | It's common for a class to have a debugging mechanism. For example, |
474 | you might want to see when objects are created or destroyed. To do that, |
475 | add a debugging variable as a file-scoped lexical. For this, we'll pull |
476 | in the standard Carp module to emit our warnings and fatal messages. |
477 | That way messages will come out with the caller's filename and |
478 | line number instead of our own; if we wanted them to be from our own |
479 | perspective, we'd just use die() and warn() directly instead of croak() |
480 | and carp() respectively. |
481 | |
482 | use Carp; |
483 | my $Debugging = 0; |
484 | |
485 | Now add a new class method to access the variable. |
486 | |
487 | sub debug { |
488 | my $class = shift; |
489 | if (ref $class) { confess "Class method called as object method" } |
490 | unless (@_ == 1) { confess "usage: CLASSNAME->debug(level)" } |
491 | $Debugging = shift; |
492 | } |
493 | |
494 | Now fix up DESTROY to murmur a bit as the moribund object expires: |
495 | |
496 | sub DESTROY { |
497 | my $self = shift; |
498 | if ($Debugging) { carp "Destroying $self " . $self->name } |
499 | -- ${ $self->{"_CENSUS"} }; |
500 | } |
501 | |
502 | One could conceivably make a per-object debug state. That |
503 | way you could call both of these: |
504 | |
505 | Person->debug(1); # entire class |
506 | $him->debug(1); # just this object |
507 | |
508 | To do so, we need our debugging method to be a "bimodal" one, one that |
509 | works on both classes I<and> objects. Therefore, adjust the debug() |
510 | and DESTROY methods as follows: |
511 | |
512 | sub debug { |
513 | my $self = shift; |
514 | confess "usage: thing->debug(level)" unless @_ == 1; |
515 | my $level = shift; |
516 | if (ref($self)) { |
517 | $self->{"_DEBUG"} = $level; # just myself |
518 | } else { |
519 | $Debugging = $level; # whole class |
520 | } |
521 | } |
522 | |
523 | sub DESTROY { |
524 | my $self = shift; |
525 | if ($Debugging || $self->{"_DEBUG"}) { |
526 | carp "Destroying $self " . $self->name; |
527 | } |
528 | -- ${ $self->{"_CENSUS"} }; |
529 | } |
530 | |
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531 | What happens if a derived class (which we'll call Employee) inherits |
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532 | methods from this Person base class? Then C<< Employee->debug() >>, when called |
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533 | as a class method, manipulates $Person::Debugging not $Employee::Debugging. |
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534 | |
535 | =head2 Class Destructors |
536 | |
537 | The object destructor handles the death of each distinct object. But sometimes |
538 | you want a bit of cleanup when the entire class is shut down, which |
539 | currently only happens when the program exits. To make such a |
540 | I<class destructor>, create a function in that class's package named |
541 | END. This works just like the END function in traditional modules, |
542 | meaning that it gets called whenever your program exits unless it execs |
543 | or dies of an uncaught signal. For example, |
544 | |
545 | sub END { |
546 | if ($Debugging) { |
547 | print "All persons are going away now.\n"; |
548 | } |
549 | } |
550 | |
551 | When the program exits, all the class destructors (END functions) are |
552 | be called in the opposite order that they were loaded in (LIFO order). |
553 | |
554 | =head2 Documenting the Interface |
555 | |
556 | And there you have it: we've just shown you the I<implementation> of this |
557 | Person class. Its I<interface> would be its documentation. Usually this |
558 | means putting it in pod ("plain old documentation") format right there |
559 | in the same file. In our Person example, we would place the following |
560 | docs anywhere in the Person.pm file. Even though it looks mostly like |
561 | code, it's not. It's embedded documentation such as would be used by |
562 | the pod2man, pod2html, or pod2text programs. The Perl compiler ignores |
563 | pods entirely, just as the translators ignore code. Here's an example of |
564 | some pods describing the informal interface: |
565 | |
566 | =head1 NAME |
567 | |
568 | Person - class to implement people |
569 | |
570 | =head1 SYNOPSIS |
571 | |
572 | use Person; |
573 | |
574 | ################# |
575 | # class methods # |
576 | ################# |
577 | $ob = Person->new; |
578 | $count = Person->population; |
579 | |
580 | ####################### |
581 | # object data methods # |
582 | ####################### |
583 | |
584 | ### get versions ### |
585 | $who = $ob->name; |
586 | $years = $ob->age; |
587 | @pals = $ob->peers; |
588 | |
589 | ### set versions ### |
590 | $ob->name("Jason"); |
591 | $ob->age(23); |
592 | $ob->peers( "Norbert", "Rhys", "Phineas" ); |
593 | |
594 | ######################## |
595 | # other object methods # |
596 | ######################## |
597 | |
598 | $phrase = $ob->exclaim; |
599 | $ob->happy_birthday; |
600 | |
601 | =head1 DESCRIPTION |
602 | |
603 | The Person class implements dah dee dah dee dah.... |
604 | |
605 | That's all there is to the matter of interface versus implementation. |
606 | A programmer who opens up the module and plays around with all the private |
607 | little shiny bits that were safely locked up behind the interface contract |
608 | has voided the warranty, and you shouldn't worry about their fate. |
609 | |
610 | =head1 Aggregation |
611 | |
612 | Suppose you later want to change the class to implement better names. |
613 | Perhaps you'd like to support both given names (called Christian names, |
614 | irrespective of one's religion) and family names (called surnames), plus |
615 | nicknames and titles. If users of your Person class have been properly |
616 | accessing it through its documented interface, then you can easily change |
617 | the underlying implementation. If they haven't, then they lose and |
618 | it's their fault for breaking the contract and voiding their warranty. |
619 | |
620 | To do this, we'll make another class, this one called Fullname. What's |
621 | the Fullname class look like? To answer that question, you have to |
622 | first figure out how you want to use it. How about we use it this way: |
623 | |
624 | $him = Person->new(); |
625 | $him->fullname->title("St"); |
626 | $him->fullname->christian("Thomas"); |
627 | $him->fullname->surname("Aquinas"); |
628 | $him->fullname->nickname("Tommy"); |
629 | printf "His normal name is %s\n", $him->name; |
630 | printf "But his real name is %s\n", $him->fullname->as_string; |
631 | |
632 | Ok. To do this, we'll change Person::new() so that it supports |
633 | a full name field this way: |
634 | |
635 | sub new { |
eac7fe86 |
636 | my $class = shift; |
5f05dabc |
637 | my $self = {}; |
638 | $self->{FULLNAME} = Fullname->new(); |
639 | $self->{AGE} = undef; |
640 | $self->{PEERS} = []; |
641 | $self->{"_CENSUS"} = \$Census; |
642 | bless ($self, $class); |
643 | ++ ${ $self->{"_CENSUS"} }; |
644 | return $self; |
645 | } |
646 | |
647 | sub fullname { |
648 | my $self = shift; |
649 | return $self->{FULLNAME}; |
650 | } |
651 | |
652 | Then to support old code, define Person::name() this way: |
653 | |
654 | sub name { |
655 | my $self = shift; |
656 | return $self->{FULLNAME}->nickname(@_) |
657 | || $self->{FULLNAME}->christian(@_); |
658 | } |
659 | |
660 | Here's the Fullname class. We'll use the same technique |
661 | of using a hash reference to hold data fields, and methods |
662 | by the appropriate name to access them: |
663 | |
664 | package Fullname; |
665 | use strict; |
666 | |
667 | sub new { |
eac7fe86 |
668 | my $class = shift; |
5f05dabc |
669 | my $self = { |
670 | TITLE => undef, |
671 | CHRISTIAN => undef, |
672 | SURNAME => undef, |
673 | NICK => undef, |
674 | }; |
675 | bless ($self, $class); |
676 | return $self; |
677 | } |
678 | |
679 | sub christian { |
680 | my $self = shift; |
681 | if (@_) { $self->{CHRISTIAN} = shift } |
682 | return $self->{CHRISTIAN}; |
683 | } |
684 | |
685 | sub surname { |
686 | my $self = shift; |
687 | if (@_) { $self->{SURNAME} = shift } |
688 | return $self->{SURNAME}; |
689 | } |
690 | |
691 | sub nickname { |
692 | my $self = shift; |
693 | if (@_) { $self->{NICK} = shift } |
694 | return $self->{NICK}; |
695 | } |
696 | |
697 | sub title { |
698 | my $self = shift; |
699 | if (@_) { $self->{TITLE} = shift } |
700 | return $self->{TITLE}; |
701 | } |
702 | |
703 | sub as_string { |
704 | my $self = shift; |
705 | my $name = join(" ", @$self{'CHRISTIAN', 'SURNAME'}); |
706 | if ($self->{TITLE}) { |
707 | $name = $self->{TITLE} . " " . $name; |
708 | } |
709 | return $name; |
710 | } |
711 | |
712 | 1; |
713 | |
714 | Finally, here's the test program: |
715 | |
716 | #!/usr/bin/perl -w |
717 | use strict; |
718 | use Person; |
719 | sub END { show_census() } |
720 | |
721 | sub show_census () { |
722 | printf "Current population: %d\n", Person->population; |
723 | } |
724 | |
725 | Person->debug(1); |
726 | |
727 | show_census(); |
728 | |
729 | my $him = Person->new(); |
730 | |
731 | $him->fullname->christian("Thomas"); |
732 | $him->fullname->surname("Aquinas"); |
733 | $him->fullname->nickname("Tommy"); |
734 | $him->fullname->title("St"); |
735 | $him->age(1); |
736 | |
69fc43e8 |
737 | printf "%s is really %s.\n", $him->name, $him->fullname->as_string; |
5f05dabc |
738 | printf "%s's age: %d.\n", $him->name, $him->age; |
739 | $him->happy_birthday; |
740 | printf "%s's age: %d.\n", $him->name, $him->age; |
741 | |
742 | show_census(); |
743 | |
744 | =head1 Inheritance |
745 | |
746 | Object-oriented programming systems all support some notion of |
747 | inheritance. Inheritance means allowing one class to piggy-back on |
748 | top of another one so you don't have to write the same code again and |
749 | again. It's about software reuse, and therefore related to Laziness, |
750 | the principal virtue of a programmer. (The import/export mechanisms in |
751 | traditional modules are also a form of code reuse, but a simpler one than |
752 | the true inheritance that you find in object modules.) |
753 | |
754 | Sometimes the syntax of inheritance is built into the core of the |
755 | language, and sometimes it's not. Perl has no special syntax for |
756 | specifying the class (or classes) to inherit from. Instead, it's all |
757 | strictly in the semantics. Each package can have a variable called @ISA, |
758 | which governs (method) inheritance. If you try to call a method on an |
759 | object or class, and that method is not found in that object's package, |
760 | Perl then looks to @ISA for other packages to go looking through in |
761 | search of the missing method. |
762 | |
763 | Like the special per-package variables recognized by Exporter (such as |
764 | @EXPORT, @EXPORT_OK, @EXPORT_FAIL, %EXPORT_TAGS, and $VERSION), the @ISA |
765 | array I<must> be a package-scoped global and not a file-scoped lexical |
766 | created via my(). Most classes have just one item in their @ISA array. |
767 | In this case, we have what's called "single inheritance", or SI for short. |
768 | |
769 | Consider this class: |
770 | |
771 | package Employee; |
772 | use Person; |
773 | @ISA = ("Person"); |
774 | 1; |
775 | |
776 | Not a lot to it, eh? All it's doing so far is loading in another |
777 | class and stating that this one will inherit methods from that |
778 | other class if need be. We have given it none of its own methods. |
779 | We rely upon an Employee to behave just like a Person. |
780 | |
781 | Setting up an empty class like this is called the "empty subclass test"; |
782 | that is, making a derived class that does nothing but inherit from a |
783 | base class. If the original base class has been designed properly, |
784 | then the new derived class can be used as a drop-in replacement for the |
785 | old one. This means you should be able to write a program like this: |
786 | |
3e3baf6d |
787 | use Employee; |
5f05dabc |
788 | my $empl = Employee->new(); |
789 | $empl->name("Jason"); |
790 | $empl->age(23); |
791 | printf "%s is age %d.\n", $empl->name, $empl->age; |
792 | |
793 | By proper design, we mean always using the two-argument form of bless(), |
794 | avoiding direct access of global data, and not exporting anything. If you |
795 | look back at the Person::new() function we defined above, we were careful |
796 | to do that. There's a bit of package data used in the constructor, |
797 | but the reference to this is stored on the object itself and all other |
798 | methods access package data via that reference, so we should be ok. |
799 | |
800 | What do we mean by the Person::new() function -- isn't that actually |
801 | a method? Well, in principle, yes. A method is just a function that |
802 | expects as its first argument a class name (package) or object |
803 | (blessed reference). Person::new() is the function that both the |
c47ff5f1 |
804 | C<< Person->new() >> method and the C<< Employee->new() >> method end |
5f05dabc |
805 | up calling. Understand that while a method call looks a lot like a |
806 | function call, they aren't really quite the same, and if you treat them |
807 | as the same, you'll very soon be left with nothing but broken programs. |
808 | First, the actual underlying calling conventions are different: method |
809 | calls get an extra argument. Second, function calls don't do inheritance, |
810 | but methods do. |
811 | |
812 | Method Call Resulting Function Call |
813 | ----------- ------------------------ |
814 | Person->new() Person::new("Person") |
815 | Employee->new() Person::new("Employee") |
816 | |
817 | So don't use function calls when you mean to call a method. |
818 | |
819 | If an employee is just a Person, that's not all too very interesting. |
820 | So let's add some other methods. We'll give our employee |
821 | data fields to access their salary, their employee ID, and their |
822 | start date. |
823 | |
824 | If you're getting a little tired of creating all these nearly identical |
825 | methods just to get at the object's data, do not despair. Later, |
826 | we'll describe several different convenience mechanisms for shortening |
827 | this up. Meanwhile, here's the straight-forward way: |
828 | |
829 | sub salary { |
830 | my $self = shift; |
831 | if (@_) { $self->{SALARY} = shift } |
832 | return $self->{SALARY}; |
833 | } |
834 | |
835 | sub id_number { |
836 | my $self = shift; |
837 | if (@_) { $self->{ID} = shift } |
838 | return $self->{ID}; |
839 | } |
840 | |
841 | sub start_date { |
842 | my $self = shift; |
843 | if (@_) { $self->{START_DATE} = shift } |
844 | return $self->{START_DATE}; |
845 | } |
846 | |
847 | =head2 Overridden Methods |
848 | |
849 | What happens when both a derived class and its base class have the same |
850 | method defined? Well, then you get the derived class's version of that |
851 | method. For example, let's say that we want the peers() method called on |
852 | an employee to act a bit differently. Instead of just returning the list |
853 | of peer names, let's return slightly different strings. So doing this: |
854 | |
855 | $empl->peers("Peter", "Paul", "Mary"); |
856 | printf "His peers are: %s\n", join(", ", $empl->peers); |
857 | |
858 | will produce: |
859 | |
860 | His peers are: PEON=PETER, PEON=PAUL, PEON=MARY |
861 | |
862 | To do this, merely add this definition into the Employee.pm file: |
863 | |
864 | sub peers { |
865 | my $self = shift; |
866 | if (@_) { @{ $self->{PEERS} } = @_ } |
867 | return map { "PEON=\U$_" } @{ $self->{PEERS} }; |
868 | } |
869 | |
870 | There, we've just demonstrated the high-falutin' concept known in certain |
871 | circles as I<polymorphism>. We've taken on the form and behaviour of |
872 | an existing object, and then we've altered it to suit our own purposes. |
873 | This is a form of Laziness. (Getting polymorphed is also what happens |
874 | when the wizard decides you'd look better as a frog.) |
875 | |
876 | Every now and then you'll want to have a method call trigger both its |
68dc0745 |
877 | derived class (also known as "subclass") version as well as its base class |
5f05dabc |
878 | (also known as "superclass") version. In practice, constructors and |
879 | destructors are likely to want to do this, and it probably also makes |
880 | sense in the debug() method we showed previously. |
881 | |
882 | To do this, add this to Employee.pm: |
883 | |
884 | use Carp; |
885 | my $Debugging = 0; |
886 | |
887 | sub debug { |
888 | my $self = shift; |
889 | confess "usage: thing->debug(level)" unless @_ == 1; |
890 | my $level = shift; |
891 | if (ref($self)) { |
892 | $self->{"_DEBUG"} = $level; |
893 | } else { |
894 | $Debugging = $level; # whole class |
895 | } |
896 | Person::debug($self, $Debugging); # don't really do this |
897 | } |
898 | |
899 | As you see, we turn around and call the Person package's debug() function. |
900 | But this is far too fragile for good design. What if Person doesn't |
901 | have a debug() function, but is inheriting I<its> debug() method |
902 | from elsewhere? It would have been slightly better to say |
903 | |
904 | Person->debug($Debugging); |
905 | |
906 | But even that's got too much hard-coded. It's somewhat better to say |
907 | |
908 | $self->Person::debug($Debugging); |
909 | |
910 | Which is a funny way to say to start looking for a debug() method up |
911 | in Person. This strategy is more often seen on overridden object methods |
912 | than on overridden class methods. |
913 | |
914 | There is still something a bit off here. We've hard-coded our |
915 | superclass's name. This in particular is bad if you change which classes |
916 | you inherit from, or add others. Fortunately, the pseudoclass SUPER |
917 | comes to the rescue here. |
918 | |
71be2cbc |
919 | $self->SUPER::debug($Debugging); |
5f05dabc |
920 | |
921 | This way it starts looking in my class's @ISA. This only makes sense |
922 | from I<within> a method call, though. Don't try to access anything |
923 | in SUPER:: from anywhere else, because it doesn't exist outside |
029f3b44 |
924 | an overridden method call. Note that C<SUPER> refers to the superclass of |
925 | the current package, I<not> to the superclass of C<$self>. |
5f05dabc |
926 | |
927 | Things are getting a bit complicated here. Have we done anything |
928 | we shouldn't? As before, one way to test whether we're designing |
929 | a decent class is via the empty subclass test. Since we already have |
930 | an Employee class that we're trying to check, we'd better get a new |
931 | empty subclass that can derive from Employee. Here's one: |
932 | |
933 | package Boss; |
934 | use Employee; # :-) |
935 | @ISA = qw(Employee); |
936 | |
937 | And here's the test program: |
938 | |
939 | #!/usr/bin/perl -w |
940 | use strict; |
941 | use Boss; |
942 | Boss->debug(1); |
943 | |
944 | my $boss = Boss->new(); |
945 | |
946 | $boss->fullname->title("Don"); |
947 | $boss->fullname->surname("Pichon Alvarez"); |
948 | $boss->fullname->christian("Federico Jesus"); |
949 | $boss->fullname->nickname("Fred"); |
950 | |
951 | $boss->age(47); |
952 | $boss->peers("Frank", "Felipe", "Faust"); |
953 | |
69fc43e8 |
954 | printf "%s is age %d.\n", $boss->fullname->as_string, $boss->age; |
5f05dabc |
955 | printf "His peers are: %s\n", join(", ", $boss->peers); |
956 | |
957 | Running it, we see that we're still ok. If you'd like to dump out your |
958 | object in a nice format, somewhat like the way the 'x' command works in |
959 | the debugger, you could use the Data::Dumper module from CPAN this way: |
960 | |
961 | use Data::Dumper; |
962 | print "Here's the boss:\n"; |
963 | print Dumper($boss); |
964 | |
965 | Which shows us something like this: |
966 | |
967 | Here's the boss: |
968 | $VAR1 = bless( { |
969 | _CENSUS => \1, |
970 | FULLNAME => bless( { |
971 | TITLE => 'Don', |
972 | SURNAME => 'Pichon Alvarez', |
973 | NICK => 'Fred', |
974 | CHRISTIAN => 'Federico Jesus' |
975 | }, 'Fullname' ), |
976 | AGE => 47, |
977 | PEERS => [ |
978 | 'Frank', |
979 | 'Felipe', |
980 | 'Faust' |
981 | ] |
982 | }, 'Boss' ); |
983 | |
984 | Hm.... something's missing there. What about the salary, start date, |
985 | and ID fields? Well, we never set them to anything, even undef, so they |
986 | don't show up in the hash's keys. The Employee class has no new() method |
987 | of its own, and the new() method in Person doesn't know about Employees. |
988 | (Nor should it: proper OO design dictates that a subclass be allowed to |
989 | know about its immediate superclass, but never vice-versa.) So let's |
990 | fix up Employee::new() this way: |
991 | |
992 | sub new { |
eac7fe86 |
993 | my $class = shift; |
5f05dabc |
994 | my $self = $class->SUPER::new(); |
995 | $self->{SALARY} = undef; |
996 | $self->{ID} = undef; |
997 | $self->{START_DATE} = undef; |
998 | bless ($self, $class); # reconsecrate |
999 | return $self; |
1000 | } |
1001 | |
1002 | Now if you dump out an Employee or Boss object, you'll find |
1003 | that new fields show up there now. |
1004 | |
1005 | =head2 Multiple Inheritance |
1006 | |
1007 | Ok, at the risk of confusing beginners and annoying OO gurus, it's |
1008 | time to confess that Perl's object system includes that controversial |
1009 | notion known as multiple inheritance, or MI for short. All this means |
1010 | is that rather than having just one parent class who in turn might |
1011 | itself have a parent class, etc., that you can directly inherit from |
1012 | two or more parents. It's true that some uses of MI can get you into |
1013 | trouble, although hopefully not quite so much trouble with Perl as with |
1014 | dubiously-OO languages like C++. |
1015 | |
1016 | The way it works is actually pretty simple: just put more than one package |
1017 | name in your @ISA array. When it comes time for Perl to go finding |
1018 | methods for your object, it looks at each of these packages in order. |
dd69841b |
1019 | Well, kinda. It's actually a fully recursive, depth-first order by |
1020 | default (see L<mro> for alternate method resolution orders). |
5f05dabc |
1021 | Consider a bunch of @ISA arrays like this: |
1022 | |
1023 | @First::ISA = qw( Alpha ); |
1024 | @Second::ISA = qw( Beta ); |
1025 | @Third::ISA = qw( First Second ); |
1026 | |
1027 | If you have an object of class Third: |
1028 | |
1029 | my $ob = Third->new(); |
1030 | $ob->spin(); |
1031 | |
1032 | How do we find a spin() method (or a new() method for that matter)? |
1033 | Because the search is depth-first, classes will be looked up |
1034 | in the following order: Third, First, Alpha, Second, and Beta. |
1035 | |
1036 | In practice, few class modules have been seen that actually |
1037 | make use of MI. One nearly always chooses simple containership of |
1038 | one class within another over MI. That's why our Person |
1039 | object I<contained> a Fullname object. That doesn't mean |
1040 | it I<was> one. |
1041 | |
1042 | However, there is one particular area where MI in Perl is rampant: |
1043 | borrowing another class's class methods. This is rather common, |
1044 | especially with some bundled "objectless" classes, |
1045 | like Exporter, DynaLoader, AutoLoader, and SelfLoader. These classes |
1046 | do not provide constructors; they exist only so you may inherit their |
1047 | class methods. (It's not entirely clear why inheritance was done |
1048 | here rather than traditional module importation.) |
1049 | |
1050 | For example, here is the POSIX module's @ISA: |
1051 | |
1052 | package POSIX; |
1053 | @ISA = qw(Exporter DynaLoader); |
1054 | |
1055 | The POSIX module isn't really an object module, but then, |
1056 | neither are Exporter or DynaLoader. They're just lending their |
1057 | classes' behaviours to POSIX. |
1058 | |
1059 | Why don't people use MI for object methods much? One reason is that |
1060 | it can have complicated side-effects. For one thing, your inheritance |
1061 | graph (no longer a tree) might converge back to the same base class. |
1062 | Although Perl guards against recursive inheritance, merely having parents |
1063 | who are related to each other via a common ancestor, incestuous though |
1064 | it sounds, is not forbidden. What if in our Third class shown above we |
1065 | wanted its new() method to also call both overridden constructors in its |
1066 | two parent classes? The SUPER notation would only find the first one. |
1067 | Also, what about if the Alpha and Beta classes both had a common ancestor, |
1068 | like Nought? If you kept climbing up the inheritance tree calling |
1069 | overridden methods, you'd end up calling Nought::new() twice, |
1070 | which might well be a bad idea. |
1071 | |
1072 | =head2 UNIVERSAL: The Root of All Objects |
1073 | |
1074 | Wouldn't it be convenient if all objects were rooted at some ultimate |
1075 | base class? That way you could give every object common methods without |
1076 | having to go and add it to each and every @ISA. Well, it turns out that |
1077 | you can. You don't see it, but Perl tacitly and irrevocably assumes |
1078 | that there's an extra element at the end of @ISA: the class UNIVERSAL. |
a6006777 |
1079 | In version 5.003, there were no predefined methods there, but you could put |
5f05dabc |
1080 | whatever you felt like into it. |
1081 | |
a6006777 |
1082 | However, as of version 5.004 (or some subversive releases, like 5.003_08), |
54310121 |
1083 | UNIVERSAL has some methods in it already. These are builtin to your Perl |
5f05dabc |
1084 | binary, so they don't take any extra time to load. Predefined methods |
1085 | include isa(), can(), and VERSION(). isa() tells you whether an object or |
1086 | class "is" another one without having to traverse the hierarchy yourself: |
1087 | |
1088 | $has_io = $fd->isa("IO::Handle"); |
1089 | $itza_handle = IO::Socket->isa("IO::Handle"); |
1090 | |
1091 | The can() method, called against that object or class, reports back |
1092 | whether its string argument is a callable method name in that class. |
1093 | In fact, it gives you back a function reference to that method: |
1094 | |
1095 | $his_print_method = $obj->can('as_string'); |
1096 | |
1097 | Finally, the VERSION method checks whether the class (or the object's |
1098 | class) has a package global called $VERSION that's high enough, as in: |
1099 | |
1100 | Some_Module->VERSION(3.0); |
1101 | $his_vers = $ob->VERSION(); |
1102 | |
1103 | However, we don't usually call VERSION ourselves. (Remember that an all |
68dc0745 |
1104 | uppercase function name is a Perl convention that indicates that the |
5f05dabc |
1105 | function will be automatically used by Perl in some way.) In this case, |
1106 | it happens when you say |
1107 | |
1108 | use Some_Module 3.0; |
1109 | |
8d9aa5e0 |
1110 | If you wanted to add version checking to your Person class explained |
5f05dabc |
1111 | above, just add this to Person.pm: |
1112 | |
77ca0c92 |
1113 | our $VERSION = '1.1'; |
5f05dabc |
1114 | |
0184c62d |
1115 | and then in Employee.pm you can say |
5f05dabc |
1116 | |
0184c62d |
1117 | use Person 1.1; |
5f05dabc |
1118 | |
1119 | And it would make sure that you have at least that version number or |
1120 | higher available. This is not the same as loading in that exact version |
1121 | number. No mechanism currently exists for concurrent installation of |
1122 | multiple versions of a module. Lamentably. |
1123 | |
dd69841b |
1124 | =head2 Deeper UNIVERSAL details |
1125 | |
1126 | It is also valid (though perhaps unwise in most cases) to put other |
1127 | packages' names in @UNIVERSAL::ISA. These packages will also be |
1128 | implicitly inherited by all classes, just as UNIVERSAL itself is. |
1129 | However, neither UNIVERSAL nor any of its parents from the @ISA tree |
1130 | are explicit base classes of all objects. To clarify, given the |
1131 | following: |
1132 | |
1133 | @UNIVERSAL::ISA = ('REALLYUNIVERSAL'); |
1134 | |
1135 | package REALLYUNIVERSAL; |
1136 | sub special_method { return "123" } |
1137 | |
1138 | package Foo; |
1139 | sub normal_method { return "321" } |
1140 | |
1141 | Calling Foo->special_method() will return "123", but calling |
1142 | Foo->isa('REALLYUNIVERSAL') or Foo->isa('UNIVERSAL') will return |
1143 | false. |
1144 | |
1145 | If your class is using an alternate mro like C3 (see |
1146 | L<mro>), method resolution within UNIVERSAL / @UNIVERSAL::ISA will |
1147 | still occur in the default depth-first left-to-right manner, |
1148 | after the class's C3 mro is exhausted. |
1149 | |
1150 | All of the above is made more intuitive by realizing what really |
1151 | happens during method lookup, which is roughly like this |
1152 | ugly pseudo-code: |
1153 | |
1154 | get_mro(class) { |
1155 | # recurses down the @ISA's starting at class, |
1156 | # builds a single linear array of all |
1157 | # classes to search in the appropriate order. |
1158 | # The method resolution order (mro) to use |
1159 | # for the ordering is whichever mro "class" |
1160 | # has set on it (either default (depth first |
1161 | # l-to-r) or C3 ordering). |
1162 | # The first entry in the list is the class |
1163 | # itself. |
1164 | } |
1165 | |
1166 | find_method(class, methname) { |
1167 | foreach $class (get_mro(class)) { |
1168 | if($class->has_method(methname)) { |
1169 | return ref_to($class->$methname); |
1170 | } |
1171 | } |
1172 | foreach $class (get_mro(UNIVERSAL)) { |
1173 | if($class->has_method(methname)) { |
1174 | return ref_to($class->$methname); |
1175 | } |
1176 | } |
1177 | return undef; |
1178 | } |
1179 | |
1180 | However the code that implements UNIVERSAL::isa does not |
1181 | search in UNIVERSAL itself, only in the package's actual |
1182 | @ISA. |
1183 | |
5f05dabc |
1184 | =head1 Alternate Object Representations |
1185 | |
1186 | Nothing requires objects to be implemented as hash references. An object |
1187 | can be any sort of reference so long as its referent has been suitably |
1188 | blessed. That means scalar, array, and code references are also fair |
1189 | game. |
1190 | |
1191 | A scalar would work if the object has only one datum to hold. An array |
1192 | would work for most cases, but makes inheritance a bit dodgy because |
1193 | you have to invent new indices for the derived classes. |
1194 | |
1195 | =head2 Arrays as Objects |
1196 | |
1197 | If the user of your class honors the contract and sticks to the advertised |
1198 | interface, then you can change its underlying interface if you feel |
1199 | like it. Here's another implementation that conforms to the same |
1200 | interface specification. This time we'll use an array reference |
1201 | instead of a hash reference to represent the object. |
1202 | |
1203 | package Person; |
1204 | use strict; |
1205 | |
1206 | my($NAME, $AGE, $PEERS) = ( 0 .. 2 ); |
1207 | |
1208 | ############################################ |
1209 | ## the object constructor (array version) ## |
1210 | ############################################ |
1211 | sub new { |
1212 | my $self = []; |
1213 | $self->[$NAME] = undef; # this is unnecessary |
68dc0745 |
1214 | $self->[$AGE] = undef; # as is this |
5f05dabc |
1215 | $self->[$PEERS] = []; # but this isn't, really |
1216 | bless($self); |
1217 | return $self; |
1218 | } |
1219 | |
1220 | sub name { |
1221 | my $self = shift; |
1222 | if (@_) { $self->[$NAME] = shift } |
1223 | return $self->[$NAME]; |
1224 | } |
1225 | |
1226 | sub age { |
1227 | my $self = shift; |
1228 | if (@_) { $self->[$AGE] = shift } |
1229 | return $self->[$AGE]; |
1230 | } |
1231 | |
1232 | sub peers { |
1233 | my $self = shift; |
1234 | if (@_) { @{ $self->[$PEERS] } = @_ } |
1235 | return @{ $self->[$PEERS] }; |
1236 | } |
1237 | |
1238 | 1; # so the require or use succeeds |
1239 | |
8d9aa5e0 |
1240 | You might guess that the array access would be a lot faster than the |
1241 | hash access, but they're actually comparable. The array is a I<little> |
5f05dabc |
1242 | bit faster, but not more than ten or fifteen percent, even when you |
1243 | replace the variables above like $AGE with literal numbers, like 1. |
1244 | A bigger difference between the two approaches can be found in memory use. |
1245 | A hash representation takes up more memory than an array representation |
8d9aa5e0 |
1246 | because you have to allocate memory for the keys as well as for the values. |
a6006777 |
1247 | However, it really isn't that bad, especially since as of version 5.004, |
5f05dabc |
1248 | memory is only allocated once for a given hash key, no matter how many |
1249 | hashes have that key. It's expected that sometime in the future, even |
1250 | these differences will fade into obscurity as more efficient underlying |
1251 | representations are devised. |
1252 | |
1253 | Still, the tiny edge in speed (and somewhat larger one in memory) |
1254 | is enough to make some programmers choose an array representation |
1255 | for simple classes. There's still a little problem with |
1256 | scalability, though, because later in life when you feel |
1257 | like creating subclasses, you'll find that hashes just work |
1258 | out better. |
1259 | |
1260 | =head2 Closures as Objects |
1261 | |
1262 | Using a code reference to represent an object offers some fascinating |
1263 | possibilities. We can create a new anonymous function (closure) who |
1264 | alone in all the world can see the object's data. This is because we |
1265 | put the data into an anonymous hash that's lexically visible only to |
1266 | the closure we create, bless, and return as the object. This object's |
1267 | methods turn around and call the closure as a regular subroutine call, |
1268 | passing it the field we want to affect. (Yes, |
1269 | the double-function call is slow, but if you wanted fast, you wouldn't |
1270 | be using objects at all, eh? :-) |
1271 | |
1272 | Use would be similar to before: |
1273 | |
1274 | use Person; |
1275 | $him = Person->new(); |
1276 | $him->name("Jason"); |
1277 | $him->age(23); |
1278 | $him->peers( [ "Norbert", "Rhys", "Phineas" ] ); |
1279 | printf "%s is %d years old.\n", $him->name, $him->age; |
1280 | print "His peers are: ", join(", ", @{$him->peers}), "\n"; |
1281 | |
1282 | but the implementation would be radically, perhaps even sublimely |
1283 | different: |
1284 | |
1285 | package Person; |
1286 | |
1287 | sub new { |
eac7fe86 |
1288 | my $class = shift; |
5f05dabc |
1289 | my $self = { |
1290 | NAME => undef, |
1291 | AGE => undef, |
1292 | PEERS => [], |
1293 | }; |
1294 | my $closure = sub { |
1295 | my $field = shift; |
1296 | if (@_) { $self->{$field} = shift } |
1297 | return $self->{$field}; |
1298 | }; |
1299 | bless($closure, $class); |
1300 | return $closure; |
1301 | } |
1302 | |
1303 | sub name { &{ $_[0] }("NAME", @_[ 1 .. $#_ ] ) } |
1304 | sub age { &{ $_[0] }("AGE", @_[ 1 .. $#_ ] ) } |
1305 | sub peers { &{ $_[0] }("PEERS", @_[ 1 .. $#_ ] ) } |
1306 | |
1307 | 1; |
1308 | |
1309 | Because this object is hidden behind a code reference, it's probably a bit |
1310 | mysterious to those whose background is more firmly rooted in standard |
1311 | procedural or object-based programming languages than in functional |
1312 | programming languages whence closures derive. The object |
1313 | created and returned by the new() method is itself not a data reference |
1314 | as we've seen before. It's an anonymous code reference that has within |
1315 | it access to a specific version (lexical binding and instantiation) |
1316 | of the object's data, which are stored in the private variable $self. |
1317 | Although this is the same function each time, it contains a different |
1318 | version of $self. |
1319 | |
1320 | When a method like C<$him-E<gt>name("Jason")> is called, its implicit |
8d9aa5e0 |
1321 | zeroth argument is the invoking object--just as it is with all method |
5f05dabc |
1322 | calls. But in this case, it's our code reference (something like a |
1323 | function pointer in C++, but with deep binding of lexical variables). |
1324 | There's not a lot to be done with a code reference beyond calling it, so |
1325 | that's just what we do when we say C<&{$_[0]}>. This is just a regular |
1326 | function call, not a method call. The initial argument is the string |
1327 | "NAME", and any remaining arguments are whatever had been passed to the |
1328 | method itself. |
1329 | |
1330 | Once we're executing inside the closure that had been created in new(), |
1331 | the $self hash reference suddenly becomes visible. The closure grabs |
1332 | its first argument ("NAME" in this case because that's what the name() |
1333 | method passed it), and uses that string to subscript into the private |
1334 | hash hidden in its unique version of $self. |
1335 | |
1336 | Nothing under the sun will allow anyone outside the executing method to |
1337 | be able to get at this hidden data. Well, nearly nothing. You I<could> |
1338 | single step through the program using the debugger and find out the |
1339 | pieces while you're in the method, but everyone else is out of luck. |
1340 | |
1341 | There, if that doesn't excite the Scheme folks, then I just don't know |
1342 | what will. Translation of this technique into C++, Java, or any other |
1343 | braindead-static language is left as a futile exercise for aficionados |
1344 | of those camps. |
1345 | |
1346 | You could even add a bit of nosiness via the caller() function and |
1347 | make the closure refuse to operate unless called via its own package. |
1348 | This would no doubt satisfy certain fastidious concerns of programming |
1349 | police and related puritans. |
1350 | |
1351 | If you were wondering when Hubris, the third principle virtue of a |
1352 | programmer, would come into play, here you have it. (More seriously, |
1353 | Hubris is just the pride in craftsmanship that comes from having written |
1354 | a sound bit of well-designed code.) |
1355 | |
1356 | =head1 AUTOLOAD: Proxy Methods |
1357 | |
1358 | Autoloading is a way to intercept calls to undefined methods. An autoload |
1359 | routine may choose to create a new function on the fly, either loaded |
1360 | from disk or perhaps just eval()ed right there. This define-on-the-fly |
1361 | strategy is why it's called autoloading. |
1362 | |
1363 | But that's only one possible approach. Another one is to just |
1364 | have the autoloaded method itself directly provide the |
1365 | requested service. When used in this way, you may think |
1366 | of autoloaded methods as "proxy" methods. |
1367 | |
1368 | When Perl tries to call an undefined function in a particular package |
1369 | and that function is not defined, it looks for a function in |
1370 | that same package called AUTOLOAD. If one exists, it's called |
1371 | with the same arguments as the original function would have had. |
1372 | The fully-qualified name of the function is stored in that package's |
1373 | global variable $AUTOLOAD. Once called, the function can do anything |
1374 | it would like, including defining a new function by the right name, and |
1375 | then doing a really fancy kind of C<goto> right to it, erasing itself |
1376 | from the call stack. |
1377 | |
1378 | What does this have to do with objects? After all, we keep talking about |
1379 | functions, not methods. Well, since a method is just a function with |
1380 | an extra argument and some fancier semantics about where it's found, |
1381 | we can use autoloading for methods, too. Perl doesn't start looking |
1382 | for an AUTOLOAD method until it has exhausted the recursive hunt up |
1383 | through @ISA, though. Some programmers have even been known to define |
1384 | a UNIVERSAL::AUTOLOAD method to trap unresolved method calls to any |
1385 | kind of object. |
1386 | |
1387 | =head2 Autoloaded Data Methods |
1388 | |
1389 | You probably began to get a little suspicious about the duplicated |
1390 | code way back earlier when we first showed you the Person class, and |
1391 | then later the Employee class. Each method used to access the |
1392 | hash fields looked virtually identical. This should have tickled |
1393 | that great programming virtue, Impatience, but for the time, |
1394 | we let Laziness win out, and so did nothing. Proxy methods can cure |
1395 | this. |
1396 | |
1397 | Instead of writing a new function every time we want a new data field, |
1398 | we'll use the autoload mechanism to generate (actually, mimic) methods on |
1399 | the fly. To verify that we're accessing a valid member, we will check |
1400 | against an C<_permitted> (pronounced "under-permitted") field, which |
1401 | is a reference to a file-scoped lexical (like a C file static) hash of permitted fields in this record |
1402 | called %fields. Why the underscore? For the same reason as the _CENSUS |
1403 | field we once used: as a marker that means "for internal use only". |
1404 | |
1405 | Here's what the module initialization code and class |
1406 | constructor will look like when taking this approach: |
1407 | |
1408 | package Person; |
1409 | use Carp; |
77ca0c92 |
1410 | our $AUTOLOAD; # it's a package global |
5f05dabc |
1411 | |
1412 | my %fields = ( |
1413 | name => undef, |
1414 | age => undef, |
1415 | peers => undef, |
1416 | ); |
1417 | |
1418 | sub new { |
eac7fe86 |
1419 | my $class = shift; |
5f05dabc |
1420 | my $self = { |
1421 | _permitted => \%fields, |
1422 | %fields, |
1423 | }; |
1424 | bless $self, $class; |
1425 | return $self; |
1426 | } |
1427 | |
1428 | If we wanted our record to have default values, we could fill those in |
1429 | where current we have C<undef> in the %fields hash. |
1430 | |
1431 | Notice how we saved a reference to our class data on the object itself? |
1432 | Remember that it's important to access class data through the object |
1433 | itself instead of having any method reference %fields directly, or else |
1434 | you won't have a decent inheritance. |
1435 | |
1436 | The real magic, though, is going to reside in our proxy method, which |
1437 | will handle all calls to undefined methods for objects of class Person |
1438 | (or subclasses of Person). It has to be called AUTOLOAD. Again, it's |
1439 | all caps because it's called for us implicitly by Perl itself, not by |
1440 | a user directly. |
1441 | |
1442 | sub AUTOLOAD { |
1443 | my $self = shift; |
1444 | my $type = ref($self) |
1445 | or croak "$self is not an object"; |
1446 | |
1447 | my $name = $AUTOLOAD; |
1448 | $name =~ s/.*://; # strip fully-qualified portion |
1449 | |
1450 | unless (exists $self->{_permitted}->{$name} ) { |
1451 | croak "Can't access `$name' field in class $type"; |
1452 | } |
1453 | |
1454 | if (@_) { |
1455 | return $self->{$name} = shift; |
1456 | } else { |
1457 | return $self->{$name}; |
1458 | } |
1459 | } |
1460 | |
1461 | Pretty nifty, eh? All we have to do to add new data fields |
1462 | is modify %fields. No new functions need be written. |
1463 | |
1464 | I could have avoided the C<_permitted> field entirely, but I |
1465 | wanted to demonstrate how to store a reference to class data on the |
54310121 |
1466 | object so you wouldn't have to access that class data |
5f05dabc |
1467 | directly from an object method. |
1468 | |
1469 | =head2 Inherited Autoloaded Data Methods |
1470 | |
1471 | But what about inheritance? Can we define our Employee |
1472 | class similarly? Yes, so long as we're careful enough. |
1473 | |
1474 | Here's how to be careful: |
1475 | |
1476 | package Employee; |
1477 | use Person; |
1478 | use strict; |
77ca0c92 |
1479 | our @ISA = qw(Person); |
5f05dabc |
1480 | |
1481 | my %fields = ( |
1482 | id => undef, |
1483 | salary => undef, |
1484 | ); |
1485 | |
1486 | sub new { |
eac7fe86 |
1487 | my $class = shift; |
1488 | my $self = $class->SUPER::new(); |
5f05dabc |
1489 | my($element); |
1490 | foreach $element (keys %fields) { |
1491 | $self->{_permitted}->{$element} = $fields{$element}; |
1492 | } |
1493 | @{$self}{keys %fields} = values %fields; |
1494 | return $self; |
1495 | } |
1496 | |
1497 | Once we've done this, we don't even need to have an |
1498 | AUTOLOAD function in the Employee package, because |
1499 | we'll grab Person's version of that via inheritance, |
1500 | and it will all work out just fine. |
1501 | |
1502 | =head1 Metaclassical Tools |
1503 | |
1504 | Even though proxy methods can provide a more convenient approach to making |
1505 | more struct-like classes than tediously coding up data methods as |
1506 | functions, it still leaves a bit to be desired. For one thing, it means |
1507 | you have to handle bogus calls that you don't mean to trap via your proxy. |
1508 | It also means you have to be quite careful when dealing with inheritance, |
1509 | as detailed above. |
1510 | |
1511 | Perl programmers have responded to this by creating several different |
1512 | class construction classes. These metaclasses are classes |
1513 | that create other classes. A couple worth looking at are |
8cc95fdb |
1514 | Class::Struct and Alias. These and other related metaclasses can be |
5f05dabc |
1515 | found in the modules directory on CPAN. |
1516 | |
8cc95fdb |
1517 | =head2 Class::Struct |
5f05dabc |
1518 | |
8cc95fdb |
1519 | One of the older ones is Class::Struct. In fact, its syntax and |
5f05dabc |
1520 | interface were sketched out long before perl5 even solidified into a |
8d9aa5e0 |
1521 | real thing. What it does is provide you a way to "declare" a class |
1522 | as having objects whose fields are of a specific type. The function |
1523 | that does this is called, not surprisingly enough, struct(). Because |
1524 | structures or records are not base types in Perl, each time you want to |
1525 | create a class to provide a record-like data object, you yourself have |
1526 | to define a new() method, plus separate data-access methods for each of |
1527 | that record's fields. You'll quickly become bored with this process. |
8cc95fdb |
1528 | The Class::Struct::struct() function alleviates this tedium. |
5f05dabc |
1529 | |
1530 | Here's a simple example of using it: |
1531 | |
8cc95fdb |
1532 | use Class::Struct qw(struct); |
5f05dabc |
1533 | use Jobbie; # user-defined; see below |
1534 | |
1535 | struct 'Fred' => { |
1536 | one => '$', |
1537 | many => '@', |
0184c62d |
1538 | profession => 'Jobbie', # does not call Jobbie->new() |
5f05dabc |
1539 | }; |
1540 | |
0184c62d |
1541 | $ob = Fred->new(profession => Jobbie->new()); |
5f05dabc |
1542 | $ob->one("hmmmm"); |
1543 | |
1544 | $ob->many(0, "here"); |
1545 | $ob->many(1, "you"); |
1546 | $ob->many(2, "go"); |
1547 | print "Just set: ", $ob->many(2), "\n"; |
1548 | |
1549 | $ob->profession->salary(10_000); |
1550 | |
1551 | You can declare types in the struct to be basic Perl types, or |
1552 | user-defined types (classes). User types will be initialized by calling |
1553 | that class's new() method. |
1554 | |
0184c62d |
1555 | Take care that the C<Jobbie> object is not created automatically by the |
1556 | C<Fred> class's new() method, so you should specify a C<Jobbie> object |
1557 | when you create an instance of C<Fred>. |
1558 | |
5f05dabc |
1559 | Here's a real-world example of using struct generation. Let's say you |
1560 | wanted to override Perl's idea of gethostbyname() and gethostbyaddr() so |
1561 | that they would return objects that acted like C structures. We don't |
1562 | care about high-falutin' OO gunk. All we want is for these objects to |
1563 | act like structs in the C sense. |
1564 | |
1565 | use Socket; |
1566 | use Net::hostent; |
1567 | $h = gethostbyname("perl.com"); # object return |
1568 | printf "perl.com's real name is %s, address %s\n", |
1569 | $h->name, inet_ntoa($h->addr); |
1570 | |
8cc95fdb |
1571 | Here's how to do this using the Class::Struct module. |
5f05dabc |
1572 | The crux is going to be this call: |
1573 | |
1574 | struct 'Net::hostent' => [ # note bracket |
1575 | name => '$', |
1576 | aliases => '@', |
1577 | addrtype => '$', |
1578 | 'length' => '$', |
1579 | addr_list => '@', |
1580 | ]; |
1581 | |
1582 | Which creates object methods of those names and types. |
1583 | It even creates a new() method for us. |
1584 | |
1585 | We could also have implemented our object this way: |
1586 | |
1587 | struct 'Net::hostent' => { # note brace |
1588 | name => '$', |
1589 | aliases => '@', |
1590 | addrtype => '$', |
1591 | 'length' => '$', |
1592 | addr_list => '@', |
1593 | }; |
1594 | |
8cc95fdb |
1595 | and then Class::Struct would have used an anonymous hash as the object |
5f05dabc |
1596 | type, instead of an anonymous array. The array is faster and smaller, |
1597 | but the hash works out better if you eventually want to do inheritance. |
1598 | Since for this struct-like object we aren't planning on inheritance, |
1599 | this time we'll opt for better speed and size over better flexibility. |
1600 | |
1601 | Here's the whole implementation: |
1602 | |
1603 | package Net::hostent; |
1604 | use strict; |
1605 | |
1606 | BEGIN { |
1607 | use Exporter (); |
77ca0c92 |
1608 | our @EXPORT = qw(gethostbyname gethostbyaddr gethost); |
1609 | our @EXPORT_OK = qw( |
1610 | $h_name @h_aliases |
1611 | $h_addrtype $h_length |
1612 | @h_addr_list $h_addr |
1613 | ); |
1614 | our %EXPORT_TAGS = ( FIELDS => [ @EXPORT_OK, @EXPORT ] ); |
5f05dabc |
1615 | } |
77ca0c92 |
1616 | our @EXPORT_OK; |
5f05dabc |
1617 | |
8cc95fdb |
1618 | # Class::Struct forbids use of @ISA |
1619 | sub import { goto &Exporter::import } |
1620 | |
1621 | use Class::Struct qw(struct); |
5f05dabc |
1622 | struct 'Net::hostent' => [ |
1623 | name => '$', |
1624 | aliases => '@', |
1625 | addrtype => '$', |
1626 | 'length' => '$', |
1627 | addr_list => '@', |
1628 | ]; |
1629 | |
1630 | sub addr { shift->addr_list->[0] } |
1631 | |
1632 | sub populate (@) { |
1633 | return unless @_; |
8cc95fdb |
1634 | my $hob = new(); # Class::Struct made this! |
5f05dabc |
1635 | $h_name = $hob->[0] = $_[0]; |
1636 | @h_aliases = @{ $hob->[1] } = split ' ', $_[1]; |
1637 | $h_addrtype = $hob->[2] = $_[2]; |
1638 | $h_length = $hob->[3] = $_[3]; |
1639 | $h_addr = $_[4]; |
1640 | @h_addr_list = @{ $hob->[4] } = @_[ (4 .. $#_) ]; |
1641 | return $hob; |
1642 | } |
1643 | |
1644 | sub gethostbyname ($) { populate(CORE::gethostbyname(shift)) } |
1645 | |
1646 | sub gethostbyaddr ($;$) { |
1647 | my ($addr, $addrtype); |
1648 | $addr = shift; |
1649 | require Socket unless @_; |
1650 | $addrtype = @_ ? shift : Socket::AF_INET(); |
1651 | populate(CORE::gethostbyaddr($addr, $addrtype)) |
1652 | } |
1653 | |
1654 | sub gethost($) { |
1655 | if ($_[0] =~ /^\d+(?:\.\d+(?:\.\d+(?:\.\d+)?)?)?$/) { |
1656 | require Socket; |
1657 | &gethostbyaddr(Socket::inet_aton(shift)); |
1658 | } else { |
1659 | &gethostbyname; |
1660 | } |
1661 | } |
1662 | |
1663 | 1; |
1664 | |
1665 | We've snuck in quite a fair bit of other concepts besides just dynamic |
1666 | class creation, like overriding core functions, import/export bits, |
8cc95fdb |
1667 | function prototyping, short-cut function call via C<&whatever>, and |
1668 | function replacement with C<goto &whatever>. These all mostly make |
1669 | sense from the perspective of a traditional module, but as you can see, |
1670 | we can also use them in an object module. |
5f05dabc |
1671 | |
1672 | You can look at other object-based, struct-like overrides of core |
1673 | functions in the 5.004 release of Perl in File::stat, Net::hostent, |
1674 | Net::netent, Net::protoent, Net::servent, Time::gmtime, Time::localtime, |
1675 | User::grent, and User::pwent. These modules have a final component |
68dc0745 |
1676 | that's all lowercase, by convention reserved for compiler pragmas, |
54310121 |
1677 | because they affect the compilation and change a builtin function. |
5f05dabc |
1678 | They also have the type names that a C programmer would most expect. |
1679 | |
1680 | =head2 Data Members as Variables |
1681 | |
1682 | If you're used to C++ objects, then you're accustomed to being able to |
1683 | get at an object's data members as simple variables from within a method. |
1684 | The Alias module provides for this, as well as a good bit more, such |
1685 | as the possibility of private methods that the object can call but folks |
1686 | outside the class cannot. |
1687 | |
1688 | Here's an example of creating a Person using the Alias module. |
1689 | When you update these magical instance variables, you automatically |
1690 | update value fields in the hash. Convenient, eh? |
1691 | |
1692 | package Person; |
1693 | |
1694 | # this is the same as before... |
1695 | sub new { |
eac7fe86 |
1696 | my $class = shift; |
5f05dabc |
1697 | my $self = { |
1698 | NAME => undef, |
1699 | AGE => undef, |
1700 | PEERS => [], |
1701 | }; |
1702 | bless($self, $class); |
1703 | return $self; |
1704 | } |
1705 | |
1706 | use Alias qw(attr); |
77ca0c92 |
1707 | our ($NAME, $AGE, $PEERS); |
5f05dabc |
1708 | |
1709 | sub name { |
1710 | my $self = attr shift; |
1711 | if (@_) { $NAME = shift; } |
1712 | return $NAME; |
1713 | } |
1714 | |
1715 | sub age { |
1716 | my $self = attr shift; |
1717 | if (@_) { $AGE = shift; } |
1718 | return $AGE; |
1719 | } |
1720 | |
1721 | sub peers { |
1722 | my $self = attr shift; |
1723 | if (@_) { @PEERS = @_; } |
1724 | return @PEERS; |
1725 | } |
1726 | |
1727 | sub exclaim { |
1728 | my $self = attr shift; |
1729 | return sprintf "Hi, I'm %s, age %d, working with %s", |
1730 | $NAME, $AGE, join(", ", @PEERS); |
1731 | } |
1732 | |
1733 | sub happy_birthday { |
1734 | my $self = attr shift; |
1735 | return ++$AGE; |
1736 | } |
1737 | |
77ca0c92 |
1738 | The need for the C<our> declaration is because what Alias does |
5f05dabc |
1739 | is play with package globals with the same name as the fields. To use |
54310121 |
1740 | globals while C<use strict> is in effect, you have to predeclare them. |
5f05dabc |
1741 | These package variables are localized to the block enclosing the attr() |
1742 | call just as if you'd used a local() on them. However, that means that |
1743 | they're still considered global variables with temporary values, just |
1744 | as with any other local(). |
1745 | |
1746 | It would be nice to combine Alias with |
8cc95fdb |
1747 | something like Class::Struct or Class::MethodMaker. |
5f05dabc |
1748 | |
13a2d996 |
1749 | =head1 NOTES |
5f05dabc |
1750 | |
1751 | =head2 Object Terminology |
1752 | |
1753 | In the various OO literature, it seems that a lot of different words |
1754 | are used to describe only a few different concepts. If you're not |
1755 | already an object programmer, then you don't need to worry about all |
1756 | these fancy words. But if you are, then you might like to know how to |
1757 | get at the same concepts in Perl. |
1758 | |
1759 | For example, it's common to call an object an I<instance> of a class |
1760 | and to call those objects' methods I<instance methods>. Data fields |
1761 | peculiar to each object are often called I<instance data> or I<object |
1762 | attributes>, and data fields common to all members of that class are |
1763 | I<class data>, I<class attributes>, or I<static data members>. |
1764 | |
1765 | Also, I<base class>, I<generic class>, and I<superclass> all describe |
1766 | the same notion, whereas I<derived class>, I<specific class>, and |
1767 | I<subclass> describe the other related one. |
1768 | |
1769 | C++ programmers have I<static methods> and I<virtual methods>, |
1770 | but Perl only has I<class methods> and I<object methods>. |
1771 | Actually, Perl only has methods. Whether a method gets used |
1772 | as a class or object method is by usage only. You could accidentally |
1773 | call a class method (one expecting a string argument) on an |
1774 | object (one expecting a reference), or vice versa. |
1775 | |
aaa2bbb1 |
1776 | From the C++ perspective, all methods in Perl are virtual. |
5f05dabc |
1777 | This, by the way, is why they are never checked for function |
54310121 |
1778 | prototypes in the argument list as regular builtin and user-defined |
5f05dabc |
1779 | functions can be. |
1780 | |
1781 | Because a class is itself something of an object, Perl's classes can be |
1782 | taken as describing both a "class as meta-object" (also called I<object |
1783 | factory>) philosophy and the "class as type definition" (I<declaring> |
1784 | behaviour, not I<defining> mechanism) idea. C++ supports the latter |
1785 | notion, but not the former. |
1786 | |
1787 | =head1 SEE ALSO |
1788 | |
54310121 |
1789 | The following manpages will doubtless provide more |
5f05dabc |
1790 | background for this one: |
1791 | L<perlmod>, |
1792 | L<perlref>, |
1793 | L<perlobj>, |
1794 | L<perlbot>, |
1795 | L<perltie>, |
1796 | and |
1797 | L<overload>. |
1798 | |
8257a158 |
1799 | L<perlboot> is a kinder, gentler introduction to object-oriented |
1800 | programming. |
1801 | |
890a53b9 |
1802 | L<perltooc> provides more detail on class data. |
8257a158 |
1803 | |
1804 | Some modules which might prove interesting are Class::Accessor, |
1805 | Class::Class, Class::Contract, Class::Data::Inheritable, |
1806 | Class::MethodMaker and Tie::SecureHash |
1807 | |
1808 | |
5a964f20 |
1809 | =head1 AUTHOR AND COPYRIGHT |
1810 | |
1811 | Copyright (c) 1997, 1998 Tom Christiansen |
1812 | All rights reserved. |
1813 | |
5a7beb56 |
1814 | This documentation is free; you can redistribute it and/or modify it |
1815 | under the same terms as Perl itself. |
5f05dabc |
1816 | |
5a964f20 |
1817 | Irrespective of its distribution, all code examples in this file |
1818 | are hereby placed into the public domain. You are permitted and |
1819 | encouraged to use this code in your own programs for fun |
1820 | or for profit as you see fit. A simple comment in the code giving |
1821 | credit would be courteous but is not required. |
1822 | |
1823 | =head1 COPYRIGHT |
5f05dabc |
1824 | |
1825 | =head2 Acknowledgments |
1826 | |
1827 | Thanks to |
1828 | Larry Wall, |
1829 | Roderick Schertler, |
1830 | Gurusamy Sarathy, |
1831 | Dean Roehrich, |
1832 | Raphael Manfredi, |
1833 | Brent Halsey, |
1834 | Greg Bacon, |
1835 | Brad Appleton, |
1836 | and many others for their helpful comments. |