| 1 | =head1 NAME |
| 2 | |
| 3 | perltooc - Tom's OO Tutorial for Class Data in Perl |
| 4 | |
| 5 | =head1 DESCRIPTION |
| 6 | |
| 7 | When designing an object class, you are sometimes faced with the situation |
| 8 | of wanting common state shared by all objects of that class. |
| 9 | Such I<class attributes> act somewhat like global variables for the entire |
| 10 | class, but unlike program-wide globals, class attributes have meaning only to |
| 11 | the class itself. |
| 12 | |
| 13 | Here are a few examples where class attributes might come in handy: |
| 14 | |
| 15 | =over 4 |
| 16 | |
| 17 | =item * |
| 18 | |
| 19 | to keep a count of the objects you've created, or how many are |
| 20 | still extant. |
| 21 | |
| 22 | =item * |
| 23 | |
| 24 | to extract the name or file descriptor for a logfile used by a debugging |
| 25 | method. |
| 26 | |
| 27 | =item * |
| 28 | |
| 29 | to access collective data, like the total amount of cash dispensed by |
| 30 | all ATMs in a network in a given day. |
| 31 | |
| 32 | =item * |
| 33 | |
| 34 | to access the last object created by a class, or the most accessed object, |
| 35 | or to retrieve a list of all objects. |
| 36 | |
| 37 | =back |
| 38 | |
| 39 | Unlike a true global, class attributes should not be accessed directly. |
| 40 | Instead, their state should be inspected, and perhaps altered, only |
| 41 | through the mediated access of I<class methods>. These class attributes |
| 42 | accessor methods are similar in spirit and function to accessors used |
| 43 | to manipulate the state of instance attributes on an object. They provide a |
| 44 | clear firewall between interface and implementation. |
| 45 | |
| 46 | You should allow access to class attributes through either the class |
| 47 | name or any object of that class. If we assume that $an_object is of |
| 48 | type Some_Class, and the &Some_Class::population_count method accesses |
| 49 | class attributes, then these two invocations should both be possible, |
| 50 | and almost certainly equivalent. |
| 51 | |
| 52 | Some_Class->population_count() |
| 53 | $an_object->population_count() |
| 54 | |
| 55 | The question is, where do you store the state which that method accesses? |
| 56 | Unlike more restrictive languages like C++, where these are called |
| 57 | static data members, Perl provides no syntactic mechanism to declare |
| 58 | class attributes, any more than it provides a syntactic mechanism to |
| 59 | declare instance attributes. Perl provides the developer with a broad |
| 60 | set of powerful but flexible features that can be uniquely crafted to |
| 61 | the particular demands of the situation. |
| 62 | |
| 63 | A class in Perl is typically implemented in a module. A module consists |
| 64 | of two complementary feature sets: a package for interfacing with the |
| 65 | outside world, and a lexical file scope for privacy. Either of these |
| 66 | two mechanisms can be used to implement class attributes. That means you |
| 67 | get to decide whether to put your class attributes in package variables |
| 68 | or to put them in lexical variables. |
| 69 | |
| 70 | And those aren't the only decisions to make. If you choose to use package |
| 71 | variables, you can make your class attribute accessor methods either ignorant |
| 72 | of inheritance or sensitive to it. If you choose lexical variables, |
| 73 | you can elect to permit access to them from anywhere in the entire file |
| 74 | scope, or you can limit direct data access exclusively to the methods |
| 75 | implementing those attributes. |
| 76 | |
| 77 | =head1 Class Data in a Can |
| 78 | |
| 79 | One of the easiest ways to solve a hard problem is to let someone else |
| 80 | do it for you! In this case, Class::Data::Inheritable (available on a |
| 81 | CPAN near you) offers a canned solution to the class data problem |
| 82 | using closures. So before you wade into this document, consider |
| 83 | having a look at that module. |
| 84 | |
| 85 | |
| 86 | =head1 Class Data as Package Variables |
| 87 | |
| 88 | Because a class in Perl is really just a package, using package variables |
| 89 | to hold class attributes is the most natural choice. This makes it simple |
| 90 | for each class to have its own class attributes. Let's say you have a class |
| 91 | called Some_Class that needs a couple of different attributes that you'd |
| 92 | like to be global to the entire class. The simplest thing to do is to |
| 93 | use package variables like $Some_Class::CData1 and $Some_Class::CData2 |
| 94 | to hold these attributes. But we certainly don't want to encourage |
| 95 | outsiders to touch those data directly, so we provide methods |
| 96 | to mediate access. |
| 97 | |
| 98 | In the accessor methods below, we'll for now just ignore the first |
| 99 | argument--that part to the left of the arrow on method invocation, which |
| 100 | is either a class name or an object reference. |
| 101 | |
| 102 | package Some_Class; |
| 103 | sub CData1 { |
| 104 | shift; # XXX: ignore calling class/object |
| 105 | $Some_Class::CData1 = shift if @_; |
| 106 | return $Some_Class::CData1; |
| 107 | } |
| 108 | sub CData2 { |
| 109 | shift; # XXX: ignore calling class/object |
| 110 | $Some_Class::CData2 = shift if @_; |
| 111 | return $Some_Class::CData2; |
| 112 | } |
| 113 | |
| 114 | This technique is highly legible and should be completely straightforward |
| 115 | to even the novice Perl programmer. By fully qualifying the package |
| 116 | variables, they stand out clearly when reading the code. Unfortunately, |
| 117 | if you misspell one of these, you've introduced an error that's hard |
| 118 | to catch. It's also somewhat disconcerting to see the class name itself |
| 119 | hard-coded in so many places. |
| 120 | |
| 121 | Both these problems can be easily fixed. Just add the C<use strict> |
| 122 | pragma, then pre-declare your package variables. (The C<our> operator |
| 123 | will be new in 5.6, and will work for package globals just like C<my> |
| 124 | works for scoped lexicals.) |
| 125 | |
| 126 | package Some_Class; |
| 127 | use strict; |
| 128 | our($CData1, $CData2); # our() is new to perl5.6 |
| 129 | sub CData1 { |
| 130 | shift; # XXX: ignore calling class/object |
| 131 | $CData1 = shift if @_; |
| 132 | return $CData1; |
| 133 | } |
| 134 | sub CData2 { |
| 135 | shift; # XXX: ignore calling class/object |
| 136 | $CData2 = shift if @_; |
| 137 | return $CData2; |
| 138 | } |
| 139 | |
| 140 | |
| 141 | As with any other global variable, some programmers prefer to start their |
| 142 | package variables with capital letters. This helps clarity somewhat, but |
| 143 | by no longer fully qualifying the package variables, their significance |
| 144 | can be lost when reading the code. You can fix this easily enough by |
| 145 | choosing better names than were used here. |
| 146 | |
| 147 | =head2 Putting All Your Eggs in One Basket |
| 148 | |
| 149 | Just as the mindless enumeration of accessor methods for instance attributes |
| 150 | grows tedious after the first few (see L<perltoot>), so too does the |
| 151 | repetition begin to grate when listing out accessor methods for class |
| 152 | data. Repetition runs counter to the primary virtue of a programmer: |
| 153 | Laziness, here manifesting as that innate urge every programmer feels |
| 154 | to factor out duplicate code whenever possible. |
| 155 | |
| 156 | Here's what to do. First, make just one hash to hold all class attributes. |
| 157 | |
| 158 | package Some_Class; |
| 159 | use strict; |
| 160 | our %ClassData = ( # our() is new to perl5.6 |
| 161 | CData1 => "", |
| 162 | CData2 => "", |
| 163 | ); |
| 164 | |
| 165 | Using closures (see L<perlref>) and direct access to the package symbol |
| 166 | table (see L<perlmod>), now clone an accessor method for each key in |
| 167 | the %ClassData hash. Each of these methods is used to fetch or store |
| 168 | values to the specific, named class attribute. |
| 169 | |
| 170 | for my $datum (keys %ClassData) { |
| 171 | no strict "refs"; # to register new methods in package |
| 172 | *$datum = sub { |
| 173 | shift; # XXX: ignore calling class/object |
| 174 | $ClassData{$datum} = shift if @_; |
| 175 | return $ClassData{$datum}; |
| 176 | } |
| 177 | } |
| 178 | |
| 179 | It's true that you could work out a solution employing an &AUTOLOAD |
| 180 | method, but this approach is unlikely to prove satisfactory. Your |
| 181 | function would have to distinguish between class attributes and object |
| 182 | attributes; it could interfere with inheritance; and it would have to |
| 183 | careful about DESTROY. Such complexity is uncalled for in most cases, |
| 184 | and certainly in this one. |
| 185 | |
| 186 | You may wonder why we're rescinding strict refs for the loop. We're |
| 187 | manipulating the package's symbol table to introduce new function names |
| 188 | using symbolic references (indirect naming), which the strict pragma |
| 189 | would otherwise forbid. Normally, symbolic references are a dodgy |
| 190 | notion at best. This isn't just because they can be used accidentally |
| 191 | when you aren't meaning to. It's also because for most uses |
| 192 | to which beginning Perl programmers attempt to put symbolic references, |
| 193 | we have much better approaches, like nested hashes or hashes of arrays. |
| 194 | But there's nothing wrong with using symbolic references to manipulate |
| 195 | something that is meaningful only from the perspective of the package |
| 196 | symbol table, like method names or package variables. In other |
| 197 | words, when you want to refer to the symbol table, use symbol references. |
| 198 | |
| 199 | Clustering all the class attributes in one place has several advantages. |
| 200 | They're easy to spot, initialize, and change. The aggregation also |
| 201 | makes them convenient to access externally, such as from a debugger |
| 202 | or a persistence package. The only possible problem is that we don't |
| 203 | automatically know the name of each class's class object, should it have |
| 204 | one. This issue is addressed below in L<"The Eponymous Meta-Object">. |
| 205 | |
| 206 | =head2 Inheritance Concerns |
| 207 | |
| 208 | Suppose you have an instance of a derived class, and you access class |
| 209 | data using an inherited method call. Should that end up referring |
| 210 | to the base class's attributes, or to those in the derived class? |
| 211 | How would it work in the earlier examples? The derived class inherits |
| 212 | all the base class's methods, including those that access class attributes. |
| 213 | But what package are the class attributes stored in? |
| 214 | |
| 215 | The answer is that, as written, class attributes are stored in the package into |
| 216 | which those methods were compiled. When you invoke the &CData1 method |
| 217 | on the name of the derived class or on one of that class's objects, the |
| 218 | version shown above is still run, so you'll access $Some_Class::CData1--or |
| 219 | in the method cloning version, C<$Some_Class::ClassData{CData1}>. |
| 220 | |
| 221 | Think of these class methods as executing in the context of their base |
| 222 | class, not in that of their derived class. Sometimes this is exactly |
| 223 | what you want. If Feline subclasses Carnivore, then the population of |
| 224 | Carnivores in the world should go up when a new Feline is born. |
| 225 | But what if you wanted to figure out how many Felines you have apart |
| 226 | from Carnivores? The current approach doesn't support that. |
| 227 | |
| 228 | You'll have to decide on a case-by-case basis whether it makes any sense |
| 229 | for class attributes to be package-relative. If you want it to be so, |
| 230 | then stop ignoring the first argument to the function. Either it will |
| 231 | be a package name if the method was invoked directly on a class name, |
| 232 | or else it will be an object reference if the method was invoked on an |
| 233 | object reference. In the latter case, the ref() function provides the |
| 234 | class of that object. |
| 235 | |
| 236 | package Some_Class; |
| 237 | sub CData1 { |
| 238 | my $obclass = shift; |
| 239 | my $class = ref($obclass) || $obclass; |
| 240 | my $varname = $class . "::CData1"; |
| 241 | no strict "refs"; # to access package data symbolically |
| 242 | $$varname = shift if @_; |
| 243 | return $$varname; |
| 244 | } |
| 245 | |
| 246 | And then do likewise for all other class attributes (such as CData2, |
| 247 | etc.) that you wish to access as package variables in the invoking package |
| 248 | instead of the compiling package as we had previously. |
| 249 | |
| 250 | Once again we temporarily disable the strict references ban, because |
| 251 | otherwise we couldn't use the fully-qualified symbolic name for |
| 252 | the package global. This is perfectly reasonable: since all package |
| 253 | variables by definition live in a package, there's nothing wrong with |
| 254 | accessing them via that package's symbol table. That's what it's there |
| 255 | for (well, somewhat). |
| 256 | |
| 257 | What about just using a single hash for everything and then cloning |
| 258 | methods? What would that look like? The only difference would be the |
| 259 | closure used to produce new method entries for the class's symbol table. |
| 260 | |
| 261 | no strict "refs"; |
| 262 | *$datum = sub { |
| 263 | my $obclass = shift; |
| 264 | my $class = ref($obclass) || $obclass; |
| 265 | my $varname = $class . "::ClassData"; |
| 266 | $varname->{$datum} = shift if @_; |
| 267 | return $varname->{$datum}; |
| 268 | } |
| 269 | |
| 270 | =head2 The Eponymous Meta-Object |
| 271 | |
| 272 | It could be argued that the %ClassData hash in the previous example is |
| 273 | neither the most imaginative nor the most intuitive of names. Is there |
| 274 | something else that might make more sense, be more useful, or both? |
| 275 | |
| 276 | As it happens, yes, there is. For the "class meta-object", we'll use |
| 277 | a package variable of the same name as the package itself. Within the |
| 278 | scope of a package Some_Class declaration, we'll use the eponymously |
| 279 | named hash %Some_Class as that class's meta-object. (Using an eponymously |
| 280 | named hash is somewhat reminiscent of classes that name their constructors |
| 281 | eponymously in the Python or C++ fashion. That is, class Some_Class would |
| 282 | use &Some_Class::Some_Class as a constructor, probably even exporting that |
| 283 | name as well. The StrNum class in Recipe 13.14 in I<The Perl Cookbook> |
| 284 | does this, if you're looking for an example.) |
| 285 | |
| 286 | This predictable approach has many benefits, including having a well-known |
| 287 | identifier to aid in debugging, transparent persistence, |
| 288 | or checkpointing. It's also the obvious name for monadic classes and |
| 289 | translucent attributes, discussed later. |
| 290 | |
| 291 | Here's an example of such a class. Notice how the name of the |
| 292 | hash storing the meta-object is the same as the name of the package |
| 293 | used to implement the class. |
| 294 | |
| 295 | package Some_Class; |
| 296 | use strict; |
| 297 | |
| 298 | # create class meta-object using that most perfect of names |
| 299 | our %Some_Class = ( # our() is new to perl5.6 |
| 300 | CData1 => "", |
| 301 | CData2 => "", |
| 302 | ); |
| 303 | |
| 304 | # this accessor is calling-package-relative |
| 305 | sub CData1 { |
| 306 | my $obclass = shift; |
| 307 | my $class = ref($obclass) || $obclass; |
| 308 | no strict "refs"; # to access eponymous meta-object |
| 309 | $class->{CData1} = shift if @_; |
| 310 | return $class->{CData1}; |
| 311 | } |
| 312 | |
| 313 | # but this accessor is not |
| 314 | sub CData2 { |
| 315 | shift; # XXX: ignore calling class/object |
| 316 | no strict "refs"; # to access eponymous meta-object |
| 317 | __PACKAGE__ -> {CData2} = shift if @_; |
| 318 | return __PACKAGE__ -> {CData2}; |
| 319 | } |
| 320 | |
| 321 | In the second accessor method, the __PACKAGE__ notation was used for |
| 322 | two reasons. First, to avoid hardcoding the literal package name |
| 323 | in the code in case we later want to change that name. Second, to |
| 324 | clarify to the reader that what matters here is the package currently |
| 325 | being compiled into, not the package of the invoking object or class. |
| 326 | If the long sequence of non-alphabetic characters bothers you, you can |
| 327 | always put the __PACKAGE__ in a variable first. |
| 328 | |
| 329 | sub CData2 { |
| 330 | shift; # XXX: ignore calling class/object |
| 331 | no strict "refs"; # to access eponymous meta-object |
| 332 | my $class = __PACKAGE__; |
| 333 | $class->{CData2} = shift if @_; |
| 334 | return $class->{CData2}; |
| 335 | } |
| 336 | |
| 337 | Even though we're using symbolic references for good not evil, some |
| 338 | folks tend to become unnerved when they see so many places with strict |
| 339 | ref checking disabled. Given a symbolic reference, you can always |
| 340 | produce a real reference (the reverse is not true, though). So we'll |
| 341 | create a subroutine that does this conversion for us. If invoked as a |
| 342 | function of no arguments, it returns a reference to the compiling class's |
| 343 | eponymous hash. Invoked as a class method, it returns a reference to |
| 344 | the eponymous hash of its caller. And when invoked as an object method, |
| 345 | this function returns a reference to the eponymous hash for whatever |
| 346 | class the object belongs to. |
| 347 | |
| 348 | package Some_Class; |
| 349 | use strict; |
| 350 | |
| 351 | our %Some_Class = ( # our() is new to perl5.6 |
| 352 | CData1 => "", |
| 353 | CData2 => "", |
| 354 | ); |
| 355 | |
| 356 | # tri-natured: function, class method, or object method |
| 357 | sub _classobj { |
| 358 | my $obclass = shift || __PACKAGE__; |
| 359 | my $class = ref($obclass) || $obclass; |
| 360 | no strict "refs"; # to convert sym ref to real one |
| 361 | return \%$class; |
| 362 | } |
| 363 | |
| 364 | for my $datum (keys %{ _classobj() } ) { |
| 365 | # turn off strict refs so that we can |
| 366 | # register a method in the symbol table |
| 367 | no strict "refs"; |
| 368 | *$datum = sub { |
| 369 | use strict "refs"; |
| 370 | my $self = shift->_classobj(); |
| 371 | $self->{$datum} = shift if @_; |
| 372 | return $self->{$datum}; |
| 373 | } |
| 374 | } |
| 375 | |
| 376 | =head2 Indirect References to Class Data |
| 377 | |
| 378 | A reasonably common strategy for handling class attributes is to store |
| 379 | a reference to each package variable on the object itself. This is |
| 380 | a strategy you've probably seen before, such as in L<perltoot> and |
| 381 | L<perlbot>, but there may be variations in the example below that you |
| 382 | haven't thought of before. |
| 383 | |
| 384 | package Some_Class; |
| 385 | our($CData1, $CData2); # our() is new to perl5.6 |
| 386 | |
| 387 | sub new { |
| 388 | my $obclass = shift; |
| 389 | return bless my $self = { |
| 390 | ObData1 => "", |
| 391 | ObData2 => "", |
| 392 | CData1 => \$CData1, |
| 393 | CData2 => \$CData2, |
| 394 | } => (ref $obclass || $obclass); |
| 395 | } |
| 396 | |
| 397 | sub ObData1 { |
| 398 | my $self = shift; |
| 399 | $self->{ObData1} = shift if @_; |
| 400 | return $self->{ObData1}; |
| 401 | } |
| 402 | |
| 403 | sub ObData2 { |
| 404 | my $self = shift; |
| 405 | $self->{ObData2} = shift if @_; |
| 406 | return $self->{ObData2}; |
| 407 | } |
| 408 | |
| 409 | sub CData1 { |
| 410 | my $self = shift; |
| 411 | my $dataref = ref $self |
| 412 | ? $self->{CData1} |
| 413 | : \$CData1; |
| 414 | $$dataref = shift if @_; |
| 415 | return $$dataref; |
| 416 | } |
| 417 | |
| 418 | sub CData2 { |
| 419 | my $self = shift; |
| 420 | my $dataref = ref $self |
| 421 | ? $self->{CData2} |
| 422 | : \$CData2; |
| 423 | $$dataref = shift if @_; |
| 424 | return $$dataref; |
| 425 | } |
| 426 | |
| 427 | As written above, a derived class will inherit these methods, which |
| 428 | will consequently access package variables in the base class's package. |
| 429 | This is not necessarily expected behavior in all circumstances. Here's an |
| 430 | example that uses a variable meta-object, taking care to access the |
| 431 | proper package's data. |
| 432 | |
| 433 | package Some_Class; |
| 434 | use strict; |
| 435 | |
| 436 | our %Some_Class = ( # our() is new to perl5.6 |
| 437 | CData1 => "", |
| 438 | CData2 => "", |
| 439 | ); |
| 440 | |
| 441 | sub _classobj { |
| 442 | my $self = shift; |
| 443 | my $class = ref($self) || $self; |
| 444 | no strict "refs"; |
| 445 | # get (hard) ref to eponymous meta-object |
| 446 | return \%$class; |
| 447 | } |
| 448 | |
| 449 | sub new { |
| 450 | my $obclass = shift; |
| 451 | my $classobj = $obclass->_classobj(); |
| 452 | bless my $self = { |
| 453 | ObData1 => "", |
| 454 | ObData2 => "", |
| 455 | CData1 => \$classobj->{CData1}, |
| 456 | CData2 => \$classobj->{CData2}, |
| 457 | } => (ref $obclass || $obclass); |
| 458 | return $self; |
| 459 | } |
| 460 | |
| 461 | sub ObData1 { |
| 462 | my $self = shift; |
| 463 | $self->{ObData1} = shift if @_; |
| 464 | return $self->{ObData1}; |
| 465 | } |
| 466 | |
| 467 | sub ObData2 { |
| 468 | my $self = shift; |
| 469 | $self->{ObData2} = shift if @_; |
| 470 | return $self->{ObData2}; |
| 471 | } |
| 472 | |
| 473 | sub CData1 { |
| 474 | my $self = shift; |
| 475 | $self = $self->_classobj() unless ref $self; |
| 476 | my $dataref = $self->{CData1}; |
| 477 | $$dataref = shift if @_; |
| 478 | return $$dataref; |
| 479 | } |
| 480 | |
| 481 | sub CData2 { |
| 482 | my $self = shift; |
| 483 | $self = $self->_classobj() unless ref $self; |
| 484 | my $dataref = $self->{CData2}; |
| 485 | $$dataref = shift if @_; |
| 486 | return $$dataref; |
| 487 | } |
| 488 | |
| 489 | Not only are we now strict refs clean, using an eponymous meta-object |
| 490 | seems to make the code cleaner. Unlike the previous version, this one |
| 491 | does something interesting in the face of inheritance: it accesses the |
| 492 | class meta-object in the invoking class instead of the one into which |
| 493 | the method was initially compiled. |
| 494 | |
| 495 | You can easily access data in the class meta-object, making |
| 496 | it easy to dump the complete class state using an external mechanism such |
| 497 | as when debugging or implementing a persistent class. This works because |
| 498 | the class meta-object is a package variable, has a well-known name, and |
| 499 | clusters all its data together. (Transparent persistence |
| 500 | is not always feasible, but it's certainly an appealing idea.) |
| 501 | |
| 502 | There's still no check that object accessor methods have not been |
| 503 | invoked on a class name. If strict ref checking is enabled, you'd |
| 504 | blow up. If not, then you get the eponymous meta-object. What you do |
| 505 | with--or about--this is up to you. The next two sections demonstrate |
| 506 | innovative uses for this powerful feature. |
| 507 | |
| 508 | =head2 Monadic Classes |
| 509 | |
| 510 | Some of the standard modules shipped with Perl provide class interfaces |
| 511 | without any attribute methods whatsoever. The most commonly used module |
| 512 | not numbered amongst the pragmata, the Exporter module, is a class with |
| 513 | neither constructors nor attributes. Its job is simply to provide a |
| 514 | standard interface for modules wishing to export part of their namespace |
| 515 | into that of their caller. Modules use the Exporter's &import method by |
| 516 | setting their inheritance list in their package's @ISA array to mention |
| 517 | "Exporter". But class Exporter provides no constructor, so you can't |
| 518 | have several instances of the class. In fact, you can't have any--it |
| 519 | just doesn't make any sense. All you get is its methods. Its interface |
| 520 | contains no statefulness, so state data is wholly superfluous. |
| 521 | |
| 522 | Another sort of class that pops up from time to time is one that supports |
| 523 | a unique instance. Such classes are called I<monadic classes>, or less |
| 524 | formally, I<singletons> or I<highlander classes>. |
| 525 | |
| 526 | If a class is monadic, where do you store its state, that is, |
| 527 | its attributes? How do you make sure that there's never more than |
| 528 | one instance? While you could merely use a slew of package variables, |
| 529 | it's a lot cleaner to use the eponymously named hash. Here's a complete |
| 530 | example of a monadic class: |
| 531 | |
| 532 | package Cosmos; |
| 533 | %Cosmos = (); |
| 534 | |
| 535 | # accessor method for "name" attribute |
| 536 | sub name { |
| 537 | my $self = shift; |
| 538 | $self->{name} = shift if @_; |
| 539 | return $self->{name}; |
| 540 | } |
| 541 | |
| 542 | # read-only accessor method for "birthday" attribute |
| 543 | sub birthday { |
| 544 | my $self = shift; |
| 545 | die "can't reset birthday" if @_; # XXX: croak() is better |
| 546 | return $self->{birthday}; |
| 547 | } |
| 548 | |
| 549 | # accessor method for "stars" attribute |
| 550 | sub stars { |
| 551 | my $self = shift; |
| 552 | $self->{stars} = shift if @_; |
| 553 | return $self->{stars}; |
| 554 | } |
| 555 | |
| 556 | # oh my - one of our stars just went out! |
| 557 | sub supernova { |
| 558 | my $self = shift; |
| 559 | my $count = $self->stars(); |
| 560 | $self->stars($count - 1) if $count > 0; |
| 561 | } |
| 562 | |
| 563 | # constructor/initializer method - fix by reboot |
| 564 | sub bigbang { |
| 565 | my $self = shift; |
| 566 | %$self = ( |
| 567 | name => "the world according to tchrist", |
| 568 | birthday => time(), |
| 569 | stars => 0, |
| 570 | ); |
| 571 | return $self; # yes, it's probably a class. SURPRISE! |
| 572 | } |
| 573 | |
| 574 | # After the class is compiled, but before any use or require |
| 575 | # returns, we start off the universe with a bang. |
| 576 | __PACKAGE__ -> bigbang(); |
| 577 | |
| 578 | Hold on, that doesn't look like anything special. Those attribute |
| 579 | accessors look no different than they would if this were a regular class |
| 580 | instead of a monadic one. The crux of the matter is there's nothing |
| 581 | that says that $self must hold a reference to a blessed object. It merely |
| 582 | has to be something you can invoke methods on. Here the package name |
| 583 | itself, Cosmos, works as an object. Look at the &supernova method. Is that |
| 584 | a class method or an object method? The answer is that static analysis |
| 585 | cannot reveal the answer. Perl doesn't care, and neither should you. |
| 586 | In the three attribute methods, C<%$self> is really accessing the %Cosmos |
| 587 | package variable. |
| 588 | |
| 589 | If like Stephen Hawking, you posit the existence of multiple, sequential, |
| 590 | and unrelated universes, then you can invoke the &bigbang method yourself |
| 591 | at any time to start everything all over again. You might think of |
| 592 | &bigbang as more of an initializer than a constructor, since the function |
| 593 | doesn't allocate new memory; it only initializes what's already there. |
| 594 | But like any other constructor, it does return a scalar value to use |
| 595 | for later method invocations. |
| 596 | |
| 597 | Imagine that some day in the future, you decide that one universe just |
| 598 | isn't enough. You could write a new class from scratch, but you already |
| 599 | have an existing class that does what you want--except that it's monadic, |
| 600 | and you want more than just one cosmos. |
| 601 | |
| 602 | That's what code reuse via subclassing is all about. Look how short |
| 603 | the new code is: |
| 604 | |
| 605 | package Multiverse; |
| 606 | use Cosmos; |
| 607 | @ISA = qw(Cosmos); |
| 608 | |
| 609 | sub new { |
| 610 | my $protoverse = shift; |
| 611 | my $class = ref($protoverse) || $protoverse; |
| 612 | my $self = {}; |
| 613 | return bless($self, $class)->bigbang(); |
| 614 | } |
| 615 | 1; |
| 616 | |
| 617 | Because we were careful to be good little creators when we designed our |
| 618 | Cosmos class, we can now reuse it without touching a single line of code |
| 619 | when it comes time to write our Multiverse class. The same code that |
| 620 | worked when invoked as a class method continues to work perfectly well |
| 621 | when invoked against separate instances of a derived class. |
| 622 | |
| 623 | The astonishing thing about the Cosmos class above is that the value |
| 624 | returned by the &bigbang "constructor" is not a reference to a blessed |
| 625 | object at all. It's just the class's own name. A class name is, for |
| 626 | virtually all intents and purposes, a perfectly acceptable object. |
| 627 | It has state, behavior, and identify, the three crucial components |
| 628 | of an object system. It even manifests inheritance, polymorphism, |
| 629 | and encapsulation. And what more can you ask of an object? |
| 630 | |
| 631 | To understand object orientation in Perl, it's important to recognize the |
| 632 | unification of what other programming languages might think of as class |
| 633 | methods and object methods into just plain methods. "Class methods" |
| 634 | and "object methods" are distinct only in the compartmentalizing mind |
| 635 | of the Perl programmer, not in the Perl language itself. |
| 636 | |
| 637 | Along those same lines, a constructor is nothing special either, which |
| 638 | is one reason why Perl has no pre-ordained name for them. "Constructor" |
| 639 | is just an informal term loosely used to describe a method that returns |
| 640 | a scalar value that you can make further method calls against. So long |
| 641 | as it's either a class name or an object reference, that's good enough. |
| 642 | It doesn't even have to be a reference to a brand new object. |
| 643 | |
| 644 | You can have as many--or as few--constructors as you want, and you can |
| 645 | name them whatever you care to. Blindly and obediently using new() |
| 646 | for each and every constructor you ever write is to speak Perl with |
| 647 | such a severe C++ accent that you do a disservice to both languages. |
| 648 | There's no reason to insist that each class have but one constructor, |
| 649 | or that a constructor be named new(), or that a constructor be |
| 650 | used solely as a class method and not an object method. |
| 651 | |
| 652 | The next section shows how useful it can be to further distance ourselves |
| 653 | from any formal distinction between class method calls and object method |
| 654 | calls, both in constructors and in accessor methods. |
| 655 | |
| 656 | =head2 Translucent Attributes |
| 657 | |
| 658 | A package's eponymous hash can be used for more than just containing |
| 659 | per-class, global state data. It can also serve as a sort of template |
| 660 | containing default settings for object attributes. These default |
| 661 | settings can then be used in constructors for initialization of a |
| 662 | particular object. The class's eponymous hash can also be used to |
| 663 | implement I<translucent attributes>. A translucent attribute is one |
| 664 | that has a class-wide default. Each object can set its own value for the |
| 665 | attribute, in which case C<< $object->attribute() >> returns that value. |
| 666 | But if no value has been set, then C<< $object->attribute() >> returns |
| 667 | the class-wide default. |
| 668 | |
| 669 | We'll apply something of a copy-on-write approach to these translucent |
| 670 | attributes. If you're just fetching values from them, you get |
| 671 | translucency. But if you store a new value to them, that new value is |
| 672 | set on the current object. On the other hand, if you use the class as |
| 673 | an object and store the attribute value directly on the class, then the |
| 674 | meta-object's value changes, and later fetch operations on objects with |
| 675 | uninitialized values for those attributes will retrieve the meta-object's |
| 676 | new values. Objects with their own initialized values, however, won't |
| 677 | see any change. |
| 678 | |
| 679 | Let's look at some concrete examples of using these properties before we |
| 680 | show how to implement them. Suppose that a class named Some_Class |
| 681 | had a translucent data attribute called "color". First you set the color |
| 682 | in the meta-object, then you create three objects using a constructor |
| 683 | that happens to be named &spawn. |
| 684 | |
| 685 | use Vermin; |
| 686 | Vermin->color("vermilion"); |
| 687 | |
| 688 | $ob1 = Vermin->spawn(); # so that's where Jedi come from |
| 689 | $ob2 = Vermin->spawn(); |
| 690 | $ob3 = Vermin->spawn(); |
| 691 | |
| 692 | print $obj3->color(); # prints "vermilion" |
| 693 | |
| 694 | Each of these objects' colors is now "vermilion", because that's the |
| 695 | meta-object's value that attribute, and these objects do not have |
| 696 | individual color values set. |
| 697 | |
| 698 | Changing the attribute on one object has no effect on other objects |
| 699 | previously created. |
| 700 | |
| 701 | $ob3->color("chartreuse"); |
| 702 | print $ob3->color(); # prints "chartreuse" |
| 703 | print $ob1->color(); # prints "vermilion", translucently |
| 704 | |
| 705 | If you now use $ob3 to spawn off another object, the new object will |
| 706 | take the color its parent held, which now happens to be "chartreuse". |
| 707 | That's because the constructor uses the invoking object as its template |
| 708 | for initializing attributes. When that invoking object is the |
| 709 | class name, the object used as a template is the eponymous meta-object. |
| 710 | When the invoking object is a reference to an instantiated object, the |
| 711 | &spawn constructor uses that existing object as a template. |
| 712 | |
| 713 | $ob4 = $ob3->spawn(); # $ob3 now template, not %Vermin |
| 714 | print $ob4->color(); # prints "chartreuse" |
| 715 | |
| 716 | Any actual values set on the template object will be copied to the |
| 717 | new object. But attributes undefined in the template object, being |
| 718 | translucent, will remain undefined and consequently translucent in the |
| 719 | new one as well. |
| 720 | |
| 721 | Now let's change the color attribute on the entire class: |
| 722 | |
| 723 | Vermin->color("azure"); |
| 724 | print $ob1->color(); # prints "azure" |
| 725 | print $ob2->color(); # prints "azure" |
| 726 | print $ob3->color(); # prints "chartreuse" |
| 727 | print $ob4->color(); # prints "chartreuse" |
| 728 | |
| 729 | That color change took effect only in the first pair of objects, which |
| 730 | were still translucently accessing the meta-object's values. The second |
| 731 | pair had per-object initialized colors, and so didn't change. |
| 732 | |
| 733 | One important question remains. Changes to the meta-object are reflected |
| 734 | in translucent attributes in the entire class, but what about |
| 735 | changes to discrete objects? If you change the color of $ob3, does the |
| 736 | value of $ob4 see that change? Or vice-versa. If you change the color |
| 737 | of $ob4, does then the value of $ob3 shift? |
| 738 | |
| 739 | $ob3->color("amethyst"); |
| 740 | print $ob3->color(); # prints "amethyst" |
| 741 | print $ob4->color(); # hmm: "chartreuse" or "amethyst"? |
| 742 | |
| 743 | While one could argue that in certain rare cases it should, let's not |
| 744 | do that. Good taste aside, we want the answer to the question posed in |
| 745 | the comment above to be "chartreuse", not "amethyst". So we'll treat |
| 746 | these attributes similar to the way process attributes like environment |
| 747 | variables, user and group IDs, or the current working directory are |
| 748 | treated across a fork(). You can change only yourself, but you will see |
| 749 | those changes reflected in your unspawned children. Changes to one object |
| 750 | will propagate neither up to the parent nor down to any existing child objects. |
| 751 | Those objects made later, however, will see the changes. |
| 752 | |
| 753 | If you have an object with an actual attribute value, and you want to |
| 754 | make that object's attribute value translucent again, what do you do? |
| 755 | Let's design the class so that when you invoke an accessor method with |
| 756 | C<undef> as its argument, that attribute returns to translucency. |
| 757 | |
| 758 | $ob4->color(undef); # back to "azure" |
| 759 | |
| 760 | Here's a complete implementation of Vermin as described above. |
| 761 | |
| 762 | package Vermin; |
| 763 | |
| 764 | # here's the class meta-object, eponymously named. |
| 765 | # it holds all class attributes, and also all instance attributes |
| 766 | # so the latter can be used for both initialization |
| 767 | # and translucency. |
| 768 | |
| 769 | our %Vermin = ( # our() is new to perl5.6 |
| 770 | PopCount => 0, # capital for class attributes |
| 771 | color => "beige", # small for instance attributes |
| 772 | ); |
| 773 | |
| 774 | # constructor method |
| 775 | # invoked as class method or object method |
| 776 | sub spawn { |
| 777 | my $obclass = shift; |
| 778 | my $class = ref($obclass) || $obclass; |
| 779 | my $self = {}; |
| 780 | bless($self, $class); |
| 781 | $class->{PopCount}++; |
| 782 | # init fields from invoking object, or omit if |
| 783 | # invoking object is the class to provide translucency |
| 784 | %$self = %$obclass if ref $obclass; |
| 785 | return $self; |
| 786 | } |
| 787 | |
| 788 | # translucent accessor for "color" attribute |
| 789 | # invoked as class method or object method |
| 790 | sub color { |
| 791 | my $self = shift; |
| 792 | my $class = ref($self) || $self; |
| 793 | |
| 794 | # handle class invocation |
| 795 | unless (ref $self) { |
| 796 | $class->{color} = shift if @_; |
| 797 | return $class->{color} |
| 798 | } |
| 799 | |
| 800 | # handle object invocation |
| 801 | $self->{color} = shift if @_; |
| 802 | if (defined $self->{color}) { # not exists! |
| 803 | return $self->{color}; |
| 804 | } else { |
| 805 | return $class->{color}; |
| 806 | } |
| 807 | } |
| 808 | |
| 809 | # accessor for "PopCount" class attribute |
| 810 | # invoked as class method or object method |
| 811 | # but uses object solely to locate meta-object |
| 812 | sub population { |
| 813 | my $obclass = shift; |
| 814 | my $class = ref($obclass) || $obclass; |
| 815 | return $class->{PopCount}; |
| 816 | } |
| 817 | |
| 818 | # instance destructor |
| 819 | # invoked only as object method |
| 820 | sub DESTROY { |
| 821 | my $self = shift; |
| 822 | my $class = ref $self; |
| 823 | $class->{PopCount}--; |
| 824 | } |
| 825 | |
| 826 | Here are a couple of helper methods that might be convenient. They aren't |
| 827 | accessor methods at all. They're used to detect accessibility of data |
| 828 | attributes. The &is_translucent method determines whether a particular |
| 829 | object attribute is coming from the meta-object. The &has_attribute |
| 830 | method detects whether a class implements a particular property at all. |
| 831 | It could also be used to distinguish undefined properties from non-existent |
| 832 | ones. |
| 833 | |
| 834 | # detect whether an object attribute is translucent |
| 835 | # (typically?) invoked only as object method |
| 836 | sub is_translucent { |
| 837 | my($self, $attr) = @_; |
| 838 | return !defined $self->{$attr}; |
| 839 | } |
| 840 | |
| 841 | # test for presence of attribute in class |
| 842 | # invoked as class method or object method |
| 843 | sub has_attribute { |
| 844 | my($self, $attr) = @_; |
| 845 | my $class = ref $self if $self; |
| 846 | return exists $class->{$attr}; |
| 847 | } |
| 848 | |
| 849 | If you prefer to install your accessors more generically, you can make |
| 850 | use of the upper-case versus lower-case convention to register into the |
| 851 | package appropriate methods cloned from generic closures. |
| 852 | |
| 853 | for my $datum (keys %{ +__PACKAGE__ }) { |
| 854 | *$datum = ($datum =~ /^[A-Z]/) |
| 855 | ? sub { # install class accessor |
| 856 | my $obclass = shift; |
| 857 | my $class = ref($obclass) || $obclass; |
| 858 | return $class->{$datum}; |
| 859 | } |
| 860 | : sub { # install translucent accessor |
| 861 | my $self = shift; |
| 862 | my $class = ref($self) || $self; |
| 863 | unless (ref $self) { |
| 864 | $class->{$datum} = shift if @_; |
| 865 | return $class->{$datum} |
| 866 | } |
| 867 | $self->{$datum} = shift if @_; |
| 868 | return defined $self->{$datum} |
| 869 | ? $self -> {$datum} |
| 870 | : $class -> {$datum} |
| 871 | } |
| 872 | } |
| 873 | |
| 874 | Translations of this closure-based approach into C++, Java, and Python |
| 875 | have been left as exercises for the reader. Be sure to send us mail as |
| 876 | soon as you're done. |
| 877 | |
| 878 | =head1 Class Data as Lexical Variables |
| 879 | |
| 880 | =head2 Privacy and Responsibility |
| 881 | |
| 882 | Unlike conventions used by some Perl programmers, in the previous |
| 883 | examples, we didn't prefix the package variables used for class attributes |
| 884 | with an underscore, nor did we do so for the names of the hash keys used |
| 885 | for instance attributes. You don't need little markers on data names to |
| 886 | suggest nominal privacy on attribute variables or hash keys, because these |
| 887 | are B<already> notionally private! Outsiders have no business whatsoever |
| 888 | playing with anything within a class save through the mediated access of |
| 889 | its documented interface; in other words, through method invocations. |
| 890 | And not even through just any method, either. Methods that begin with |
| 891 | an underscore are traditionally considered off-limits outside the class. |
| 892 | If outsiders skip the documented method interface to poke around the |
| 893 | internals of your class and end up breaking something, that's not your |
| 894 | fault--it's theirs. |
| 895 | |
| 896 | Perl believes in individual responsibility rather than mandated control. |
| 897 | Perl respects you enough to let you choose your own preferred level of |
| 898 | pain, or of pleasure. Perl believes that you are creative, intelligent, |
| 899 | and capable of making your own decisions--and fully expects you to |
| 900 | take complete responsibility for your own actions. In a perfect world, |
| 901 | these admonitions alone would suffice, and everyone would be intelligent, |
| 902 | responsible, happy, and creative. And careful. One probably shouldn't |
| 903 | forget careful, and that's a good bit harder to expect. Even Einstein |
| 904 | would take wrong turns by accident and end up lost in the wrong part |
| 905 | of town. |
| 906 | |
| 907 | Some folks get the heebie-jeebies when they see package variables |
| 908 | hanging out there for anyone to reach over and alter them. Some folks |
| 909 | live in constant fear that someone somewhere might do something wicked. |
| 910 | The solution to that problem is simply to fire the wicked, of course. |
| 911 | But unfortunately, it's not as simple as all that. These cautious |
| 912 | types are also afraid that they or others will do something not so |
| 913 | much wicked as careless, whether by accident or out of desperation. |
| 914 | If we fire everyone who ever gets careless, pretty soon there won't be |
| 915 | anybody left to get any work done. |
| 916 | |
| 917 | Whether it's needless paranoia or sensible caution, this uneasiness can |
| 918 | be a problem for some people. We can take the edge off their discomfort |
| 919 | by providing the option of storing class attributes as lexical variables |
| 920 | instead of as package variables. The my() operator is the source of |
| 921 | all privacy in Perl, and it is a powerful form of privacy indeed. |
| 922 | |
| 923 | It is widely perceived, and indeed has often been written, that Perl |
| 924 | provides no data hiding, that it affords the class designer no privacy |
| 925 | nor isolation, merely a rag-tag assortment of weak and unenforcible |
| 926 | social conventions instead. This perception is demonstrably false and |
| 927 | easily disproven. In the next section, we show how to implement forms |
| 928 | of privacy that are far stronger than those provided in nearly any |
| 929 | other object-oriented language. |
| 930 | |
| 931 | =head2 File-Scoped Lexicals |
| 932 | |
| 933 | A lexical variable is visible only through the end of its static scope. |
| 934 | That means that the only code able to access that variable is code |
| 935 | residing textually below the my() operator through the end of its block |
| 936 | if it has one, or through the end of the current file if it doesn't. |
| 937 | |
| 938 | Starting again with our simplest example given at the start of this |
| 939 | document, we replace our() variables with my() versions. |
| 940 | |
| 941 | package Some_Class; |
| 942 | my($CData1, $CData2); # file scope, not in any package |
| 943 | sub CData1 { |
| 944 | shift; # XXX: ignore calling class/object |
| 945 | $CData1 = shift if @_; |
| 946 | return $CData1; |
| 947 | } |
| 948 | sub CData2 { |
| 949 | shift; # XXX: ignore calling class/object |
| 950 | $CData2 = shift if @_; |
| 951 | return $CData2; |
| 952 | } |
| 953 | |
| 954 | So much for that old $Some_Class::CData1 package variable and its brethren! |
| 955 | Those are gone now, replaced with lexicals. No one outside the |
| 956 | scope can reach in and alter the class state without resorting to the |
| 957 | documented interface. Not even subclasses or superclasses of |
| 958 | this one have unmediated access to $CData1. They have to invoke the &CData1 |
| 959 | method against Some_Class or an instance thereof, just like anybody else. |
| 960 | |
| 961 | To be scrupulously honest, that last statement assumes you haven't packed |
| 962 | several classes together into the same file scope, nor strewn your class |
| 963 | implementation across several different files. Accessibility of those |
| 964 | variables is based uniquely on the static file scope. It has nothing to |
| 965 | do with the package. That means that code in a different file but |
| 966 | the same package (class) could not access those variables, yet code in the |
| 967 | same file but a different package (class) could. There are sound reasons |
| 968 | why we usually suggest a one-to-one mapping between files and packages |
| 969 | and modules and classes. You don't have to stick to this suggestion if |
| 970 | you really know what you're doing, but you're apt to confuse yourself |
| 971 | otherwise, especially at first. |
| 972 | |
| 973 | If you'd like to aggregate your class attributes into one lexically scoped, |
| 974 | composite structure, you're perfectly free to do so. |
| 975 | |
| 976 | package Some_Class; |
| 977 | my %ClassData = ( |
| 978 | CData1 => "", |
| 979 | CData2 => "", |
| 980 | ); |
| 981 | sub CData1 { |
| 982 | shift; # XXX: ignore calling class/object |
| 983 | $ClassData{CData1} = shift if @_; |
| 984 | return $ClassData{CData1}; |
| 985 | } |
| 986 | sub CData2 { |
| 987 | shift; # XXX: ignore calling class/object |
| 988 | $ClassData{CData2} = shift if @_; |
| 989 | return $ClassData{CData2}; |
| 990 | } |
| 991 | |
| 992 | To make this more scalable as other class attributes are added, we can |
| 993 | again register closures into the package symbol table to create accessor |
| 994 | methods for them. |
| 995 | |
| 996 | package Some_Class; |
| 997 | my %ClassData = ( |
| 998 | CData1 => "", |
| 999 | CData2 => "", |
| 1000 | ); |
| 1001 | for my $datum (keys %ClassData) { |
| 1002 | no strict "refs"; |
| 1003 | *$datum = sub { |
| 1004 | shift; # XXX: ignore calling class/object |
| 1005 | $ClassData{$datum} = shift if @_; |
| 1006 | return $ClassData{$datum}; |
| 1007 | }; |
| 1008 | } |
| 1009 | |
| 1010 | Requiring even your own class to use accessor methods like anybody else is |
| 1011 | probably a good thing. But demanding and expecting that everyone else, |
| 1012 | be they subclass or superclass, friend or foe, will all come to your |
| 1013 | object through mediation is more than just a good idea. It's absolutely |
| 1014 | critical to the model. Let there be in your mind no such thing as |
| 1015 | "public" data, nor even "protected" data, which is a seductive but |
| 1016 | ultimately destructive notion. Both will come back to bite at you. |
| 1017 | That's because as soon as you take that first step out of the solid |
| 1018 | position in which all state is considered completely private, save from the |
| 1019 | perspective of its own accessor methods, you have violated the envelope. |
| 1020 | And, having pierced that encapsulating envelope, you shall doubtless |
| 1021 | someday pay the price when future changes in the implementation break |
| 1022 | unrelated code. Considering that avoiding this infelicitous outcome was |
| 1023 | precisely why you consented to suffer the slings and arrows of obsequious |
| 1024 | abstraction by turning to object orientation in the first place, such |
| 1025 | breakage seems unfortunate in the extreme. |
| 1026 | |
| 1027 | =head2 More Inheritance Concerns |
| 1028 | |
| 1029 | Suppose that Some_Class were used as a base class from which to derive |
| 1030 | Another_Class. If you invoke a &CData method on the derived class or |
| 1031 | on an object of that class, what do you get? Would the derived class |
| 1032 | have its own state, or would it piggyback on its base class's versions |
| 1033 | of the class attributes? |
| 1034 | |
| 1035 | The answer is that under the scheme outlined above, the derived class |
| 1036 | would B<not> have its own state data. As before, whether you consider |
| 1037 | this a good thing or a bad one depends on the semantics of the classes |
| 1038 | involved. |
| 1039 | |
| 1040 | The cleanest, sanest, simplest way to address per-class state in a |
| 1041 | lexical is for the derived class to override its base class's version |
| 1042 | of the method that accesses the class attributes. Since the actual method |
| 1043 | called is the one in the object's derived class if this exists, you |
| 1044 | automatically get per-class state this way. Any urge to provide an |
| 1045 | unadvertised method to sneak out a reference to the %ClassData hash |
| 1046 | should be strenuously resisted. |
| 1047 | |
| 1048 | As with any other overridden method, the implementation in the |
| 1049 | derived class always has the option of invoking its base class's |
| 1050 | version of the method in addition to its own. Here's an example: |
| 1051 | |
| 1052 | package Another_Class; |
| 1053 | @ISA = qw(Some_Class); |
| 1054 | |
| 1055 | my %ClassData = ( |
| 1056 | CData1 => "", |
| 1057 | ); |
| 1058 | |
| 1059 | sub CData1 { |
| 1060 | my($self, $newvalue) = @_; |
| 1061 | if (@_ > 1) { |
| 1062 | # set locally first |
| 1063 | $ClassData{CData1} = $newvalue; |
| 1064 | |
| 1065 | # then pass the buck up to the first |
| 1066 | # overridden version, if there is one |
| 1067 | if ($self->can("SUPER::CData1")) { |
| 1068 | $self->SUPER::CData1($newvalue); |
| 1069 | } |
| 1070 | } |
| 1071 | return $ClassData{CData1}; |
| 1072 | } |
| 1073 | |
| 1074 | Those dabbling in multiple inheritance might be concerned |
| 1075 | about there being more than one override. |
| 1076 | |
| 1077 | for my $parent (@ISA) { |
| 1078 | my $methname = $parent . "::CData1"; |
| 1079 | if ($self->can($methname)) { |
| 1080 | $self->$methname($newvalue); |
| 1081 | } |
| 1082 | } |
| 1083 | |
| 1084 | Because the &UNIVERSAL::can method returns a reference |
| 1085 | to the function directly, you can use this directly |
| 1086 | for a significant performance improvement: |
| 1087 | |
| 1088 | for my $parent (@ISA) { |
| 1089 | if (my $coderef = $self->can($parent . "::CData1")) { |
| 1090 | $self->$coderef($newvalue); |
| 1091 | } |
| 1092 | } |
| 1093 | |
| 1094 | =head2 Locking the Door and Throwing Away the Key |
| 1095 | |
| 1096 | As currently implemented, any code within the same scope as the |
| 1097 | file-scoped lexical %ClassData can alter that hash directly. Is that |
| 1098 | ok? Is it acceptable or even desirable to allow other parts of the |
| 1099 | implementation of this class to access class attributes directly? |
| 1100 | |
| 1101 | That depends on how careful you want to be. Think back to the Cosmos |
| 1102 | class. If the &supernova method had directly altered $Cosmos::Stars or |
| 1103 | C<$Cosmos::Cosmos{stars}>, then we wouldn't have been able to reuse the |
| 1104 | class when it came to inventing a Multiverse. So letting even the class |
| 1105 | itself access its own class attributes without the mediating intervention of |
| 1106 | properly designed accessor methods is probably not a good idea after all. |
| 1107 | |
| 1108 | Restricting access to class attributes from the class itself is usually |
| 1109 | not enforcible even in strongly object-oriented languages. But in Perl, |
| 1110 | you can. |
| 1111 | |
| 1112 | Here's one way: |
| 1113 | |
| 1114 | package Some_Class; |
| 1115 | |
| 1116 | { # scope for hiding $CData1 |
| 1117 | my $CData1; |
| 1118 | sub CData1 { |
| 1119 | shift; # XXX: unused |
| 1120 | $CData1 = shift if @_; |
| 1121 | return $CData1; |
| 1122 | } |
| 1123 | } |
| 1124 | |
| 1125 | { # scope for hiding $CData2 |
| 1126 | my $CData2; |
| 1127 | sub CData2 { |
| 1128 | shift; # XXX: unused |
| 1129 | $CData2 = shift if @_; |
| 1130 | return $CData2; |
| 1131 | } |
| 1132 | } |
| 1133 | |
| 1134 | No one--absolutely no one--is allowed to read or write the class |
| 1135 | attributes without the mediation of the managing accessor method, since |
| 1136 | only that method has access to the lexical variable it's managing. |
| 1137 | This use of mediated access to class attributes is a form of privacy far |
| 1138 | stronger than most OO languages provide. |
| 1139 | |
| 1140 | The repetition of code used to create per-datum accessor methods chafes |
| 1141 | at our Laziness, so we'll again use closures to create similar |
| 1142 | methods. |
| 1143 | |
| 1144 | package Some_Class; |
| 1145 | |
| 1146 | { # scope for ultra-private meta-object for class attributes |
| 1147 | my %ClassData = ( |
| 1148 | CData1 => "", |
| 1149 | CData2 => "", |
| 1150 | ); |
| 1151 | |
| 1152 | for my $datum (keys %ClassData ) { |
| 1153 | no strict "refs"; |
| 1154 | *$datum = sub { |
| 1155 | use strict "refs"; |
| 1156 | my ($self, $newvalue) = @_; |
| 1157 | $ClassData{$datum} = $newvalue if @_ > 1; |
| 1158 | return $ClassData{$datum}; |
| 1159 | } |
| 1160 | } |
| 1161 | |
| 1162 | } |
| 1163 | |
| 1164 | The closure above can be modified to take inheritance into account using |
| 1165 | the &UNIVERSAL::can method and SUPER as shown previously. |
| 1166 | |
| 1167 | =head2 Translucency Revisited |
| 1168 | |
| 1169 | The Vermin class demonstrates translucency using a package variable, |
| 1170 | eponymously named %Vermin, as its meta-object. If you prefer to |
| 1171 | use absolutely no package variables beyond those necessary to appease |
| 1172 | inheritance or possibly the Exporter, this strategy is closed to you. |
| 1173 | That's too bad, because translucent attributes are an appealing |
| 1174 | technique, so it would be valuable to devise an implementation using |
| 1175 | only lexicals. |
| 1176 | |
| 1177 | There's a second reason why you might wish to avoid the eponymous |
| 1178 | package hash. If you use class names with double-colons in them, you |
| 1179 | would end up poking around somewhere you might not have meant to poke. |
| 1180 | |
| 1181 | package Vermin; |
| 1182 | $class = "Vermin"; |
| 1183 | $class->{PopCount}++; |
| 1184 | # accesses $Vermin::Vermin{PopCount} |
| 1185 | |
| 1186 | package Vermin::Noxious; |
| 1187 | $class = "Vermin::Noxious"; |
| 1188 | $class->{PopCount}++; |
| 1189 | # accesses $Vermin::Noxious{PopCount} |
| 1190 | |
| 1191 | In the first case, because the class name had no double-colons, we got |
| 1192 | the hash in the current package. But in the second case, instead of |
| 1193 | getting some hash in the current package, we got the hash %Noxious in |
| 1194 | the Vermin package. (The noxious vermin just invaded another package and |
| 1195 | sprayed their data around it. :-) Perl doesn't support relative packages |
| 1196 | in its naming conventions, so any double-colons trigger a fully-qualified |
| 1197 | lookup instead of just looking in the current package. |
| 1198 | |
| 1199 | In practice, it is unlikely that the Vermin class had an existing |
| 1200 | package variable named %Noxious that you just blew away. If you're |
| 1201 | still mistrustful, you could always stake out your own territory |
| 1202 | where you know the rules, such as using Eponymous::Vermin::Noxious or |
| 1203 | Hieronymus::Vermin::Boschious or Leave_Me_Alone::Vermin::Noxious as class |
| 1204 | names instead. Sure, it's in theory possible that someone else has |
| 1205 | a class named Eponymous::Vermin with its own %Noxious hash, but this |
| 1206 | kind of thing is always true. There's no arbiter of package names. |
| 1207 | It's always the case that globals like @Cwd::ISA would collide if more |
| 1208 | than one class uses the same Cwd package. |
| 1209 | |
| 1210 | If this still leaves you with an uncomfortable twinge of paranoia, |
| 1211 | we have another solution for you. There's nothing that says that you |
| 1212 | have to have a package variable to hold a class meta-object, either for |
| 1213 | monadic classes or for translucent attributes. Just code up the methods |
| 1214 | so that they access a lexical instead. |
| 1215 | |
| 1216 | Here's another implementation of the Vermin class with semantics identical |
| 1217 | to those given previously, but this time using no package variables. |
| 1218 | |
| 1219 | package Vermin; |
| 1220 | |
| 1221 | |
| 1222 | # Here's the class meta-object, eponymously named. |
| 1223 | # It holds all class data, and also all instance data |
| 1224 | # so the latter can be used for both initialization |
| 1225 | # and translucency. it's a template. |
| 1226 | my %ClassData = ( |
| 1227 | PopCount => 0, # capital for class attributes |
| 1228 | color => "beige", # small for instance attributes |
| 1229 | ); |
| 1230 | |
| 1231 | # constructor method |
| 1232 | # invoked as class method or object method |
| 1233 | sub spawn { |
| 1234 | my $obclass = shift; |
| 1235 | my $class = ref($obclass) || $obclass; |
| 1236 | my $self = {}; |
| 1237 | bless($self, $class); |
| 1238 | $ClassData{PopCount}++; |
| 1239 | # init fields from invoking object, or omit if |
| 1240 | # invoking object is the class to provide translucency |
| 1241 | %$self = %$obclass if ref $obclass; |
| 1242 | return $self; |
| 1243 | } |
| 1244 | |
| 1245 | # translucent accessor for "color" attribute |
| 1246 | # invoked as class method or object method |
| 1247 | sub color { |
| 1248 | my $self = shift; |
| 1249 | |
| 1250 | # handle class invocation |
| 1251 | unless (ref $self) { |
| 1252 | $ClassData{color} = shift if @_; |
| 1253 | return $ClassData{color} |
| 1254 | } |
| 1255 | |
| 1256 | # handle object invocation |
| 1257 | $self->{color} = shift if @_; |
| 1258 | if (defined $self->{color}) { # not exists! |
| 1259 | return $self->{color}; |
| 1260 | } else { |
| 1261 | return $ClassData{color}; |
| 1262 | } |
| 1263 | } |
| 1264 | |
| 1265 | # class attribute accessor for "PopCount" attribute |
| 1266 | # invoked as class method or object method |
| 1267 | sub population { |
| 1268 | return $ClassData{PopCount}; |
| 1269 | } |
| 1270 | |
| 1271 | # instance destructor; invoked only as object method |
| 1272 | sub DESTROY { |
| 1273 | $ClassData{PopCount}--; |
| 1274 | } |
| 1275 | |
| 1276 | # detect whether an object attribute is translucent |
| 1277 | # (typically?) invoked only as object method |
| 1278 | sub is_translucent { |
| 1279 | my($self, $attr) = @_; |
| 1280 | $self = \%ClassData if !ref $self; |
| 1281 | return !defined $self->{$attr}; |
| 1282 | } |
| 1283 | |
| 1284 | # test for presence of attribute in class |
| 1285 | # invoked as class method or object method |
| 1286 | sub has_attribute { |
| 1287 | my($self, $attr) = @_; |
| 1288 | return exists $ClassData{$attr}; |
| 1289 | } |
| 1290 | |
| 1291 | =head1 NOTES |
| 1292 | |
| 1293 | Inheritance is a powerful but subtle device, best used only after careful |
| 1294 | forethought and design. Aggregation instead of inheritance is often a |
| 1295 | better approach. |
| 1296 | |
| 1297 | We use the hypothetical our() syntax for package variables. It works |
| 1298 | like C<use vars>, but looks like my(). It should be in this summer's |
| 1299 | major release (5.6) of perl--we hope. |
| 1300 | |
| 1301 | You can't use file-scoped lexicals in conjunction with the SelfLoader |
| 1302 | or the AutoLoader, because they alter the lexical scope in which the |
| 1303 | module's methods wind up getting compiled. |
| 1304 | |
| 1305 | The usual mealy-mouthed package-mungeing doubtless applies to setting |
| 1306 | up names of object attributes. For example, C<< $self->{ObData1} >> |
| 1307 | should probably be C<< $self->{ __PACKAGE__ . "_ObData1" } >>, but that |
| 1308 | would just confuse the examples. |
| 1309 | |
| 1310 | =head1 SEE ALSO |
| 1311 | |
| 1312 | L<perltoot>, L<perlobj>, L<perlmod>, and L<perlbot>. |
| 1313 | |
| 1314 | The Tie::SecureHash and Class::Data::Inheritable modules from CPAN are |
| 1315 | worth checking out. |
| 1316 | |
| 1317 | =head1 AUTHOR AND COPYRIGHT |
| 1318 | |
| 1319 | Copyright (c) 1999 Tom Christiansen. |
| 1320 | All rights reserved. |
| 1321 | |
| 1322 | This documentation is free; you can redistribute it and/or modify it |
| 1323 | under the same terms as Perl itself. |
| 1324 | |
| 1325 | Irrespective of its distribution, all code examples in this file |
| 1326 | are hereby placed into the public domain. You are permitted and |
| 1327 | encouraged to use this code in your own programs for fun |
| 1328 | or for profit as you see fit. A simple comment in the code giving |
| 1329 | credit would be courteous but is not required. |
| 1330 | |
| 1331 | =head1 ACKNOWLEDGEMENTS |
| 1332 | |
| 1333 | Russ Allbery, Jon Orwant, Randy Ray, Larry Rosler, Nat Torkington, |
| 1334 | and Stephen Warren all contributed suggestions and corrections to this |
| 1335 | piece. Thanks especially to Damian Conway for his ideas and feedback, |
| 1336 | and without whose indirect prodding I might never have taken the time |
| 1337 | to show others how much Perl has to offer in the way of objects once |
| 1338 | you start thinking outside the tiny little box that today's "popular" |
| 1339 | object-oriented languages enforce. |
| 1340 | |
| 1341 | =head1 HISTORY |
| 1342 | |
| 1343 | Last edit: Sun Feb 4 20:50:28 EST 2001 |