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1 | =head1 NAME |
2 | X<tie> | |
3 | ||
4 | perltie - how to hide an object class in a simple variable | |
5 | ||
6 | =head1 SYNOPSIS | |
7 | ||
8 | tie VARIABLE, CLASSNAME, LIST | |
9 | ||
10 | $object = tied VARIABLE | |
11 | ||
12 | untie VARIABLE | |
13 | ||
14 | =head1 DESCRIPTION | |
15 | ||
16 | Prior to release 5.0 of Perl, a programmer could use dbmopen() | |
17 | to connect an on-disk database in the standard Unix dbm(3x) | |
18 | format magically to a %HASH in their program. However, their Perl was either | |
19 | built with one particular dbm library or another, but not both, and | |
20 | you couldn't extend this mechanism to other packages or types of variables. | |
21 | ||
22 | Now you can. | |
23 | ||
24 | The tie() function binds a variable to a class (package) that will provide | |
25 | the implementation for access methods for that variable. Once this magic | |
26 | has been performed, accessing a tied variable automatically triggers | |
27 | method calls in the proper class. The complexity of the class is | |
28 | hidden behind magic methods calls. The method names are in ALL CAPS, | |
29 | which is a convention that Perl uses to indicate that they're called | |
30 | implicitly rather than explicitly--just like the BEGIN() and END() | |
31 | functions. | |
32 | ||
33 | In the tie() call, C<VARIABLE> is the name of the variable to be | |
34 | enchanted. C<CLASSNAME> is the name of a class implementing objects of | |
35 | the correct type. Any additional arguments in the C<LIST> are passed to | |
36 | the appropriate constructor method for that class--meaning TIESCALAR(), | |
37 | TIEARRAY(), TIEHASH(), or TIEHANDLE(). (Typically these are arguments | |
38 | such as might be passed to the dbminit() function of C.) The object | |
39 | returned by the "new" method is also returned by the tie() function, | |
40 | which would be useful if you wanted to access other methods in | |
41 | C<CLASSNAME>. (You don't actually have to return a reference to a right | |
42 | "type" (e.g., HASH or C<CLASSNAME>) so long as it's a properly blessed | |
43 | object.) You can also retrieve a reference to the underlying object | |
44 | using the tied() function. | |
45 | ||
46 | Unlike dbmopen(), the tie() function will not C<use> or C<require> a module | |
47 | for you--you need to do that explicitly yourself. | |
48 | ||
49 | =head2 Tying Scalars | |
50 | X<scalar, tying> | |
51 | ||
52 | A class implementing a tied scalar should define the following methods: | |
53 | TIESCALAR, FETCH, STORE, and possibly UNTIE and/or DESTROY. | |
54 | ||
55 | Let's look at each in turn, using as an example a tie class for | |
56 | scalars that allows the user to do something like: | |
57 | ||
58 | tie $his_speed, 'Nice', getppid(); | |
59 | tie $my_speed, 'Nice', $$; | |
60 | ||
61 | And now whenever either of those variables is accessed, its current | |
62 | system priority is retrieved and returned. If those variables are set, | |
63 | then the process's priority is changed! | |
64 | ||
65 | We'll use Jarkko Hietaniemi <F<jhi@iki.fi>>'s BSD::Resource class (not | |
66 | included) to access the PRIO_PROCESS, PRIO_MIN, and PRIO_MAX constants | |
67 | from your system, as well as the getpriority() and setpriority() system | |
68 | calls. Here's the preamble of the class. | |
69 | ||
70 | package Nice; | |
71 | use Carp; | |
72 | use BSD::Resource; | |
73 | use strict; | |
74 | $Nice::DEBUG = 0 unless defined $Nice::DEBUG; | |
75 | ||
76 | =over 4 | |
77 | ||
78 | =item TIESCALAR classname, LIST | |
79 | X<TIESCALAR> | |
80 | ||
81 | This is the constructor for the class. That means it is | |
82 | expected to return a blessed reference to a new scalar | |
83 | (probably anonymous) that it's creating. For example: | |
84 | ||
85 | sub TIESCALAR { | |
86 | my $class = shift; | |
87 | my $pid = shift || $$; # 0 means me | |
88 | ||
89 | if ($pid !~ /^\d+$/) { | |
90 | carp "Nice::Tie::Scalar got non-numeric pid $pid" if $^W; | |
91 | return undef; | |
92 | } | |
93 | ||
94 | unless (kill 0, $pid) { # EPERM or ERSCH, no doubt | |
95 | carp "Nice::Tie::Scalar got bad pid $pid: $!" if $^W; | |
96 | return undef; | |
97 | } | |
98 | ||
99 | return bless \$pid, $class; | |
100 | } | |
101 | ||
102 | This tie class has chosen to return an error rather than raising an | |
103 | exception if its constructor should fail. While this is how dbmopen() works, | |
104 | other classes may well not wish to be so forgiving. It checks the global | |
105 | variable C<$^W> to see whether to emit a bit of noise anyway. | |
106 | ||
107 | =item FETCH this | |
108 | X<FETCH> | |
109 | ||
110 | This method will be triggered every time the tied variable is accessed | |
111 | (read). It takes no arguments beyond its self reference, which is the | |
112 | object representing the scalar we're dealing with. Because in this case | |
113 | we're using just a SCALAR ref for the tied scalar object, a simple $$self | |
114 | allows the method to get at the real value stored there. In our example | |
115 | below, that real value is the process ID to which we've tied our variable. | |
116 | ||
117 | sub FETCH { | |
118 | my $self = shift; | |
119 | confess "wrong type" unless ref $self; | |
120 | croak "usage error" if @_; | |
121 | my $nicety; | |
122 | local($!) = 0; | |
123 | $nicety = getpriority(PRIO_PROCESS, $$self); | |
124 | if ($!) { croak "getpriority failed: $!" } | |
125 | return $nicety; | |
126 | } | |
127 | ||
128 | This time we've decided to blow up (raise an exception) if the renice | |
129 | fails--there's no place for us to return an error otherwise, and it's | |
130 | probably the right thing to do. | |
131 | ||
132 | =item STORE this, value | |
133 | X<STORE> | |
134 | ||
135 | This method will be triggered every time the tied variable is set | |
136 | (assigned). Beyond its self reference, it also expects one (and only one) | |
137 | argument--the new value the user is trying to assign. Don't worry about | |
138 | returning a value from STORE -- the semantic of assignment returning the | |
139 | assigned value is implemented with FETCH. | |
140 | ||
141 | sub STORE { | |
142 | my $self = shift; | |
143 | confess "wrong type" unless ref $self; | |
144 | my $new_nicety = shift; | |
145 | croak "usage error" if @_; | |
146 | ||
147 | if ($new_nicety < PRIO_MIN) { | |
148 | carp sprintf | |
149 | "WARNING: priority %d less than minimum system priority %d", | |
150 | $new_nicety, PRIO_MIN if $^W; | |
151 | $new_nicety = PRIO_MIN; | |
152 | } | |
153 | ||
154 | if ($new_nicety > PRIO_MAX) { | |
155 | carp sprintf | |
156 | "WARNING: priority %d greater than maximum system priority %d", | |
157 | $new_nicety, PRIO_MAX if $^W; | |
158 | $new_nicety = PRIO_MAX; | |
159 | } | |
160 | ||
161 | unless (defined setpriority(PRIO_PROCESS, $$self, $new_nicety)) { | |
162 | confess "setpriority failed: $!"; | |
163 | } | |
164 | } | |
165 | ||
166 | =item UNTIE this | |
167 | X<UNTIE> | |
168 | ||
169 | This method will be triggered when the C<untie> occurs. This can be useful | |
170 | if the class needs to know when no further calls will be made. (Except DESTROY | |
171 | of course.) See L<The C<untie> Gotcha> below for more details. | |
172 | ||
173 | =item DESTROY this | |
174 | X<DESTROY> | |
175 | ||
176 | This method will be triggered when the tied variable needs to be destructed. | |
177 | As with other object classes, such a method is seldom necessary, because Perl | |
178 | deallocates its moribund object's memory for you automatically--this isn't | |
179 | C++, you know. We'll use a DESTROY method here for debugging purposes only. | |
180 | ||
181 | sub DESTROY { | |
182 | my $self = shift; | |
183 | confess "wrong type" unless ref $self; | |
184 | carp "[ Nice::DESTROY pid $$self ]" if $Nice::DEBUG; | |
185 | } | |
186 | ||
187 | =back | |
188 | ||
189 | That's about all there is to it. Actually, it's more than all there | |
190 | is to it, because we've done a few nice things here for the sake | |
191 | of completeness, robustness, and general aesthetics. Simpler | |
192 | TIESCALAR classes are certainly possible. | |
193 | ||
194 | =head2 Tying Arrays | |
195 | X<array, tying> | |
196 | ||
197 | A class implementing a tied ordinary array should define the following | |
198 | methods: TIEARRAY, FETCH, STORE, FETCHSIZE, STORESIZE and perhaps UNTIE and/or DESTROY. | |
199 | ||
200 | FETCHSIZE and STORESIZE are used to provide C<$#array> and | |
201 | equivalent C<scalar(@array)> access. | |
202 | ||
203 | The methods POP, PUSH, SHIFT, UNSHIFT, SPLICE, DELETE, and EXISTS are | |
204 | required if the perl operator with the corresponding (but lowercase) name | |
205 | is to operate on the tied array. The B<Tie::Array> class can be used as a | |
206 | base class to implement the first five of these in terms of the basic | |
207 | methods above. The default implementations of DELETE and EXISTS in | |
208 | B<Tie::Array> simply C<croak>. | |
209 | ||
210 | In addition EXTEND will be called when perl would have pre-extended | |
211 | allocation in a real array. | |
212 | ||
213 | For this discussion, we'll implement an array whose elements are a fixed | |
214 | size at creation. If you try to create an element larger than the fixed | |
215 | size, you'll take an exception. For example: | |
216 | ||
217 | use FixedElem_Array; | |
218 | tie @array, 'FixedElem_Array', 3; | |
219 | $array[0] = 'cat'; # ok. | |
220 | $array[1] = 'dogs'; # exception, length('dogs') > 3. | |
221 | ||
222 | The preamble code for the class is as follows: | |
223 | ||
224 | package FixedElem_Array; | |
225 | use Carp; | |
226 | use strict; | |
227 | ||
228 | =over 4 | |
229 | ||
230 | =item TIEARRAY classname, LIST | |
231 | X<TIEARRAY> | |
232 | ||
233 | This is the constructor for the class. That means it is expected to | |
234 | return a blessed reference through which the new array (probably an | |
235 | anonymous ARRAY ref) will be accessed. | |
236 | ||
237 | In our example, just to show you that you don't I<really> have to return an | |
238 | ARRAY reference, we'll choose a HASH reference to represent our object. | |
239 | A HASH works out well as a generic record type: the C<{ELEMSIZE}> field will | |
240 | store the maximum element size allowed, and the C<{ARRAY}> field will hold the | |
241 | true ARRAY ref. If someone outside the class tries to dereference the | |
242 | object returned (doubtless thinking it an ARRAY ref), they'll blow up. | |
243 | This just goes to show you that you should respect an object's privacy. | |
244 | ||
245 | sub TIEARRAY { | |
246 | my $class = shift; | |
247 | my $elemsize = shift; | |
248 | if ( @_ || $elemsize =~ /\D/ ) { | |
249 | croak "usage: tie ARRAY, '" . __PACKAGE__ . "', elem_size"; | |
250 | } | |
251 | return bless { | |
252 | ELEMSIZE => $elemsize, | |
253 | ARRAY => [], | |
254 | }, $class; | |
255 | } | |
256 | ||
257 | =item FETCH this, index | |
258 | X<FETCH> | |
259 | ||
260 | This method will be triggered every time an individual element the tied array | |
261 | is accessed (read). It takes one argument beyond its self reference: the | |
262 | index whose value we're trying to fetch. | |
263 | ||
264 | sub FETCH { | |
265 | my $self = shift; | |
266 | my $index = shift; | |
267 | return $self->{ARRAY}->[$index]; | |
268 | } | |
269 | ||
270 | If a negative array index is used to read from an array, the index | |
271 | will be translated to a positive one internally by calling FETCHSIZE | |
272 | before being passed to FETCH. You may disable this feature by | |
273 | assigning a true value to the variable C<$NEGATIVE_INDICES> in the | |
274 | tied array class. | |
275 | ||
276 | As you may have noticed, the name of the FETCH method (et al.) is the same | |
277 | for all accesses, even though the constructors differ in names (TIESCALAR | |
278 | vs TIEARRAY). While in theory you could have the same class servicing | |
279 | several tied types, in practice this becomes cumbersome, and it's easiest | |
280 | to keep them at simply one tie type per class. | |
281 | ||
282 | =item STORE this, index, value | |
283 | X<STORE> | |
284 | ||
285 | This method will be triggered every time an element in the tied array is set | |
286 | (written). It takes two arguments beyond its self reference: the index at | |
287 | which we're trying to store something and the value we're trying to put | |
288 | there. | |
289 | ||
290 | In our example, C<undef> is really C<$self-E<gt>{ELEMSIZE}> number of | |
291 | spaces so we have a little more work to do here: | |
292 | ||
293 | sub STORE { | |
294 | my $self = shift; | |
295 | my( $index, $value ) = @_; | |
296 | if ( length $value > $self->{ELEMSIZE} ) { | |
297 | croak "length of $value is greater than $self->{ELEMSIZE}"; | |
298 | } | |
299 | # fill in the blanks | |
300 | $self->EXTEND( $index ) if $index > $self->FETCHSIZE(); | |
301 | # right justify to keep element size for smaller elements | |
302 | $self->{ARRAY}->[$index] = sprintf "%$self->{ELEMSIZE}s", $value; | |
303 | } | |
304 | ||
305 | Negative indexes are treated the same as with FETCH. | |
306 | ||
307 | =item FETCHSIZE this | |
308 | X<FETCHSIZE> | |
309 | ||
310 | Returns the total number of items in the tied array associated with | |
311 | object I<this>. (Equivalent to C<scalar(@array)>). For example: | |
312 | ||
313 | sub FETCHSIZE { | |
314 | my $self = shift; | |
315 | return scalar @{$self->{ARRAY}}; | |
316 | } | |
317 | ||
318 | =item STORESIZE this, count | |
319 | X<STORESIZE> | |
320 | ||
321 | Sets the total number of items in the tied array associated with | |
322 | object I<this> to be I<count>. If this makes the array larger then | |
323 | class's mapping of C<undef> should be returned for new positions. | |
324 | If the array becomes smaller then entries beyond count should be | |
325 | deleted. | |
326 | ||
327 | In our example, 'undef' is really an element containing | |
328 | C<$self-E<gt>{ELEMSIZE}> number of spaces. Observe: | |
329 | ||
330 | sub STORESIZE { | |
331 | my $self = shift; | |
332 | my $count = shift; | |
333 | if ( $count > $self->FETCHSIZE() ) { | |
334 | foreach ( $count - $self->FETCHSIZE() .. $count ) { | |
335 | $self->STORE( $_, '' ); | |
336 | } | |
337 | } elsif ( $count < $self->FETCHSIZE() ) { | |
338 | foreach ( 0 .. $self->FETCHSIZE() - $count - 2 ) { | |
339 | $self->POP(); | |
340 | } | |
341 | } | |
342 | } | |
343 | ||
344 | =item EXTEND this, count | |
345 | X<EXTEND> | |
346 | ||
347 | Informative call that array is likely to grow to have I<count> entries. | |
348 | Can be used to optimize allocation. This method need do nothing. | |
349 | ||
350 | In our example, we want to make sure there are no blank (C<undef>) | |
351 | entries, so C<EXTEND> will make use of C<STORESIZE> to fill elements | |
352 | as needed: | |
353 | ||
354 | sub EXTEND { | |
355 | my $self = shift; | |
356 | my $count = shift; | |
357 | $self->STORESIZE( $count ); | |
358 | } | |
359 | ||
360 | =item EXISTS this, key | |
361 | X<EXISTS> | |
362 | ||
363 | Verify that the element at index I<key> exists in the tied array I<this>. | |
364 | ||
365 | In our example, we will determine that if an element consists of | |
366 | C<$self-E<gt>{ELEMSIZE}> spaces only, it does not exist: | |
367 | ||
368 | sub EXISTS { | |
369 | my $self = shift; | |
370 | my $index = shift; | |
371 | return 0 if ! defined $self->{ARRAY}->[$index] || | |
372 | $self->{ARRAY}->[$index] eq ' ' x $self->{ELEMSIZE}; | |
373 | return 1; | |
374 | } | |
375 | ||
376 | =item DELETE this, key | |
377 | X<DELETE> | |
378 | ||
379 | Delete the element at index I<key> from the tied array I<this>. | |
380 | ||
381 | In our example, a deleted item is C<$self-E<gt>{ELEMSIZE}> spaces: | |
382 | ||
383 | sub DELETE { | |
384 | my $self = shift; | |
385 | my $index = shift; | |
386 | return $self->STORE( $index, '' ); | |
387 | } | |
388 | ||
389 | =item CLEAR this | |
390 | X<CLEAR> | |
391 | ||
392 | Clear (remove, delete, ...) all values from the tied array associated with | |
393 | object I<this>. For example: | |
394 | ||
395 | sub CLEAR { | |
396 | my $self = shift; | |
397 | return $self->{ARRAY} = []; | |
398 | } | |
399 | ||
400 | =item PUSH this, LIST | |
401 | X<PUSH> | |
402 | ||
403 | Append elements of I<LIST> to the array. For example: | |
404 | ||
405 | sub PUSH { | |
406 | my $self = shift; | |
407 | my @list = @_; | |
408 | my $last = $self->FETCHSIZE(); | |
409 | $self->STORE( $last + $_, $list[$_] ) foreach 0 .. $#list; | |
410 | return $self->FETCHSIZE(); | |
411 | } | |
412 | ||
413 | =item POP this | |
414 | X<POP> | |
415 | ||
416 | Remove last element of the array and return it. For example: | |
417 | ||
418 | sub POP { | |
419 | my $self = shift; | |
420 | return pop @{$self->{ARRAY}}; | |
421 | } | |
422 | ||
423 | =item SHIFT this | |
424 | X<SHIFT> | |
425 | ||
426 | Remove the first element of the array (shifting other elements down) | |
427 | and return it. For example: | |
428 | ||
429 | sub SHIFT { | |
430 | my $self = shift; | |
431 | return shift @{$self->{ARRAY}}; | |
432 | } | |
433 | ||
434 | =item UNSHIFT this, LIST | |
435 | X<UNSHIFT> | |
436 | ||
437 | Insert LIST elements at the beginning of the array, moving existing elements | |
438 | up to make room. For example: | |
439 | ||
440 | sub UNSHIFT { | |
441 | my $self = shift; | |
442 | my @list = @_; | |
443 | my $size = scalar( @list ); | |
444 | # make room for our list | |
445 | @{$self->{ARRAY}}[ $size .. $#{$self->{ARRAY}} + $size ] | |
446 | = @{$self->{ARRAY}}; | |
447 | $self->STORE( $_, $list[$_] ) foreach 0 .. $#list; | |
448 | } | |
449 | ||
450 | =item SPLICE this, offset, length, LIST | |
451 | X<SPLICE> | |
452 | ||
453 | Perform the equivalent of C<splice> on the array. | |
454 | ||
455 | I<offset> is optional and defaults to zero, negative values count back | |
456 | from the end of the array. | |
457 | ||
458 | I<length> is optional and defaults to rest of the array. | |
459 | ||
460 | I<LIST> may be empty. | |
461 | ||
462 | Returns a list of the original I<length> elements at I<offset>. | |
463 | ||
464 | In our example, we'll use a little shortcut if there is a I<LIST>: | |
465 | ||
466 | sub SPLICE { | |
467 | my $self = shift; | |
468 | my $offset = shift || 0; | |
469 | my $length = shift || $self->FETCHSIZE() - $offset; | |
470 | my @list = (); | |
471 | if ( @_ ) { | |
472 | tie @list, __PACKAGE__, $self->{ELEMSIZE}; | |
473 | @list = @_; | |
474 | } | |
475 | return splice @{$self->{ARRAY}}, $offset, $length, @list; | |
476 | } | |
477 | ||
478 | =item UNTIE this | |
479 | X<UNTIE> | |
480 | ||
481 | Will be called when C<untie> happens. (See L<The C<untie> Gotcha> below.) | |
482 | ||
483 | =item DESTROY this | |
484 | X<DESTROY> | |
485 | ||
486 | This method will be triggered when the tied variable needs to be destructed. | |
487 | As with the scalar tie class, this is almost never needed in a | |
488 | language that does its own garbage collection, so this time we'll | |
489 | just leave it out. | |
490 | ||
491 | =back | |
492 | ||
493 | =head2 Tying Hashes | |
494 | X<hash, tying> | |
495 | ||
496 | Hashes were the first Perl data type to be tied (see dbmopen()). A class | |
497 | implementing a tied hash should define the following methods: TIEHASH is | |
498 | the constructor. FETCH and STORE access the key and value pairs. EXISTS | |
499 | reports whether a key is present in the hash, and DELETE deletes one. | |
500 | CLEAR empties the hash by deleting all the key and value pairs. FIRSTKEY | |
501 | and NEXTKEY implement the keys() and each() functions to iterate over all | |
502 | the keys. SCALAR is triggered when the tied hash is evaluated in scalar | |
503 | context. UNTIE is called when C<untie> happens, and DESTROY is called when | |
504 | the tied variable is garbage collected. | |
505 | ||
506 | If this seems like a lot, then feel free to inherit from merely the | |
507 | standard Tie::StdHash module for most of your methods, redefining only the | |
508 | interesting ones. See L<Tie::Hash> for details. | |
509 | ||
510 | Remember that Perl distinguishes between a key not existing in the hash, | |
511 | and the key existing in the hash but having a corresponding value of | |
512 | C<undef>. The two possibilities can be tested with the C<exists()> and | |
513 | C<defined()> functions. | |
514 | ||
515 | Here's an example of a somewhat interesting tied hash class: it gives you | |
516 | a hash representing a particular user's dot files. You index into the hash | |
517 | with the name of the file (minus the dot) and you get back that dot file's | |
518 | contents. For example: | |
519 | ||
520 | use DotFiles; | |
521 | tie %dot, 'DotFiles'; | |
522 | if ( $dot{profile} =~ /MANPATH/ || | |
523 | $dot{login} =~ /MANPATH/ || | |
524 | $dot{cshrc} =~ /MANPATH/ ) | |
525 | { | |
526 | print "you seem to set your MANPATH\n"; | |
527 | } | |
528 | ||
529 | Or here's another sample of using our tied class: | |
530 | ||
531 | tie %him, 'DotFiles', 'daemon'; | |
532 | foreach $f ( keys %him ) { | |
533 | printf "daemon dot file %s is size %d\n", | |
534 | $f, length $him{$f}; | |
535 | } | |
536 | ||
537 | In our tied hash DotFiles example, we use a regular | |
538 | hash for the object containing several important | |
539 | fields, of which only the C<{LIST}> field will be what the | |
540 | user thinks of as the real hash. | |
541 | ||
542 | =over 5 | |
543 | ||
544 | =item USER | |
545 | ||
546 | whose dot files this object represents | |
547 | ||
548 | =item HOME | |
549 | ||
550 | where those dot files live | |
551 | ||
552 | =item CLOBBER | |
553 | ||
554 | whether we should try to change or remove those dot files | |
555 | ||
556 | =item LIST | |
557 | ||
558 | the hash of dot file names and content mappings | |
559 | ||
560 | =back | |
561 | ||
562 | Here's the start of F<Dotfiles.pm>: | |
563 | ||
564 | package DotFiles; | |
565 | use Carp; | |
566 | sub whowasi { (caller(1))[3] . '()' } | |
567 | my $DEBUG = 0; | |
568 | sub debug { $DEBUG = @_ ? shift : 1 } | |
569 | ||
570 | For our example, we want to be able to emit debugging info to help in tracing | |
571 | during development. We keep also one convenience function around | |
572 | internally to help print out warnings; whowasi() returns the function name | |
573 | that calls it. | |
574 | ||
575 | Here are the methods for the DotFiles tied hash. | |
576 | ||
577 | =over 4 | |
578 | ||
579 | =item TIEHASH classname, LIST | |
580 | X<TIEHASH> | |
581 | ||
582 | This is the constructor for the class. That means it is expected to | |
583 | return a blessed reference through which the new object (probably but not | |
584 | necessarily an anonymous hash) will be accessed. | |
585 | ||
586 | Here's the constructor: | |
587 | ||
588 | sub TIEHASH { | |
589 | my $self = shift; | |
590 | my $user = shift || $>; | |
591 | my $dotdir = shift || ''; | |
592 | croak "usage: @{[&whowasi]} [USER [DOTDIR]]" if @_; | |
593 | $user = getpwuid($user) if $user =~ /^\d+$/; | |
594 | my $dir = (getpwnam($user))[7] | |
595 | || croak "@{[&whowasi]}: no user $user"; | |
596 | $dir .= "/$dotdir" if $dotdir; | |
597 | ||
598 | my $node = { | |
599 | USER => $user, | |
600 | HOME => $dir, | |
601 | LIST => {}, | |
602 | CLOBBER => 0, | |
603 | }; | |
604 | ||
605 | opendir(DIR, $dir) | |
606 | || croak "@{[&whowasi]}: can't opendir $dir: $!"; | |
607 | foreach $dot ( grep /^\./ && -f "$dir/$_", readdir(DIR)) { | |
608 | $dot =~ s/^\.//; | |
609 | $node->{LIST}{$dot} = undef; | |
610 | } | |
611 | closedir DIR; | |
612 | return bless $node, $self; | |
613 | } | |
614 | ||
615 | It's probably worth mentioning that if you're going to filetest the | |
616 | return values out of a readdir, you'd better prepend the directory | |
617 | in question. Otherwise, because we didn't chdir() there, it would | |
618 | have been testing the wrong file. | |
619 | ||
620 | =item FETCH this, key | |
621 | X<FETCH> | |
622 | ||
623 | This method will be triggered every time an element in the tied hash is | |
624 | accessed (read). It takes one argument beyond its self reference: the key | |
625 | whose value we're trying to fetch. | |
626 | ||
627 | Here's the fetch for our DotFiles example. | |
628 | ||
629 | sub FETCH { | |
630 | carp &whowasi if $DEBUG; | |
631 | my $self = shift; | |
632 | my $dot = shift; | |
633 | my $dir = $self->{HOME}; | |
634 | my $file = "$dir/.$dot"; | |
635 | ||
636 | unless (exists $self->{LIST}->{$dot} || -f $file) { | |
637 | carp "@{[&whowasi]}: no $dot file" if $DEBUG; | |
638 | return undef; | |
639 | } | |
640 | ||
641 | if (defined $self->{LIST}->{$dot}) { | |
642 | return $self->{LIST}->{$dot}; | |
643 | } else { | |
644 | return $self->{LIST}->{$dot} = `cat $dir/.$dot`; | |
645 | } | |
646 | } | |
647 | ||
648 | It was easy to write by having it call the Unix cat(1) command, but it | |
649 | would probably be more portable to open the file manually (and somewhat | |
650 | more efficient). Of course, because dot files are a Unixy concept, we're | |
651 | not that concerned. | |
652 | ||
653 | =item STORE this, key, value | |
654 | X<STORE> | |
655 | ||
656 | This method will be triggered every time an element in the tied hash is set | |
657 | (written). It takes two arguments beyond its self reference: the index at | |
658 | which we're trying to store something, and the value we're trying to put | |
659 | there. | |
660 | ||
661 | Here in our DotFiles example, we'll be careful not to let | |
662 | them try to overwrite the file unless they've called the clobber() | |
663 | method on the original object reference returned by tie(). | |
664 | ||
665 | sub STORE { | |
666 | carp &whowasi if $DEBUG; | |
667 | my $self = shift; | |
668 | my $dot = shift; | |
669 | my $value = shift; | |
670 | my $file = $self->{HOME} . "/.$dot"; | |
671 | my $user = $self->{USER}; | |
672 | ||
673 | croak "@{[&whowasi]}: $file not clobberable" | |
674 | unless $self->{CLOBBER}; | |
675 | ||
676 | open(F, "> $file") || croak "can't open $file: $!"; | |
677 | print F $value; | |
678 | close(F); | |
679 | } | |
680 | ||
681 | If they wanted to clobber something, they might say: | |
682 | ||
683 | $ob = tie %daemon_dots, 'daemon'; | |
684 | $ob->clobber(1); | |
685 | $daemon_dots{signature} = "A true daemon\n"; | |
686 | ||
687 | Another way to lay hands on a reference to the underlying object is to | |
688 | use the tied() function, so they might alternately have set clobber | |
689 | using: | |
690 | ||
691 | tie %daemon_dots, 'daemon'; | |
692 | tied(%daemon_dots)->clobber(1); | |
693 | ||
694 | The clobber method is simply: | |
695 | ||
696 | sub clobber { | |
697 | my $self = shift; | |
698 | $self->{CLOBBER} = @_ ? shift : 1; | |
699 | } | |
700 | ||
701 | =item DELETE this, key | |
702 | X<DELETE> | |
703 | ||
704 | This method is triggered when we remove an element from the hash, | |
705 | typically by using the delete() function. Again, we'll | |
706 | be careful to check whether they really want to clobber files. | |
707 | ||
708 | sub DELETE { | |
709 | carp &whowasi if $DEBUG; | |
710 | ||
711 | my $self = shift; | |
712 | my $dot = shift; | |
713 | my $file = $self->{HOME} . "/.$dot"; | |
714 | croak "@{[&whowasi]}: won't remove file $file" | |
715 | unless $self->{CLOBBER}; | |
716 | delete $self->{LIST}->{$dot}; | |
717 | my $success = unlink($file); | |
718 | carp "@{[&whowasi]}: can't unlink $file: $!" unless $success; | |
719 | $success; | |
720 | } | |
721 | ||
722 | The value returned by DELETE becomes the return value of the call | |
723 | to delete(). If you want to emulate the normal behavior of delete(), | |
724 | you should return whatever FETCH would have returned for this key. | |
725 | In this example, we have chosen instead to return a value which tells | |
726 | the caller whether the file was successfully deleted. | |
727 | ||
728 | =item CLEAR this | |
729 | X<CLEAR> | |
730 | ||
731 | This method is triggered when the whole hash is to be cleared, usually by | |
732 | assigning the empty list to it. | |
733 | ||
734 | In our example, that would remove all the user's dot files! It's such a | |
735 | dangerous thing that they'll have to set CLOBBER to something higher than | |
736 | 1 to make it happen. | |
737 | ||
738 | sub CLEAR { | |
739 | carp &whowasi if $DEBUG; | |
740 | my $self = shift; | |
741 | croak "@{[&whowasi]}: won't remove all dot files for $self->{USER}" | |
742 | unless $self->{CLOBBER} > 1; | |
743 | my $dot; | |
744 | foreach $dot ( keys %{$self->{LIST}}) { | |
745 | $self->DELETE($dot); | |
746 | } | |
747 | } | |
748 | ||
749 | =item EXISTS this, key | |
750 | X<EXISTS> | |
751 | ||
752 | This method is triggered when the user uses the exists() function | |
753 | on a particular hash. In our example, we'll look at the C<{LIST}> | |
754 | hash element for this: | |
755 | ||
756 | sub EXISTS { | |
757 | carp &whowasi if $DEBUG; | |
758 | my $self = shift; | |
759 | my $dot = shift; | |
760 | return exists $self->{LIST}->{$dot}; | |
761 | } | |
762 | ||
763 | =item FIRSTKEY this | |
764 | X<FIRSTKEY> | |
765 | ||
766 | This method will be triggered when the user is going | |
767 | to iterate through the hash, such as via a keys() or each() | |
768 | call. | |
769 | ||
770 | sub FIRSTKEY { | |
771 | carp &whowasi if $DEBUG; | |
772 | my $self = shift; | |
773 | my $a = keys %{$self->{LIST}}; # reset each() iterator | |
774 | each %{$self->{LIST}} | |
775 | } | |
776 | ||
777 | =item NEXTKEY this, lastkey | |
778 | X<NEXTKEY> | |
779 | ||
780 | This method gets triggered during a keys() or each() iteration. It has a | |
781 | second argument which is the last key that had been accessed. This is | |
782 | useful if you're carrying about ordering or calling the iterator from more | |
783 | than one sequence, or not really storing things in a hash anywhere. | |
784 | ||
785 | For our example, we're using a real hash so we'll do just the simple | |
786 | thing, but we'll have to go through the LIST field indirectly. | |
787 | ||
788 | sub NEXTKEY { | |
789 | carp &whowasi if $DEBUG; | |
790 | my $self = shift; | |
791 | return each %{ $self->{LIST} } | |
792 | } | |
793 | ||
794 | =item SCALAR this | |
795 | X<SCALAR> | |
796 | ||
797 | This is called when the hash is evaluated in scalar context. In order | |
798 | to mimic the behaviour of untied hashes, this method should return a | |
799 | false value when the tied hash is considered empty. If this method does | |
800 | not exist, perl will make some educated guesses and return true when | |
801 | the hash is inside an iteration. If this isn't the case, FIRSTKEY is | |
802 | called, and the result will be a false value if FIRSTKEY returns the empty | |
803 | list, true otherwise. | |
804 | ||
805 | However, you should B<not> blindly rely on perl always doing the right | |
806 | thing. Particularly, perl will mistakenly return true when you clear the | |
807 | hash by repeatedly calling DELETE until it is empty. You are therefore | |
808 | advised to supply your own SCALAR method when you want to be absolutely | |
809 | sure that your hash behaves nicely in scalar context. | |
810 | ||
811 | In our example we can just call C<scalar> on the underlying hash | |
812 | referenced by C<$self-E<gt>{LIST}>: | |
813 | ||
814 | sub SCALAR { | |
815 | carp &whowasi if $DEBUG; | |
816 | my $self = shift; | |
817 | return scalar %{ $self->{LIST} } | |
818 | } | |
819 | ||
820 | =item UNTIE this | |
821 | X<UNTIE> | |
822 | ||
823 | This is called when C<untie> occurs. See L<The C<untie> Gotcha> below. | |
824 | ||
825 | =item DESTROY this | |
826 | X<DESTROY> | |
827 | ||
828 | This method is triggered when a tied hash is about to go out of | |
829 | scope. You don't really need it unless you're trying to add debugging | |
830 | or have auxiliary state to clean up. Here's a very simple function: | |
831 | ||
832 | sub DESTROY { | |
833 | carp &whowasi if $DEBUG; | |
834 | } | |
835 | ||
836 | =back | |
837 | ||
838 | Note that functions such as keys() and values() may return huge lists | |
839 | when used on large objects, like DBM files. You may prefer to use the | |
840 | each() function to iterate over such. Example: | |
841 | ||
842 | # print out history file offsets | |
843 | use NDBM_File; | |
844 | tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0); | |
845 | while (($key,$val) = each %HIST) { | |
846 | print $key, ' = ', unpack('L',$val), "\n"; | |
847 | } | |
848 | untie(%HIST); | |
849 | ||
850 | =head2 Tying FileHandles | |
851 | X<filehandle, tying> | |
852 | ||
853 | This is partially implemented now. | |
854 | ||
855 | A class implementing a tied filehandle should define the following | |
856 | methods: TIEHANDLE, at least one of PRINT, PRINTF, WRITE, READLINE, GETC, | |
857 | READ, and possibly CLOSE, UNTIE and DESTROY. The class can also provide: BINMODE, | |
858 | OPEN, EOF, FILENO, SEEK, TELL - if the corresponding perl operators are | |
859 | used on the handle. | |
860 | ||
861 | When STDERR is tied, its PRINT method will be called to issue warnings | |
862 | and error messages. This feature is temporarily disabled during the call, | |
863 | which means you can use C<warn()> inside PRINT without starting a recursive | |
864 | loop. And just like C<__WARN__> and C<__DIE__> handlers, STDERR's PRINT | |
865 | method may be called to report parser errors, so the caveats mentioned under | |
866 | L<perlvar/%SIG> apply. | |
867 | ||
868 | All of this is especially useful when perl is embedded in some other | |
869 | program, where output to STDOUT and STDERR may have to be redirected | |
870 | in some special way. See nvi and the Apache module for examples. | |
871 | ||
872 | In our example we're going to create a shouting handle. | |
873 | ||
874 | package Shout; | |
875 | ||
876 | =over 4 | |
877 | ||
878 | =item TIEHANDLE classname, LIST | |
879 | X<TIEHANDLE> | |
880 | ||
881 | This is the constructor for the class. That means it is expected to | |
882 | return a blessed reference of some sort. The reference can be used to | |
883 | hold some internal information. | |
884 | ||
885 | sub TIEHANDLE { print "<shout>\n"; my $i; bless \$i, shift } | |
886 | ||
887 | =item WRITE this, LIST | |
888 | X<WRITE> | |
889 | ||
890 | This method will be called when the handle is written to via the | |
891 | C<syswrite> function. | |
892 | ||
893 | sub WRITE { | |
894 | $r = shift; | |
895 | my($buf,$len,$offset) = @_; | |
896 | print "WRITE called, \$buf=$buf, \$len=$len, \$offset=$offset"; | |
897 | } | |
898 | ||
899 | =item PRINT this, LIST | |
900 | X<PRINT> | |
901 | ||
902 | This method will be triggered every time the tied handle is printed to | |
903 | with the C<print()> function. | |
904 | Beyond its self reference it also expects the list that was passed to | |
905 | the print function. | |
906 | ||
907 | sub PRINT { $r = shift; $$r++; print join($,,map(uc($_),@_)),$\ } | |
908 | ||
909 | =item PRINTF this, LIST | |
910 | X<PRINTF> | |
911 | ||
912 | This method will be triggered every time the tied handle is printed to | |
913 | with the C<printf()> function. | |
914 | Beyond its self reference it also expects the format and list that was | |
915 | passed to the printf function. | |
916 | ||
917 | sub PRINTF { | |
918 | shift; | |
919 | my $fmt = shift; | |
920 | print sprintf($fmt, @_); | |
921 | } | |
922 | ||
923 | =item READ this, LIST | |
924 | X<READ> | |
925 | ||
926 | This method will be called when the handle is read from via the C<read> | |
927 | or C<sysread> functions. | |
928 | ||
929 | sub READ { | |
930 | my $self = shift; | |
931 | my $bufref = \$_[0]; | |
932 | my(undef,$len,$offset) = @_; | |
933 | print "READ called, \$buf=$bufref, \$len=$len, \$offset=$offset"; | |
934 | # add to $$bufref, set $len to number of characters read | |
935 | $len; | |
936 | } | |
937 | ||
938 | =item READLINE this | |
939 | X<READLINE> | |
940 | ||
941 | This method will be called when the handle is read from via <HANDLE>. | |
942 | The method should return undef when there is no more data. | |
943 | ||
944 | sub READLINE { $r = shift; "READLINE called $$r times\n"; } | |
945 | ||
946 | =item GETC this | |
947 | X<GETC> | |
948 | ||
949 | This method will be called when the C<getc> function is called. | |
950 | ||
951 | sub GETC { print "Don't GETC, Get Perl"; return "a"; } | |
952 | ||
953 | =item CLOSE this | |
954 | X<CLOSE> | |
955 | ||
956 | This method will be called when the handle is closed via the C<close> | |
957 | function. | |
958 | ||
959 | sub CLOSE { print "CLOSE called.\n" } | |
960 | ||
961 | =item UNTIE this | |
962 | X<UNTIE> | |
963 | ||
964 | As with the other types of ties, this method will be called when C<untie> happens. | |
965 | It may be appropriate to "auto CLOSE" when this occurs. See | |
966 | L<The C<untie> Gotcha> below. | |
967 | ||
968 | =item DESTROY this | |
969 | X<DESTROY> | |
970 | ||
971 | As with the other types of ties, this method will be called when the | |
972 | tied handle is about to be destroyed. This is useful for debugging and | |
973 | possibly cleaning up. | |
974 | ||
975 | sub DESTROY { print "</shout>\n" } | |
976 | ||
977 | =back | |
978 | ||
979 | Here's how to use our little example: | |
980 | ||
981 | tie(*FOO,'Shout'); | |
982 | print FOO "hello\n"; | |
983 | $a = 4; $b = 6; | |
984 | print FOO $a, " plus ", $b, " equals ", $a + $b, "\n"; | |
985 | print <FOO>; | |
986 | ||
987 | =head2 UNTIE this | |
988 | X<UNTIE> | |
989 | ||
990 | You can define for all tie types an UNTIE method that will be called | |
991 | at untie(). See L<The C<untie> Gotcha> below. | |
992 | ||
993 | =head2 The C<untie> Gotcha | |
994 | X<untie> | |
995 | ||
996 | If you intend making use of the object returned from either tie() or | |
997 | tied(), and if the tie's target class defines a destructor, there is a | |
998 | subtle gotcha you I<must> guard against. | |
999 | ||
1000 | As setup, consider this (admittedly rather contrived) example of a | |
1001 | tie; all it does is use a file to keep a log of the values assigned to | |
1002 | a scalar. | |
1003 | ||
1004 | package Remember; | |
1005 | ||
1006 | use strict; | |
1007 | use warnings; | |
1008 | use IO::File; | |
1009 | ||
1010 | sub TIESCALAR { | |
1011 | my $class = shift; | |
1012 | my $filename = shift; | |
1013 | my $handle = new IO::File "> $filename" | |
1014 | or die "Cannot open $filename: $!\n"; | |
1015 | ||
1016 | print $handle "The Start\n"; | |
1017 | bless {FH => $handle, Value => 0}, $class; | |
1018 | } | |
1019 | ||
1020 | sub FETCH { | |
1021 | my $self = shift; | |
1022 | return $self->{Value}; | |
1023 | } | |
1024 | ||
1025 | sub STORE { | |
1026 | my $self = shift; | |
1027 | my $value = shift; | |
1028 | my $handle = $self->{FH}; | |
1029 | print $handle "$value\n"; | |
1030 | $self->{Value} = $value; | |
1031 | } | |
1032 | ||
1033 | sub DESTROY { | |
1034 | my $self = shift; | |
1035 | my $handle = $self->{FH}; | |
1036 | print $handle "The End\n"; | |
1037 | close $handle; | |
1038 | } | |
1039 | ||
1040 | 1; | |
1041 | ||
1042 | Here is an example that makes use of this tie: | |
1043 | ||
1044 | use strict; | |
1045 | use Remember; | |
1046 | ||
1047 | my $fred; | |
1048 | tie $fred, 'Remember', 'myfile.txt'; | |
1049 | $fred = 1; | |
1050 | $fred = 4; | |
1051 | $fred = 5; | |
1052 | untie $fred; | |
1053 | system "cat myfile.txt"; | |
1054 | ||
1055 | This is the output when it is executed: | |
1056 | ||
1057 | The Start | |
1058 | 1 | |
1059 | 4 | |
1060 | 5 | |
1061 | The End | |
1062 | ||
1063 | So far so good. Those of you who have been paying attention will have | |
1064 | spotted that the tied object hasn't been used so far. So lets add an | |
1065 | extra method to the Remember class to allow comments to be included in | |
1066 | the file -- say, something like this: | |
1067 | ||
1068 | sub comment { | |
1069 | my $self = shift; | |
1070 | my $text = shift; | |
1071 | my $handle = $self->{FH}; | |
1072 | print $handle $text, "\n"; | |
1073 | } | |
1074 | ||
1075 | And here is the previous example modified to use the C<comment> method | |
1076 | (which requires the tied object): | |
1077 | ||
1078 | use strict; | |
1079 | use Remember; | |
1080 | ||
1081 | my ($fred, $x); | |
1082 | $x = tie $fred, 'Remember', 'myfile.txt'; | |
1083 | $fred = 1; | |
1084 | $fred = 4; | |
1085 | comment $x "changing..."; | |
1086 | $fred = 5; | |
1087 | untie $fred; | |
1088 | system "cat myfile.txt"; | |
1089 | ||
1090 | When this code is executed there is no output. Here's why: | |
1091 | ||
1092 | When a variable is tied, it is associated with the object which is the | |
1093 | return value of the TIESCALAR, TIEARRAY, or TIEHASH function. This | |
1094 | object normally has only one reference, namely, the implicit reference | |
1095 | from the tied variable. When untie() is called, that reference is | |
1096 | destroyed. Then, as in the first example above, the object's | |
1097 | destructor (DESTROY) is called, which is normal for objects that have | |
1098 | no more valid references; and thus the file is closed. | |
1099 | ||
1100 | In the second example, however, we have stored another reference to | |
1101 | the tied object in $x. That means that when untie() gets called | |
1102 | there will still be a valid reference to the object in existence, so | |
1103 | the destructor is not called at that time, and thus the file is not | |
1104 | closed. The reason there is no output is because the file buffers | |
1105 | have not been flushed to disk. | |
1106 | ||
1107 | Now that you know what the problem is, what can you do to avoid it? | |
1108 | Prior to the introduction of the optional UNTIE method the only way | |
1109 | was the good old C<-w> flag. Which will spot any instances where you call | |
1110 | untie() and there are still valid references to the tied object. If | |
1111 | the second script above this near the top C<use warnings 'untie'> | |
1112 | or was run with the C<-w> flag, Perl prints this | |
1113 | warning message: | |
1114 | ||
1115 | untie attempted while 1 inner references still exist | |
1116 | ||
1117 | To get the script to work properly and silence the warning make sure | |
1118 | there are no valid references to the tied object I<before> untie() is | |
1119 | called: | |
1120 | ||
1121 | undef $x; | |
1122 | untie $fred; | |
1123 | ||
1124 | Now that UNTIE exists the class designer can decide which parts of the | |
1125 | class functionality are really associated with C<untie> and which with | |
1126 | the object being destroyed. What makes sense for a given class depends | |
1127 | on whether the inner references are being kept so that non-tie-related | |
1128 | methods can be called on the object. But in most cases it probably makes | |
1129 | sense to move the functionality that would have been in DESTROY to the UNTIE | |
1130 | method. | |
1131 | ||
1132 | If the UNTIE method exists then the warning above does not occur. Instead the | |
1133 | UNTIE method is passed the count of "extra" references and can issue its own | |
1134 | warning if appropriate. e.g. to replicate the no UNTIE case this method can | |
1135 | be used: | |
1136 | ||
1137 | sub UNTIE | |
1138 | { | |
1139 | my ($obj,$count) = @_; | |
1140 | carp "untie attempted while $count inner references still exist" if $count; | |
1141 | } | |
1142 | ||
1143 | =head1 SEE ALSO | |
1144 | ||
1145 | See L<DB_File> or L<Config> for some interesting tie() implementations. | |
1146 | A good starting point for many tie() implementations is with one of the | |
1147 | modules L<Tie::Scalar>, L<Tie::Array>, L<Tie::Hash>, or L<Tie::Handle>. | |
1148 | ||
1149 | =head1 BUGS | |
1150 | ||
1151 | The bucket usage information provided by C<scalar(%hash)> is not | |
1152 | available. What this means is that using %tied_hash in boolean | |
1153 | context doesn't work right (currently this always tests false, | |
1154 | regardless of whether the hash is empty or hash elements). | |
1155 | ||
1156 | Localizing tied arrays or hashes does not work. After exiting the | |
1157 | scope the arrays or the hashes are not restored. | |
1158 | ||
1159 | Counting the number of entries in a hash via C<scalar(keys(%hash))> | |
1160 | or C<scalar(values(%hash)>) is inefficient since it needs to iterate | |
1161 | through all the entries with FIRSTKEY/NEXTKEY. | |
1162 | ||
1163 | Tied hash/array slices cause multiple FETCH/STORE pairs, there are no | |
1164 | tie methods for slice operations. | |
1165 | ||
1166 | You cannot easily tie a multilevel data structure (such as a hash of | |
1167 | hashes) to a dbm file. The first problem is that all but GDBM and | |
1168 | Berkeley DB have size limitations, but beyond that, you also have problems | |
1169 | with how references are to be represented on disk. One experimental | |
1170 | module that does attempt to address this need is DBM::Deep. Check your | |
1171 | nearest CPAN site as described in L<perlmodlib> for source code. Note | |
1172 | that despite its name, DBM::Deep does not use dbm. Another earlier attempt | |
1173 | at solving the problem is MLDBM, which is also available on the CPAN, but | |
1174 | which has some fairly serious limitations. | |
1175 | ||
1176 | Tied filehandles are still incomplete. sysopen(), truncate(), | |
1177 | flock(), fcntl(), stat() and -X can't currently be trapped. | |
1178 | ||
1179 | =head1 AUTHOR | |
1180 | ||
1181 | Tom Christiansen | |
1182 | ||
1183 | TIEHANDLE by Sven Verdoolaege <F<skimo@dns.ufsia.ac.be>> and Doug MacEachern <F<dougm@osf.org>> | |
1184 | ||
1185 | UNTIE by Nick Ing-Simmons <F<nick@ing-simmons.net>> | |
1186 | ||
1187 | SCALAR by Tassilo von Parseval <F<tassilo.von.parseval@rwth-aachen.de>> | |
1188 | ||
1189 | Tying Arrays by Casey West <F<casey@geeknest.com>> |