Commit | Line | Data |
cb1a09d0 |
1 | =head1 NAME |
2 | |
3 | perltie - how to hide an object class in a simple variable |
4 | |
5 | =head1 SYNOPSIS |
6 | |
7 | tie VARIABLE, CLASSNAME, LIST |
8 | |
6fdf61fb |
9 | $object = tied VARIABLE |
10 | |
cb1a09d0 |
11 | untie VARIABLE |
12 | |
13 | =head1 DESCRIPTION |
14 | |
15 | Prior to release 5.0 of Perl, a programmer could use dbmopen() |
5f05dabc |
16 | to connect an on-disk database in the standard Unix dbm(3x) |
17 | format magically to a %HASH in their program. However, their Perl was either |
cb1a09d0 |
18 | built with one particular dbm library or another, but not both, and |
19 | you couldn't extend this mechanism to other packages or types of variables. |
20 | |
21 | Now you can. |
22 | |
23 | The tie() function binds a variable to a class (package) that will provide |
24 | the implementation for access methods for that variable. Once this magic |
25 | has been performed, accessing a tied variable automatically triggers |
26 | method calls in the proper class. All of the complexity of the class is |
27 | hidden behind magic methods calls. The method names are in ALL CAPS, |
28 | which is a convention that Perl uses to indicate that they're called |
29 | implicitly rather than explicitly--just like the BEGIN() and END() |
30 | functions. |
31 | |
32 | In the tie() call, C<VARIABLE> is the name of the variable to be |
33 | enchanted. C<CLASSNAME> is the name of a class implementing objects of |
34 | the correct type. Any additional arguments in the C<LIST> are passed to |
35 | the appropriate constructor method for that class--meaning TIESCALAR(), |
5f05dabc |
36 | TIEARRAY(), TIEHASH(), or TIEHANDLE(). (Typically these are arguments |
a7adf1f0 |
37 | such as might be passed to the dbminit() function of C.) The object |
38 | returned by the "new" method is also returned by the tie() function, |
39 | which would be useful if you wanted to access other methods in |
40 | C<CLASSNAME>. (You don't actually have to return a reference to a right |
5f05dabc |
41 | "type" (e.g., HASH or C<CLASSNAME>) so long as it's a properly blessed |
a7adf1f0 |
42 | object.) You can also retrieve a reference to the underlying object |
43 | using the tied() function. |
cb1a09d0 |
44 | |
45 | Unlike dbmopen(), the tie() function will not C<use> or C<require> a module |
46 | for you--you need to do that explicitly yourself. |
47 | |
48 | =head2 Tying Scalars |
49 | |
50 | A class implementing a tied scalar should define the following methods: |
51 | TIESCALAR, FETCH, STORE, and possibly DESTROY. |
52 | |
53 | Let's look at each in turn, using as an example a tie class for |
54 | scalars that allows the user to do something like: |
55 | |
56 | tie $his_speed, 'Nice', getppid(); |
57 | tie $my_speed, 'Nice', $$; |
58 | |
59 | And now whenever either of those variables is accessed, its current |
60 | system priority is retrieved and returned. If those variables are set, |
61 | then the process's priority is changed! |
62 | |
5aabfad6 |
63 | We'll use Jarkko Hietaniemi <F<jhi@iki.fi>>'s BSD::Resource class (not |
64 | included) to access the PRIO_PROCESS, PRIO_MIN, and PRIO_MAX constants |
65 | from your system, as well as the getpriority() and setpriority() system |
66 | calls. Here's the preamble of the class. |
cb1a09d0 |
67 | |
68 | package Nice; |
69 | use Carp; |
70 | use BSD::Resource; |
71 | use strict; |
72 | $Nice::DEBUG = 0 unless defined $Nice::DEBUG; |
73 | |
74 | =over |
75 | |
76 | =item TIESCALAR classname, LIST |
77 | |
78 | This is the constructor for the class. That means it is |
79 | expected to return a blessed reference to a new scalar |
80 | (probably anonymous) that it's creating. For example: |
81 | |
82 | sub TIESCALAR { |
83 | my $class = shift; |
84 | my $pid = shift || $$; # 0 means me |
85 | |
86 | if ($pid !~ /^\d+$/) { |
6fdf61fb |
87 | carp "Nice::Tie::Scalar got non-numeric pid $pid" if $^W; |
cb1a09d0 |
88 | return undef; |
89 | } |
90 | |
91 | unless (kill 0, $pid) { # EPERM or ERSCH, no doubt |
6fdf61fb |
92 | carp "Nice::Tie::Scalar got bad pid $pid: $!" if $^W; |
cb1a09d0 |
93 | return undef; |
94 | } |
95 | |
96 | return bless \$pid, $class; |
97 | } |
98 | |
99 | This tie class has chosen to return an error rather than raising an |
100 | exception if its constructor should fail. While this is how dbmopen() works, |
101 | other classes may well not wish to be so forgiving. It checks the global |
102 | variable C<$^W> to see whether to emit a bit of noise anyway. |
103 | |
104 | =item FETCH this |
105 | |
106 | This method will be triggered every time the tied variable is accessed |
107 | (read). It takes no arguments beyond its self reference, which is the |
5f05dabc |
108 | object representing the scalar we're dealing with. Because in this case |
109 | we're using just a SCALAR ref for the tied scalar object, a simple $$self |
cb1a09d0 |
110 | allows the method to get at the real value stored there. In our example |
111 | below, that real value is the process ID to which we've tied our variable. |
112 | |
113 | sub FETCH { |
114 | my $self = shift; |
115 | confess "wrong type" unless ref $self; |
116 | croak "usage error" if @_; |
117 | my $nicety; |
118 | local($!) = 0; |
119 | $nicety = getpriority(PRIO_PROCESS, $$self); |
120 | if ($!) { croak "getpriority failed: $!" } |
121 | return $nicety; |
122 | } |
123 | |
124 | This time we've decided to blow up (raise an exception) if the renice |
125 | fails--there's no place for us to return an error otherwise, and it's |
126 | probably the right thing to do. |
127 | |
128 | =item STORE this, value |
129 | |
130 | This method will be triggered every time the tied variable is set |
131 | (assigned). Beyond its self reference, it also expects one (and only one) |
132 | argument--the new value the user is trying to assign. |
133 | |
134 | sub STORE { |
135 | my $self = shift; |
136 | confess "wrong type" unless ref $self; |
137 | my $new_nicety = shift; |
138 | croak "usage error" if @_; |
139 | |
140 | if ($new_nicety < PRIO_MIN) { |
141 | carp sprintf |
142 | "WARNING: priority %d less than minimum system priority %d", |
143 | $new_nicety, PRIO_MIN if $^W; |
144 | $new_nicety = PRIO_MIN; |
145 | } |
146 | |
147 | if ($new_nicety > PRIO_MAX) { |
148 | carp sprintf |
149 | "WARNING: priority %d greater than maximum system priority %d", |
150 | $new_nicety, PRIO_MAX if $^W; |
151 | $new_nicety = PRIO_MAX; |
152 | } |
153 | |
154 | unless (defined setpriority(PRIO_PROCESS, $$self, $new_nicety)) { |
155 | confess "setpriority failed: $!"; |
156 | } |
157 | return $new_nicety; |
158 | } |
159 | |
160 | =item DESTROY this |
161 | |
162 | This method will be triggered when the tied variable needs to be destructed. |
5f05dabc |
163 | As with other object classes, such a method is seldom necessary, because Perl |
cb1a09d0 |
164 | deallocates its moribund object's memory for you automatically--this isn't |
165 | C++, you know. We'll use a DESTROY method here for debugging purposes only. |
166 | |
167 | sub DESTROY { |
168 | my $self = shift; |
169 | confess "wrong type" unless ref $self; |
170 | carp "[ Nice::DESTROY pid $$self ]" if $Nice::DEBUG; |
171 | } |
172 | |
173 | =back |
174 | |
175 | That's about all there is to it. Actually, it's more than all there |
5f05dabc |
176 | is to it, because we've done a few nice things here for the sake |
cb1a09d0 |
177 | of completeness, robustness, and general aesthetics. Simpler |
178 | TIESCALAR classes are certainly possible. |
179 | |
180 | =head2 Tying Arrays |
181 | |
182 | A class implementing a tied ordinary array should define the following |
183 | methods: TIEARRAY, FETCH, STORE, and perhaps DESTROY. |
184 | |
185 | B<WARNING>: Tied arrays are I<incomplete>. They are also distinctly lacking |
186 | something for the C<$#ARRAY> access (which is hard, as it's an lvalue), as |
187 | well as the other obvious array functions, like push(), pop(), shift(), |
188 | unshift(), and splice(). |
189 | |
190 | For this discussion, we'll implement an array whose indices are fixed at |
191 | its creation. If you try to access anything beyond those bounds, you'll |
192 | take an exception. (Well, if you access an individual element; an |
193 | aggregate assignment would be missed.) For example: |
194 | |
195 | require Bounded_Array; |
1f57c600 |
196 | tie @ary, 'Bounded_Array', 2; |
cb1a09d0 |
197 | $| = 1; |
198 | for $i (0 .. 10) { |
199 | print "setting index $i: "; |
200 | $ary[$i] = 10 * $i; |
201 | $ary[$i] = 10 * $i; |
202 | print "value of elt $i now $ary[$i]\n"; |
203 | } |
204 | |
205 | The preamble code for the class is as follows: |
206 | |
207 | package Bounded_Array; |
208 | use Carp; |
209 | use strict; |
210 | |
211 | =over |
212 | |
213 | =item TIEARRAY classname, LIST |
214 | |
215 | This is the constructor for the class. That means it is expected to |
216 | return a blessed reference through which the new array (probably an |
217 | anonymous ARRAY ref) will be accessed. |
218 | |
219 | In our example, just to show you that you don't I<really> have to return an |
220 | ARRAY reference, we'll choose a HASH reference to represent our object. |
221 | A HASH works out well as a generic record type: the C<{BOUND}> field will |
03dc9dad |
222 | store the maximum bound allowed, and the C<{ARRAY}> field will hold the |
cb1a09d0 |
223 | true ARRAY ref. If someone outside the class tries to dereference the |
224 | object returned (doubtless thinking it an ARRAY ref), they'll blow up. |
225 | This just goes to show you that you should respect an object's privacy. |
226 | |
227 | sub TIEARRAY { |
228 | my $class = shift; |
229 | my $bound = shift; |
230 | confess "usage: tie(\@ary, 'Bounded_Array', max_subscript)" |
231 | if @_ || $bound =~ /\D/; |
232 | return bless { |
233 | BOUND => $bound, |
234 | ARRAY => [], |
235 | }, $class; |
236 | } |
237 | |
238 | =item FETCH this, index |
239 | |
240 | This method will be triggered every time an individual element the tied array |
241 | is accessed (read). It takes one argument beyond its self reference: the |
242 | index whose value we're trying to fetch. |
243 | |
244 | sub FETCH { |
245 | my($self,$idx) = @_; |
246 | if ($idx > $self->{BOUND}) { |
247 | confess "Array OOB: $idx > $self->{BOUND}"; |
248 | } |
249 | return $self->{ARRAY}[$idx]; |
250 | } |
251 | |
252 | As you may have noticed, the name of the FETCH method (et al.) is the same |
253 | for all accesses, even though the constructors differ in names (TIESCALAR |
254 | vs TIEARRAY). While in theory you could have the same class servicing |
255 | several tied types, in practice this becomes cumbersome, and it's easiest |
5f05dabc |
256 | to keep them at simply one tie type per class. |
cb1a09d0 |
257 | |
258 | =item STORE this, index, value |
259 | |
260 | This method will be triggered every time an element in the tied array is set |
261 | (written). It takes two arguments beyond its self reference: the index at |
262 | which we're trying to store something and the value we're trying to put |
263 | there. For example: |
264 | |
265 | sub STORE { |
266 | my($self, $idx, $value) = @_; |
267 | print "[STORE $value at $idx]\n" if _debug; |
268 | if ($idx > $self->{BOUND} ) { |
269 | confess "Array OOB: $idx > $self->{BOUND}"; |
270 | } |
271 | return $self->{ARRAY}[$idx] = $value; |
272 | } |
273 | |
274 | =item DESTROY this |
275 | |
276 | This method will be triggered when the tied variable needs to be destructed. |
184e9718 |
277 | As with the scalar tie class, this is almost never needed in a |
cb1a09d0 |
278 | language that does its own garbage collection, so this time we'll |
279 | just leave it out. |
280 | |
281 | =back |
282 | |
283 | The code we presented at the top of the tied array class accesses many |
284 | elements of the array, far more than we've set the bounds to. Therefore, |
285 | it will blow up once they try to access beyond the 2nd element of @ary, as |
286 | the following output demonstrates: |
287 | |
288 | setting index 0: value of elt 0 now 0 |
289 | setting index 1: value of elt 1 now 10 |
290 | setting index 2: value of elt 2 now 20 |
291 | setting index 3: Array OOB: 3 > 2 at Bounded_Array.pm line 39 |
292 | Bounded_Array::FETCH called at testba line 12 |
293 | |
294 | =head2 Tying Hashes |
295 | |
aa689395 |
296 | As the first Perl data type to be tied (see dbmopen()), hashes have the |
297 | most complete and useful tie() implementation. A class implementing a |
298 | tied hash should define the following methods: TIEHASH is the constructor. |
299 | FETCH and STORE access the key and value pairs. EXISTS reports whether a |
300 | key is present in the hash, and DELETE deletes one. CLEAR empties the |
301 | hash by deleting all the key and value pairs. FIRSTKEY and NEXTKEY |
302 | implement the keys() and each() functions to iterate over all the keys. |
303 | And DESTROY is called when the tied variable is garbage collected. |
304 | |
305 | If this seems like a lot, then feel free to inherit from merely the |
306 | standard Tie::Hash module for most of your methods, redefining only the |
307 | interesting ones. See L<Tie::Hash> for details. |
cb1a09d0 |
308 | |
309 | Remember that Perl distinguishes between a key not existing in the hash, |
310 | and the key existing in the hash but having a corresponding value of |
311 | C<undef>. The two possibilities can be tested with the C<exists()> and |
312 | C<defined()> functions. |
313 | |
314 | Here's an example of a somewhat interesting tied hash class: it gives you |
5f05dabc |
315 | a hash representing a particular user's dot files. You index into the hash |
316 | with the name of the file (minus the dot) and you get back that dot file's |
cb1a09d0 |
317 | contents. For example: |
318 | |
319 | use DotFiles; |
1f57c600 |
320 | tie %dot, 'DotFiles'; |
cb1a09d0 |
321 | if ( $dot{profile} =~ /MANPATH/ || |
322 | $dot{login} =~ /MANPATH/ || |
323 | $dot{cshrc} =~ /MANPATH/ ) |
324 | { |
5f05dabc |
325 | print "you seem to set your MANPATH\n"; |
cb1a09d0 |
326 | } |
327 | |
328 | Or here's another sample of using our tied class: |
329 | |
1f57c600 |
330 | tie %him, 'DotFiles', 'daemon'; |
cb1a09d0 |
331 | foreach $f ( keys %him ) { |
332 | printf "daemon dot file %s is size %d\n", |
333 | $f, length $him{$f}; |
334 | } |
335 | |
336 | In our tied hash DotFiles example, we use a regular |
337 | hash for the object containing several important |
338 | fields, of which only the C<{LIST}> field will be what the |
339 | user thinks of as the real hash. |
340 | |
341 | =over 5 |
342 | |
343 | =item USER |
344 | |
345 | whose dot files this object represents |
346 | |
347 | =item HOME |
348 | |
5f05dabc |
349 | where those dot files live |
cb1a09d0 |
350 | |
351 | =item CLOBBER |
352 | |
353 | whether we should try to change or remove those dot files |
354 | |
355 | =item LIST |
356 | |
5f05dabc |
357 | the hash of dot file names and content mappings |
cb1a09d0 |
358 | |
359 | =back |
360 | |
361 | Here's the start of F<Dotfiles.pm>: |
362 | |
363 | package DotFiles; |
364 | use Carp; |
365 | sub whowasi { (caller(1))[3] . '()' } |
366 | my $DEBUG = 0; |
367 | sub debug { $DEBUG = @_ ? shift : 1 } |
368 | |
5f05dabc |
369 | For our example, we want to be able to emit debugging info to help in tracing |
cb1a09d0 |
370 | during development. We keep also one convenience function around |
371 | internally to help print out warnings; whowasi() returns the function name |
372 | that calls it. |
373 | |
374 | Here are the methods for the DotFiles tied hash. |
375 | |
376 | =over |
377 | |
378 | =item TIEHASH classname, LIST |
379 | |
380 | This is the constructor for the class. That means it is expected to |
381 | return a blessed reference through which the new object (probably but not |
382 | necessarily an anonymous hash) will be accessed. |
383 | |
384 | Here's the constructor: |
385 | |
386 | sub TIEHASH { |
387 | my $self = shift; |
388 | my $user = shift || $>; |
389 | my $dotdir = shift || ''; |
390 | croak "usage: @{[&whowasi]} [USER [DOTDIR]]" if @_; |
391 | $user = getpwuid($user) if $user =~ /^\d+$/; |
392 | my $dir = (getpwnam($user))[7] |
393 | || croak "@{[&whowasi]}: no user $user"; |
394 | $dir .= "/$dotdir" if $dotdir; |
395 | |
396 | my $node = { |
397 | USER => $user, |
398 | HOME => $dir, |
399 | LIST => {}, |
400 | CLOBBER => 0, |
401 | }; |
402 | |
403 | opendir(DIR, $dir) |
404 | || croak "@{[&whowasi]}: can't opendir $dir: $!"; |
405 | foreach $dot ( grep /^\./ && -f "$dir/$_", readdir(DIR)) { |
406 | $dot =~ s/^\.//; |
407 | $node->{LIST}{$dot} = undef; |
408 | } |
409 | closedir DIR; |
410 | return bless $node, $self; |
411 | } |
412 | |
413 | It's probably worth mentioning that if you're going to filetest the |
414 | return values out of a readdir, you'd better prepend the directory |
5f05dabc |
415 | in question. Otherwise, because we didn't chdir() there, it would |
2ae324a7 |
416 | have been testing the wrong file. |
cb1a09d0 |
417 | |
418 | =item FETCH this, key |
419 | |
420 | This method will be triggered every time an element in the tied hash is |
421 | accessed (read). It takes one argument beyond its self reference: the key |
422 | whose value we're trying to fetch. |
423 | |
424 | Here's the fetch for our DotFiles example. |
425 | |
426 | sub FETCH { |
427 | carp &whowasi if $DEBUG; |
428 | my $self = shift; |
429 | my $dot = shift; |
430 | my $dir = $self->{HOME}; |
431 | my $file = "$dir/.$dot"; |
432 | |
433 | unless (exists $self->{LIST}->{$dot} || -f $file) { |
434 | carp "@{[&whowasi]}: no $dot file" if $DEBUG; |
435 | return undef; |
436 | } |
437 | |
438 | if (defined $self->{LIST}->{$dot}) { |
439 | return $self->{LIST}->{$dot}; |
440 | } else { |
441 | return $self->{LIST}->{$dot} = `cat $dir/.$dot`; |
442 | } |
443 | } |
444 | |
445 | It was easy to write by having it call the Unix cat(1) command, but it |
446 | would probably be more portable to open the file manually (and somewhat |
5f05dabc |
447 | more efficient). Of course, because dot files are a Unixy concept, we're |
cb1a09d0 |
448 | not that concerned. |
449 | |
450 | =item STORE this, key, value |
451 | |
452 | This method will be triggered every time an element in the tied hash is set |
453 | (written). It takes two arguments beyond its self reference: the index at |
454 | which we're trying to store something, and the value we're trying to put |
455 | there. |
456 | |
457 | Here in our DotFiles example, we'll be careful not to let |
458 | them try to overwrite the file unless they've called the clobber() |
459 | method on the original object reference returned by tie(). |
460 | |
461 | sub STORE { |
462 | carp &whowasi if $DEBUG; |
463 | my $self = shift; |
464 | my $dot = shift; |
465 | my $value = shift; |
466 | my $file = $self->{HOME} . "/.$dot"; |
467 | my $user = $self->{USER}; |
468 | |
469 | croak "@{[&whowasi]}: $file not clobberable" |
470 | unless $self->{CLOBBER}; |
471 | |
472 | open(F, "> $file") || croak "can't open $file: $!"; |
473 | print F $value; |
474 | close(F); |
475 | } |
476 | |
477 | If they wanted to clobber something, they might say: |
478 | |
479 | $ob = tie %daemon_dots, 'daemon'; |
480 | $ob->clobber(1); |
481 | $daemon_dots{signature} = "A true daemon\n"; |
482 | |
6fdf61fb |
483 | Another way to lay hands on a reference to the underlying object is to |
484 | use the tied() function, so they might alternately have set clobber |
485 | using: |
486 | |
487 | tie %daemon_dots, 'daemon'; |
488 | tied(%daemon_dots)->clobber(1); |
489 | |
490 | The clobber method is simply: |
cb1a09d0 |
491 | |
492 | sub clobber { |
493 | my $self = shift; |
494 | $self->{CLOBBER} = @_ ? shift : 1; |
495 | } |
496 | |
497 | =item DELETE this, key |
498 | |
499 | This method is triggered when we remove an element from the hash, |
500 | typically by using the delete() function. Again, we'll |
501 | be careful to check whether they really want to clobber files. |
502 | |
503 | sub DELETE { |
504 | carp &whowasi if $DEBUG; |
505 | |
506 | my $self = shift; |
507 | my $dot = shift; |
508 | my $file = $self->{HOME} . "/.$dot"; |
509 | croak "@{[&whowasi]}: won't remove file $file" |
510 | unless $self->{CLOBBER}; |
511 | delete $self->{LIST}->{$dot}; |
1f57c600 |
512 | my $success = unlink($file); |
513 | carp "@{[&whowasi]}: can't unlink $file: $!" unless $success; |
514 | $success; |
cb1a09d0 |
515 | } |
516 | |
1f57c600 |
517 | The value returned by DELETE becomes the return value of the call |
518 | to delete(). If you want to emulate the normal behavior of delete(), |
519 | you should return whatever FETCH would have returned for this key. |
520 | In this example, we have chosen instead to return a value which tells |
521 | the caller whether the file was successfully deleted. |
522 | |
cb1a09d0 |
523 | =item CLEAR this |
524 | |
525 | This method is triggered when the whole hash is to be cleared, usually by |
526 | assigning the empty list to it. |
527 | |
5f05dabc |
528 | In our example, that would remove all the user's dot files! It's such a |
cb1a09d0 |
529 | dangerous thing that they'll have to set CLOBBER to something higher than |
530 | 1 to make it happen. |
531 | |
532 | sub CLEAR { |
533 | carp &whowasi if $DEBUG; |
534 | my $self = shift; |
5f05dabc |
535 | croak "@{[&whowasi]}: won't remove all dot files for $self->{USER}" |
cb1a09d0 |
536 | unless $self->{CLOBBER} > 1; |
537 | my $dot; |
538 | foreach $dot ( keys %{$self->{LIST}}) { |
539 | $self->DELETE($dot); |
540 | } |
541 | } |
542 | |
543 | =item EXISTS this, key |
544 | |
545 | This method is triggered when the user uses the exists() function |
546 | on a particular hash. In our example, we'll look at the C<{LIST}> |
547 | hash element for this: |
548 | |
549 | sub EXISTS { |
550 | carp &whowasi if $DEBUG; |
551 | my $self = shift; |
552 | my $dot = shift; |
553 | return exists $self->{LIST}->{$dot}; |
554 | } |
555 | |
556 | =item FIRSTKEY this |
557 | |
558 | This method will be triggered when the user is going |
559 | to iterate through the hash, such as via a keys() or each() |
560 | call. |
561 | |
562 | sub FIRSTKEY { |
563 | carp &whowasi if $DEBUG; |
564 | my $self = shift; |
6fdf61fb |
565 | my $a = keys %{$self->{LIST}}; # reset each() iterator |
cb1a09d0 |
566 | each %{$self->{LIST}} |
567 | } |
568 | |
569 | =item NEXTKEY this, lastkey |
570 | |
571 | This method gets triggered during a keys() or each() iteration. It has a |
572 | second argument which is the last key that had been accessed. This is |
573 | useful if you're carrying about ordering or calling the iterator from more |
574 | than one sequence, or not really storing things in a hash anywhere. |
575 | |
5f05dabc |
576 | For our example, we're using a real hash so we'll do just the simple |
577 | thing, but we'll have to go through the LIST field indirectly. |
cb1a09d0 |
578 | |
579 | sub NEXTKEY { |
580 | carp &whowasi if $DEBUG; |
581 | my $self = shift; |
582 | return each %{ $self->{LIST} } |
583 | } |
584 | |
585 | =item DESTROY this |
586 | |
587 | This method is triggered when a tied hash is about to go out of |
588 | scope. You don't really need it unless you're trying to add debugging |
589 | or have auxiliary state to clean up. Here's a very simple function: |
590 | |
591 | sub DESTROY { |
592 | carp &whowasi if $DEBUG; |
593 | } |
594 | |
595 | =back |
596 | |
597 | Note that functions such as keys() and values() may return huge array |
598 | values when used on large objects, like DBM files. You may prefer to |
599 | use the each() function to iterate over such. Example: |
600 | |
601 | # print out history file offsets |
602 | use NDBM_File; |
1f57c600 |
603 | tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0); |
cb1a09d0 |
604 | while (($key,$val) = each %HIST) { |
605 | print $key, ' = ', unpack('L',$val), "\n"; |
606 | } |
607 | untie(%HIST); |
608 | |
609 | =head2 Tying FileHandles |
610 | |
184e9718 |
611 | This is partially implemented now. |
a7adf1f0 |
612 | |
2ae324a7 |
613 | A class implementing a tied filehandle should define the following |
46fc3d4c |
614 | methods: TIEHANDLE, at least one of PRINT, PRINTF, READLINE, GETC, or READ, |
2ae324a7 |
615 | and possibly DESTROY. |
a7adf1f0 |
616 | |
617 | It is especially useful when perl is embedded in some other program, |
618 | where output to STDOUT and STDERR may have to be redirected in some |
619 | special way. See nvi and the Apache module for examples. |
620 | |
621 | In our example we're going to create a shouting handle. |
622 | |
623 | package Shout; |
624 | |
625 | =over |
626 | |
627 | =item TIEHANDLE classname, LIST |
628 | |
629 | This is the constructor for the class. That means it is expected to |
184e9718 |
630 | return a blessed reference of some sort. The reference can be used to |
5f05dabc |
631 | hold some internal information. |
a7adf1f0 |
632 | |
7e1af8bc |
633 | sub TIEHANDLE { print "<shout>\n"; my $i; bless \$i, shift } |
a7adf1f0 |
634 | |
635 | =item PRINT this, LIST |
636 | |
46fc3d4c |
637 | This method will be triggered every time the tied handle is printed to |
638 | with the C<print()> function. |
184e9718 |
639 | Beyond its self reference it also expects the list that was passed to |
a7adf1f0 |
640 | the print function. |
641 | |
58f51617 |
642 | sub PRINT { $r = shift; $$r++; print join($,,map(uc($_),@_)),$\ } |
643 | |
46fc3d4c |
644 | =item PRINTF this, LIST |
645 | |
646 | This method will be triggered every time the tied handle is printed to |
647 | with the C<printf()> function. |
648 | Beyond its self reference it also expects the format and list that was |
649 | passed to the printf function. |
650 | |
651 | sub PRINTF { |
652 | shift; |
653 | my $fmt = shift; |
654 | print sprintf($fmt, @_)."\n"; |
655 | } |
656 | |
2ae324a7 |
657 | =item READ this LIST |
658 | |
659 | This method will be called when the handle is read from via the C<read> |
660 | or C<sysread> functions. |
661 | |
662 | sub READ { |
663 | $r = shift; |
664 | my($buf,$len,$offset) = @_; |
665 | print "READ called, \$buf=$buf, \$len=$len, \$offset=$offset"; |
666 | } |
667 | |
58f51617 |
668 | =item READLINE this |
669 | |
2ae324a7 |
670 | This method will be called when the handle is read from via <HANDLE>. |
671 | The method should return undef when there is no more data. |
58f51617 |
672 | |
673 | sub READLINE { $r = shift; "PRINT called $$r times\n"; } |
a7adf1f0 |
674 | |
2ae324a7 |
675 | =item GETC this |
676 | |
677 | This method will be called when the C<getc> function is called. |
678 | |
679 | sub GETC { print "Don't GETC, Get Perl"; return "a"; } |
680 | |
a7adf1f0 |
681 | =item DESTROY this |
682 | |
683 | As with the other types of ties, this method will be called when the |
684 | tied handle is about to be destroyed. This is useful for debugging and |
685 | possibly cleaning up. |
686 | |
687 | sub DESTROY { print "</shout>\n" } |
688 | |
689 | =back |
690 | |
691 | Here's how to use our little example: |
692 | |
693 | tie(*FOO,'Shout'); |
694 | print FOO "hello\n"; |
695 | $a = 4; $b = 6; |
696 | print FOO $a, " plus ", $b, " equals ", $a + $b, "\n"; |
58f51617 |
697 | print <FOO>; |
cb1a09d0 |
698 | |
2752eb9f |
699 | =head2 The C<untie> Gotcha |
700 | |
701 | If you intend making use of the object returned from either tie() or |
702 | tied(), and if the tie's target class defines a destructor, there is a |
703 | subtle gotcha you I<must> guard against. |
704 | |
705 | As setup, consider this (admittedly rather contrived) example of a |
706 | tie; all it does is use a file to keep a log of the values assigned to |
707 | a scalar. |
708 | |
709 | package Remember; |
710 | |
711 | use strict; |
712 | use IO::File; |
713 | |
714 | sub TIESCALAR { |
715 | my $class = shift; |
716 | my $filename = shift; |
717 | my $handle = new IO::File "> $filename" |
718 | or die "Cannot open $filename: $!\n"; |
719 | |
720 | print $handle "The Start\n"; |
721 | bless {FH => $handle, Value => 0}, $class; |
722 | } |
723 | |
724 | sub FETCH { |
725 | my $self = shift; |
726 | return $self->{Value}; |
727 | } |
728 | |
729 | sub STORE { |
730 | my $self = shift; |
731 | my $value = shift; |
732 | my $handle = $self->{FH}; |
733 | print $handle "$value\n"; |
734 | $self->{Value} = $value; |
735 | } |
736 | |
737 | sub DESTROY { |
738 | my $self = shift; |
739 | my $handle = $self->{FH}; |
740 | print $handle "The End\n"; |
741 | close $handle; |
742 | } |
743 | |
744 | 1; |
745 | |
746 | Here is an example that makes use of this tie: |
747 | |
748 | use strict; |
749 | use Remember; |
750 | |
751 | my $fred; |
752 | tie $fred, 'Remember', 'myfile.txt'; |
753 | $fred = 1; |
754 | $fred = 4; |
755 | $fred = 5; |
756 | untie $fred; |
757 | system "cat myfile.txt"; |
758 | |
759 | This is the output when it is executed: |
760 | |
761 | The Start |
762 | 1 |
763 | 4 |
764 | 5 |
765 | The End |
766 | |
767 | So far so good. Those of you who have been paying attention will have |
768 | spotted that the tied object hasn't been used so far. So lets add an |
769 | extra method to the Remember class to allow comments to be included in |
770 | the file -- say, something like this: |
771 | |
772 | sub comment { |
773 | my $self = shift; |
774 | my $text = shift; |
775 | my $handle = $self->{FH}; |
776 | print $handle $text, "\n"; |
777 | } |
778 | |
779 | And here is the previous example modified to use the C<comment> method |
780 | (which requires the tied object): |
781 | |
782 | use strict; |
783 | use Remember; |
784 | |
785 | my ($fred, $x); |
786 | $x = tie $fred, 'Remember', 'myfile.txt'; |
787 | $fred = 1; |
788 | $fred = 4; |
789 | comment $x "changing..."; |
790 | $fred = 5; |
791 | untie $fred; |
792 | system "cat myfile.txt"; |
793 | |
794 | When this code is executed there is no output. Here's why: |
795 | |
796 | When a variable is tied, it is associated with the object which is the |
797 | return value of the TIESCALAR, TIEARRAY, or TIEHASH function. This |
798 | object normally has only one reference, namely, the implicit reference |
799 | from the tied variable. When untie() is called, that reference is |
800 | destroyed. Then, as in the first example above, the object's |
801 | destructor (DESTROY) is called, which is normal for objects that have |
802 | no more valid references; and thus the file is closed. |
803 | |
804 | In the second example, however, we have stored another reference to |
805 | the tied object in C<$x>. That means that when untie() gets called |
806 | there will still be a valid reference to the object in existence, so |
807 | the destructor is not called at that time, and thus the file is not |
808 | closed. The reason there is no output is because the file buffers |
809 | have not been flushed to disk. |
810 | |
811 | Now that you know what the problem is, what can you do to avoid it? |
812 | Well, the good old C<-w> flag will spot any instances where you call |
813 | untie() and there are still valid references to the tied object. If |
814 | the second script above is run with the C<-w> flag, Perl prints this |
815 | warning message: |
816 | |
817 | untie attempted while 1 inner references still exist |
818 | |
819 | To get the script to work properly and silence the warning make sure |
820 | there are no valid references to the tied object I<before> untie() is |
821 | called: |
822 | |
823 | undef $x; |
824 | untie $fred; |
825 | |
cb1a09d0 |
826 | =head1 SEE ALSO |
827 | |
828 | See L<DB_File> or L<Config> for some interesting tie() implementations. |
829 | |
830 | =head1 BUGS |
831 | |
832 | Tied arrays are I<incomplete>. They are also distinctly lacking something |
833 | for the C<$#ARRAY> access (which is hard, as it's an lvalue), as well as |
834 | the other obvious array functions, like push(), pop(), shift(), unshift(), |
835 | and splice(). |
836 | |
c07a80fd |
837 | You cannot easily tie a multilevel data structure (such as a hash of |
838 | hashes) to a dbm file. The first problem is that all but GDBM and |
839 | Berkeley DB have size limitations, but beyond that, you also have problems |
840 | with how references are to be represented on disk. One experimental |
5f05dabc |
841 | module that does attempt to address this need partially is the MLDBM |
f102b883 |
842 | module. Check your nearest CPAN site as described in L<perlmodlib> for |
c07a80fd |
843 | source code to MLDBM. |
844 | |
cb1a09d0 |
845 | =head1 AUTHOR |
846 | |
847 | Tom Christiansen |
a7adf1f0 |
848 | |
46fc3d4c |
849 | TIEHANDLE by Sven Verdoolaege <F<skimo@dns.ufsia.ac.be>> and Doug MacEachern <F<dougm@osf.org>> |