Commit | Line | Data |
---|---|---|
cb1a09d0 | 1 | =head1 NAME |
d74e8afc | 2 | X<tie> |
cb1a09d0 AD |
3 | |
4 | perltie - how to hide an object class in a simple variable | |
5 | ||
6 | =head1 SYNOPSIS | |
7 | ||
8 | tie VARIABLE, CLASSNAME, LIST | |
9 | ||
6fdf61fb | 10 | $object = tied VARIABLE |
11 | ||
cb1a09d0 AD |
12 | untie VARIABLE |
13 | ||
14 | =head1 DESCRIPTION | |
15 | ||
16 | Prior to release 5.0 of Perl, a programmer could use dbmopen() | |
5f05dabc | 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 | |
cb1a09d0 AD |
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 | |
5a964f20 | 27 | method calls in the proper class. The complexity of the class is |
cb1a09d0 AD |
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(), | |
5f05dabc | 37 | TIEARRAY(), TIEHASH(), or TIEHANDLE(). (Typically these are arguments |
a7adf1f0 | 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 | |
5f05dabc | 42 | "type" (e.g., HASH or C<CLASSNAME>) so long as it's a properly blessed |
a7adf1f0 | 43 | object.) You can also retrieve a reference to the underlying object |
44 | using the tied() function. | |
cb1a09d0 AD |
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 | |
d74e8afc | 50 | X<scalar, tying> |
cb1a09d0 AD |
51 | |
52 | A class implementing a tied scalar should define the following methods: | |
301e8125 | 53 | TIESCALAR, FETCH, STORE, and possibly UNTIE and/or DESTROY. |
cb1a09d0 AD |
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', $$; | |
60 | ||
61 | And now whenever either of those variables is accessed, its current | |
62 | system priority is retrieved and returned. If those variables are set, | |
63 | then the process's priority is changed! | |
64 | ||
5aabfad6 | 65 | We'll use Jarkko Hietaniemi <F<jhi@iki.fi>>'s BSD::Resource class (not |
66 | included) to access the PRIO_PROCESS, PRIO_MIN, and PRIO_MAX constants | |
67 | from your system, as well as the getpriority() and setpriority() system | |
68 | calls. Here's the preamble of the class. | |
cb1a09d0 AD |
69 | |
70 | package Nice; | |
71 | use Carp; | |
72 | use BSD::Resource; | |
73 | use strict; | |
74 | $Nice::DEBUG = 0 unless defined $Nice::DEBUG; | |
75 | ||
13a2d996 | 76 | =over 4 |
cb1a09d0 AD |
77 | |
78 | =item TIESCALAR classname, LIST | |
d74e8afc | 79 | X<TIESCALAR> |
cb1a09d0 AD |
80 | |
81 | This is the constructor for the class. That means it is | |
82 | expected to return a blessed reference to a new scalar | |
83 | (probably anonymous) that it's creating. For example: | |
84 | ||
85 | sub TIESCALAR { | |
86 | my $class = shift; | |
87 | my $pid = shift || $$; # 0 means me | |
88 | ||
89 | if ($pid !~ /^\d+$/) { | |
6fdf61fb | 90 | carp "Nice::Tie::Scalar got non-numeric pid $pid" if $^W; |
cb1a09d0 AD |
91 | return undef; |
92 | } | |
93 | ||
94 | unless (kill 0, $pid) { # EPERM or ERSCH, no doubt | |
6fdf61fb | 95 | carp "Nice::Tie::Scalar got bad pid $pid: $!" if $^W; |
cb1a09d0 AD |
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 | |
d74e8afc | 108 | X<FETCH> |
cb1a09d0 AD |
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 | |
5f05dabc | 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 $$self | |
cb1a09d0 AD |
114 | allows the method to get at the real value stored there. In our example |
115 | below, that real value is the process ID to which we've tied our variable. | |
116 | ||
117 | sub FETCH { | |
118 | my $self = shift; | |
119 | confess "wrong type" unless ref $self; | |
120 | croak "usage error" if @_; | |
121 | my $nicety; | |
122 | local($!) = 0; | |
123 | $nicety = getpriority(PRIO_PROCESS, $$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 | |
d74e8afc | 133 | X<STORE> |
cb1a09d0 AD |
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) | |
ac036724 | 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 | |
a177e38d | 139 | assigned value is implemented with FETCH. |
cb1a09d0 AD |
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, $$self, $new_nicety)) { | |
162 | confess "setpriority failed: $!"; | |
163 | } | |
cb1a09d0 AD |
164 | } |
165 | ||
301e8125 | 166 | =item UNTIE this |
d74e8afc | 167 | X<UNTIE> |
301e8125 NIS |
168 | |
169 | This method will be triggered when the C<untie> occurs. This can be useful | |
170 | if the class needs to know when no further calls will be made. (Except DESTROY | |
d5582e24 | 171 | of course.) See L<The C<untie> Gotcha> below for more details. |
301e8125 | 172 | |
cb1a09d0 | 173 | =item DESTROY this |
d74e8afc | 174 | X<DESTROY> |
cb1a09d0 AD |
175 | |
176 | This method will be triggered when the tied variable needs to be destructed. | |
5f05dabc | 177 | As with other object classes, such a method is seldom necessary, because Perl |
cb1a09d0 AD |
178 | deallocates its moribund object's memory for you automatically--this isn't |
179 | C++, you know. We'll use a DESTROY method here for debugging purposes only. | |
180 | ||
181 | sub DESTROY { | |
182 | my $self = shift; | |
183 | confess "wrong type" unless ref $self; | |
184 | carp "[ Nice::DESTROY pid $$self ]" if $Nice::DEBUG; | |
185 | } | |
186 | ||
187 | =back | |
188 | ||
189 | That's about all there is to it. Actually, it's more than all there | |
5f05dabc | 190 | is to it, because we've done a few nice things here for the sake |
cb1a09d0 AD |
191 | of completeness, robustness, and general aesthetics. Simpler |
192 | TIESCALAR classes are certainly possible. | |
193 | ||
194 | =head2 Tying Arrays | |
d74e8afc | 195 | X<array, tying> |
cb1a09d0 AD |
196 | |
197 | A class implementing a tied ordinary array should define the following | |
bf5513e0 ZA |
198 | methods: TIEARRAY, FETCH, STORE, FETCHSIZE, STORESIZE, CLEAR |
199 | and perhaps UNTIE and/or DESTROY. | |
cb1a09d0 | 200 | |
a60c0954 NIS |
201 | FETCHSIZE and STORESIZE are used to provide C<$#array> and |
202 | equivalent C<scalar(@array)> access. | |
c47ff5f1 | 203 | |
01020589 GS |
204 | The methods POP, PUSH, SHIFT, UNSHIFT, SPLICE, DELETE, and EXISTS are |
205 | required if the perl operator with the corresponding (but lowercase) name | |
206 | is to operate on the tied array. The B<Tie::Array> class can be used as a | |
207 | base class to implement the first five of these in terms of the basic | |
208 | methods above. The default implementations of DELETE and EXISTS in | |
209 | B<Tie::Array> simply C<croak>. | |
a60c0954 | 210 | |
301e8125 | 211 | In addition EXTEND will be called when perl would have pre-extended |
a60c0954 NIS |
212 | allocation in a real array. |
213 | ||
4ae85618 CT |
214 | For this discussion, we'll implement an array whose elements are a fixed |
215 | size at creation. If you try to create an element larger than the fixed | |
216 | size, you'll take an exception. For example: | |
cb1a09d0 | 217 | |
4ae85618 CT |
218 | use FixedElem_Array; |
219 | tie @array, 'FixedElem_Array', 3; | |
220 | $array[0] = 'cat'; # ok. | |
221 | $array[1] = 'dogs'; # exception, length('dogs') > 3. | |
cb1a09d0 AD |
222 | |
223 | The preamble code for the class is as follows: | |
224 | ||
4ae85618 | 225 | package FixedElem_Array; |
cb1a09d0 AD |
226 | use Carp; |
227 | use strict; | |
228 | ||
13a2d996 | 229 | =over 4 |
cb1a09d0 AD |
230 | |
231 | =item TIEARRAY classname, LIST | |
d74e8afc | 232 | X<TIEARRAY> |
cb1a09d0 AD |
233 | |
234 | This is the constructor for the class. That means it is expected to | |
235 | return a blessed reference through which the new array (probably an | |
236 | anonymous ARRAY ref) will be accessed. | |
237 | ||
238 | In our example, just to show you that you don't I<really> have to return an | |
239 | ARRAY reference, we'll choose a HASH reference to represent our object. | |
4ae85618 CT |
240 | A HASH works out well as a generic record type: the C<{ELEMSIZE}> field will |
241 | store the maximum element size allowed, and the C<{ARRAY}> field will hold the | |
cb1a09d0 AD |
242 | true ARRAY ref. If someone outside the class tries to dereference the |
243 | object returned (doubtless thinking it an ARRAY ref), they'll blow up. | |
244 | This just goes to show you that you should respect an object's privacy. | |
245 | ||
246 | sub TIEARRAY { | |
4ae85618 CT |
247 | my $class = shift; |
248 | my $elemsize = shift; | |
249 | if ( @_ || $elemsize =~ /\D/ ) { | |
250 | croak "usage: tie ARRAY, '" . __PACKAGE__ . "', elem_size"; | |
251 | } | |
252 | return bless { | |
253 | ELEMSIZE => $elemsize, | |
254 | ARRAY => [], | |
255 | }, $class; | |
cb1a09d0 AD |
256 | } |
257 | ||
258 | =item FETCH this, index | |
d74e8afc | 259 | X<FETCH> |
cb1a09d0 AD |
260 | |
261 | This method will be triggered every time an individual element the tied array | |
262 | is accessed (read). It takes one argument beyond its self reference: the | |
263 | index whose value we're trying to fetch. | |
264 | ||
265 | sub FETCH { | |
4ae85618 CT |
266 | my $self = shift; |
267 | my $index = shift; | |
268 | return $self->{ARRAY}->[$index]; | |
cb1a09d0 AD |
269 | } |
270 | ||
301e8125 | 271 | If a negative array index is used to read from an array, the index |
0b931be4 | 272 | will be translated to a positive one internally by calling FETCHSIZE |
6f12eb6d MJD |
273 | before being passed to FETCH. You may disable this feature by |
274 | assigning a true value to the variable C<$NEGATIVE_INDICES> in the | |
275 | tied array class. | |
301e8125 | 276 | |
cb1a09d0 AD |
277 | As you may have noticed, the name of the FETCH method (et al.) is the same |
278 | for all accesses, even though the constructors differ in names (TIESCALAR | |
279 | vs TIEARRAY). While in theory you could have the same class servicing | |
280 | several tied types, in practice this becomes cumbersome, and it's easiest | |
5f05dabc | 281 | to keep them at simply one tie type per class. |
cb1a09d0 AD |
282 | |
283 | =item STORE this, index, value | |
d74e8afc | 284 | X<STORE> |
cb1a09d0 AD |
285 | |
286 | This method will be triggered every time an element in the tied array is set | |
287 | (written). It takes two arguments beyond its self reference: the index at | |
288 | which we're trying to store something and the value we're trying to put | |
4ae85618 CT |
289 | there. |
290 | ||
291 | In our example, C<undef> is really C<$self-E<gt>{ELEMSIZE}> number of | |
292 | spaces so we have a little more work to do here: | |
cb1a09d0 AD |
293 | |
294 | sub STORE { | |
4ae85618 CT |
295 | my $self = shift; |
296 | my( $index, $value ) = @_; | |
297 | if ( length $value > $self->{ELEMSIZE} ) { | |
298 | croak "length of $value is greater than $self->{ELEMSIZE}"; | |
cb1a09d0 | 299 | } |
4ae85618 CT |
300 | # fill in the blanks |
301 | $self->EXTEND( $index ) if $index > $self->FETCHSIZE(); | |
302 | # right justify to keep element size for smaller elements | |
303 | $self->{ARRAY}->[$index] = sprintf "%$self->{ELEMSIZE}s", $value; | |
cb1a09d0 | 304 | } |
301e8125 NIS |
305 | |
306 | Negative indexes are treated the same as with FETCH. | |
307 | ||
4ae85618 | 308 | =item FETCHSIZE this |
d74e8afc | 309 | X<FETCHSIZE> |
4ae85618 CT |
310 | |
311 | Returns the total number of items in the tied array associated with | |
312 | object I<this>. (Equivalent to C<scalar(@array)>). For example: | |
313 | ||
314 | sub FETCHSIZE { | |
315 | my $self = shift; | |
316 | return scalar @{$self->{ARRAY}}; | |
317 | } | |
318 | ||
319 | =item STORESIZE this, count | |
d74e8afc | 320 | X<STORESIZE> |
4ae85618 CT |
321 | |
322 | Sets the total number of items in the tied array associated with | |
323 | object I<this> to be I<count>. If this makes the array larger then | |
324 | class's mapping of C<undef> should be returned for new positions. | |
325 | If the array becomes smaller then entries beyond count should be | |
326 | deleted. | |
327 | ||
328 | In our example, 'undef' is really an element containing | |
329 | C<$self-E<gt>{ELEMSIZE}> number of spaces. Observe: | |
330 | ||
f9abed49 CT |
331 | sub STORESIZE { |
332 | my $self = shift; | |
333 | my $count = shift; | |
334 | if ( $count > $self->FETCHSIZE() ) { | |
335 | foreach ( $count - $self->FETCHSIZE() .. $count ) { | |
336 | $self->STORE( $_, '' ); | |
337 | } | |
338 | } elsif ( $count < $self->FETCHSIZE() ) { | |
339 | foreach ( 0 .. $self->FETCHSIZE() - $count - 2 ) { | |
340 | $self->POP(); | |
341 | } | |
342 | } | |
343 | } | |
4ae85618 CT |
344 | |
345 | =item EXTEND this, count | |
d74e8afc | 346 | X<EXTEND> |
4ae85618 CT |
347 | |
348 | Informative call that array is likely to grow to have I<count> entries. | |
349 | Can be used to optimize allocation. This method need do nothing. | |
350 | ||
351 | In our example, we want to make sure there are no blank (C<undef>) | |
352 | entries, so C<EXTEND> will make use of C<STORESIZE> to fill elements | |
353 | as needed: | |
354 | ||
355 | sub EXTEND { | |
356 | my $self = shift; | |
357 | my $count = shift; | |
358 | $self->STORESIZE( $count ); | |
359 | } | |
360 | ||
361 | =item EXISTS this, key | |
d74e8afc | 362 | X<EXISTS> |
4ae85618 CT |
363 | |
364 | Verify that the element at index I<key> exists in the tied array I<this>. | |
365 | ||
366 | In our example, we will determine that if an element consists of | |
367 | C<$self-E<gt>{ELEMSIZE}> spaces only, it does not exist: | |
368 | ||
369 | sub EXISTS { | |
370 | my $self = shift; | |
371 | my $index = shift; | |
f9abed49 CT |
372 | return 0 if ! defined $self->{ARRAY}->[$index] || |
373 | $self->{ARRAY}->[$index] eq ' ' x $self->{ELEMSIZE}; | |
374 | return 1; | |
4ae85618 CT |
375 | } |
376 | ||
377 | =item DELETE this, key | |
d74e8afc | 378 | X<DELETE> |
4ae85618 CT |
379 | |
380 | Delete the element at index I<key> from the tied array I<this>. | |
381 | ||
ad0f383a | 382 | In our example, a deleted item is C<$self-E<gt>{ELEMSIZE}> spaces: |
4ae85618 CT |
383 | |
384 | sub DELETE { | |
385 | my $self = shift; | |
386 | my $index = shift; | |
387 | return $self->STORE( $index, '' ); | |
388 | } | |
389 | ||
390 | =item CLEAR this | |
d74e8afc | 391 | X<CLEAR> |
4ae85618 CT |
392 | |
393 | Clear (remove, delete, ...) all values from the tied array associated with | |
394 | object I<this>. For example: | |
395 | ||
396 | sub CLEAR { | |
397 | my $self = shift; | |
398 | return $self->{ARRAY} = []; | |
399 | } | |
400 | ||
401 | =item PUSH this, LIST | |
d74e8afc | 402 | X<PUSH> |
4ae85618 CT |
403 | |
404 | Append elements of I<LIST> to the array. For example: | |
405 | ||
406 | sub PUSH { | |
407 | my $self = shift; | |
408 | my @list = @_; | |
409 | my $last = $self->FETCHSIZE(); | |
410 | $self->STORE( $last + $_, $list[$_] ) foreach 0 .. $#list; | |
411 | return $self->FETCHSIZE(); | |
412 | } | |
413 | ||
414 | =item POP this | |
d74e8afc | 415 | X<POP> |
4ae85618 CT |
416 | |
417 | Remove last element of the array and return it. For example: | |
418 | ||
419 | sub POP { | |
420 | my $self = shift; | |
421 | return pop @{$self->{ARRAY}}; | |
422 | } | |
423 | ||
424 | =item SHIFT this | |
d74e8afc | 425 | X<SHIFT> |
4ae85618 CT |
426 | |
427 | Remove the first element of the array (shifting other elements down) | |
428 | and return it. For example: | |
429 | ||
430 | sub SHIFT { | |
431 | my $self = shift; | |
432 | return shift @{$self->{ARRAY}}; | |
433 | } | |
434 | ||
435 | =item UNSHIFT this, LIST | |
d74e8afc | 436 | X<UNSHIFT> |
4ae85618 CT |
437 | |
438 | Insert LIST elements at the beginning of the array, moving existing elements | |
439 | up to make room. For example: | |
440 | ||
441 | sub UNSHIFT { | |
442 | my $self = shift; | |
443 | my @list = @_; | |
444 | my $size = scalar( @list ); | |
445 | # make room for our list | |
446 | @{$self->{ARRAY}}[ $size .. $#{$self->{ARRAY}} + $size ] | |
447 | = @{$self->{ARRAY}}; | |
448 | $self->STORE( $_, $list[$_] ) foreach 0 .. $#list; | |
449 | } | |
450 | ||
451 | =item SPLICE this, offset, length, LIST | |
d74e8afc | 452 | X<SPLICE> |
4ae85618 CT |
453 | |
454 | Perform the equivalent of C<splice> on the array. | |
455 | ||
456 | I<offset> is optional and defaults to zero, negative values count back | |
457 | from the end of the array. | |
458 | ||
459 | I<length> is optional and defaults to rest of the array. | |
460 | ||
461 | I<LIST> may be empty. | |
462 | ||
463 | Returns a list of the original I<length> elements at I<offset>. | |
464 | ||
465 | In our example, we'll use a little shortcut if there is a I<LIST>: | |
466 | ||
467 | sub SPLICE { | |
468 | my $self = shift; | |
469 | my $offset = shift || 0; | |
470 | my $length = shift || $self->FETCHSIZE() - $offset; | |
471 | my @list = (); | |
472 | if ( @_ ) { | |
473 | tie @list, __PACKAGE__, $self->{ELEMSIZE}; | |
474 | @list = @_; | |
475 | } | |
476 | return splice @{$self->{ARRAY}}, $offset, $length, @list; | |
477 | } | |
478 | ||
301e8125 | 479 | =item UNTIE this |
d74e8afc | 480 | X<UNTIE> |
301e8125 | 481 | |
d5582e24 | 482 | Will be called when C<untie> happens. (See L<The C<untie> Gotcha> below.) |
cb1a09d0 AD |
483 | |
484 | =item DESTROY this | |
d74e8afc | 485 | X<DESTROY> |
cb1a09d0 AD |
486 | |
487 | This method will be triggered when the tied variable needs to be destructed. | |
184e9718 | 488 | As with the scalar tie class, this is almost never needed in a |
cb1a09d0 AD |
489 | language that does its own garbage collection, so this time we'll |
490 | just leave it out. | |
491 | ||
492 | =back | |
493 | ||
cb1a09d0 | 494 | =head2 Tying Hashes |
d74e8afc | 495 | X<hash, tying> |
cb1a09d0 | 496 | |
be3174d2 GS |
497 | Hashes were the first Perl data type to be tied (see dbmopen()). A class |
498 | implementing a tied hash should define the following methods: TIEHASH is | |
499 | the constructor. FETCH and STORE access the key and value pairs. EXISTS | |
500 | reports whether a key is present in the hash, and DELETE deletes one. | |
501 | CLEAR empties the hash by deleting all the key and value pairs. FIRSTKEY | |
502 | and NEXTKEY implement the keys() and each() functions to iterate over all | |
a3bcc51e TP |
503 | the keys. SCALAR is triggered when the tied hash is evaluated in scalar |
504 | context. UNTIE is called when C<untie> happens, and DESTROY is called when | |
301e8125 | 505 | the tied variable is garbage collected. |
aa689395 | 506 | |
507 | If this seems like a lot, then feel free to inherit from merely the | |
d5582e24 | 508 | standard Tie::StdHash module for most of your methods, redefining only the |
aa689395 | 509 | interesting ones. See L<Tie::Hash> for details. |
cb1a09d0 AD |
510 | |
511 | Remember that Perl distinguishes between a key not existing in the hash, | |
512 | and the key existing in the hash but having a corresponding value of | |
513 | C<undef>. The two possibilities can be tested with the C<exists()> and | |
514 | C<defined()> functions. | |
515 | ||
516 | Here's an example of a somewhat interesting tied hash class: it gives you | |
5f05dabc | 517 | a hash representing a particular user's dot files. You index into the hash |
518 | with the name of the file (minus the dot) and you get back that dot file's | |
cb1a09d0 AD |
519 | contents. For example: |
520 | ||
521 | use DotFiles; | |
1f57c600 | 522 | tie %dot, 'DotFiles'; |
cb1a09d0 AD |
523 | if ( $dot{profile} =~ /MANPATH/ || |
524 | $dot{login} =~ /MANPATH/ || | |
525 | $dot{cshrc} =~ /MANPATH/ ) | |
526 | { | |
5f05dabc | 527 | print "you seem to set your MANPATH\n"; |
cb1a09d0 AD |
528 | } |
529 | ||
530 | Or here's another sample of using our tied class: | |
531 | ||
1f57c600 | 532 | tie %him, 'DotFiles', 'daemon'; |
cb1a09d0 AD |
533 | foreach $f ( keys %him ) { |
534 | printf "daemon dot file %s is size %d\n", | |
535 | $f, length $him{$f}; | |
536 | } | |
537 | ||
538 | In our tied hash DotFiles example, we use a regular | |
539 | hash for the object containing several important | |
540 | fields, of which only the C<{LIST}> field will be what the | |
541 | user thinks of as the real hash. | |
542 | ||
543 | =over 5 | |
544 | ||
545 | =item USER | |
546 | ||
547 | whose dot files this object represents | |
548 | ||
549 | =item HOME | |
550 | ||
5f05dabc | 551 | where those dot files live |
cb1a09d0 AD |
552 | |
553 | =item CLOBBER | |
554 | ||
555 | whether we should try to change or remove those dot files | |
556 | ||
557 | =item LIST | |
558 | ||
5f05dabc | 559 | the hash of dot file names and content mappings |
cb1a09d0 AD |
560 | |
561 | =back | |
562 | ||
563 | Here's the start of F<Dotfiles.pm>: | |
564 | ||
565 | package DotFiles; | |
566 | use Carp; | |
567 | sub whowasi { (caller(1))[3] . '()' } | |
568 | my $DEBUG = 0; | |
569 | sub debug { $DEBUG = @_ ? shift : 1 } | |
570 | ||
5f05dabc | 571 | For our example, we want to be able to emit debugging info to help in tracing |
cb1a09d0 AD |
572 | during development. We keep also one convenience function around |
573 | internally to help print out warnings; whowasi() returns the function name | |
574 | that calls it. | |
575 | ||
576 | Here are the methods for the DotFiles tied hash. | |
577 | ||
13a2d996 | 578 | =over 4 |
cb1a09d0 AD |
579 | |
580 | =item TIEHASH classname, LIST | |
d74e8afc | 581 | X<TIEHASH> |
cb1a09d0 AD |
582 | |
583 | This is the constructor for the class. That means it is expected to | |
584 | return a blessed reference through which the new object (probably but not | |
585 | necessarily an anonymous hash) will be accessed. | |
586 | ||
587 | Here's the constructor: | |
588 | ||
589 | sub TIEHASH { | |
590 | my $self = shift; | |
591 | my $user = shift || $>; | |
592 | my $dotdir = shift || ''; | |
593 | croak "usage: @{[&whowasi]} [USER [DOTDIR]]" if @_; | |
594 | $user = getpwuid($user) if $user =~ /^\d+$/; | |
595 | my $dir = (getpwnam($user))[7] | |
596 | || croak "@{[&whowasi]}: no user $user"; | |
597 | $dir .= "/$dotdir" if $dotdir; | |
598 | ||
599 | my $node = { | |
600 | USER => $user, | |
601 | HOME => $dir, | |
602 | LIST => {}, | |
603 | CLOBBER => 0, | |
604 | }; | |
605 | ||
606 | opendir(DIR, $dir) | |
607 | || croak "@{[&whowasi]}: can't opendir $dir: $!"; | |
608 | foreach $dot ( grep /^\./ && -f "$dir/$_", readdir(DIR)) { | |
609 | $dot =~ s/^\.//; | |
610 | $node->{LIST}{$dot} = undef; | |
611 | } | |
612 | closedir DIR; | |
613 | return bless $node, $self; | |
614 | } | |
615 | ||
616 | It's probably worth mentioning that if you're going to filetest the | |
617 | return values out of a readdir, you'd better prepend the directory | |
5f05dabc | 618 | in question. Otherwise, because we didn't chdir() there, it would |
2ae324a7 | 619 | have been testing the wrong file. |
cb1a09d0 AD |
620 | |
621 | =item FETCH this, key | |
d74e8afc | 622 | X<FETCH> |
cb1a09d0 AD |
623 | |
624 | This method will be triggered every time an element in the tied hash is | |
625 | accessed (read). It takes one argument beyond its self reference: the key | |
626 | whose value we're trying to fetch. | |
627 | ||
628 | Here's the fetch for our DotFiles example. | |
629 | ||
630 | sub FETCH { | |
631 | carp &whowasi if $DEBUG; | |
632 | my $self = shift; | |
633 | my $dot = shift; | |
634 | my $dir = $self->{HOME}; | |
635 | my $file = "$dir/.$dot"; | |
636 | ||
637 | unless (exists $self->{LIST}->{$dot} || -f $file) { | |
638 | carp "@{[&whowasi]}: no $dot file" if $DEBUG; | |
639 | return undef; | |
640 | } | |
641 | ||
642 | if (defined $self->{LIST}->{$dot}) { | |
643 | return $self->{LIST}->{$dot}; | |
644 | } else { | |
645 | return $self->{LIST}->{$dot} = `cat $dir/.$dot`; | |
646 | } | |
647 | } | |
648 | ||
649 | It was easy to write by having it call the Unix cat(1) command, but it | |
650 | would probably be more portable to open the file manually (and somewhat | |
5f05dabc | 651 | more efficient). Of course, because dot files are a Unixy concept, we're |
cb1a09d0 AD |
652 | not that concerned. |
653 | ||
654 | =item STORE this, key, value | |
d74e8afc | 655 | X<STORE> |
cb1a09d0 AD |
656 | |
657 | This method will be triggered every time an element in the tied hash is set | |
658 | (written). It takes two arguments beyond its self reference: the index at | |
659 | which we're trying to store something, and the value we're trying to put | |
660 | there. | |
661 | ||
662 | Here in our DotFiles example, we'll be careful not to let | |
663 | them try to overwrite the file unless they've called the clobber() | |
664 | method on the original object reference returned by tie(). | |
665 | ||
666 | sub STORE { | |
667 | carp &whowasi if $DEBUG; | |
668 | my $self = shift; | |
669 | my $dot = shift; | |
670 | my $value = shift; | |
671 | my $file = $self->{HOME} . "/.$dot"; | |
672 | my $user = $self->{USER}; | |
673 | ||
674 | croak "@{[&whowasi]}: $file not clobberable" | |
675 | unless $self->{CLOBBER}; | |
676 | ||
67d00ddd AC |
677 | open(my $f, '>', $file) || croak "can't open $file: $!"; |
678 | print $f $value; | |
679 | close($f); | |
cb1a09d0 AD |
680 | } |
681 | ||
682 | If they wanted to clobber something, they might say: | |
683 | ||
684 | $ob = tie %daemon_dots, 'daemon'; | |
685 | $ob->clobber(1); | |
686 | $daemon_dots{signature} = "A true daemon\n"; | |
687 | ||
6fdf61fb | 688 | Another way to lay hands on a reference to the underlying object is to |
689 | use the tied() function, so they might alternately have set clobber | |
690 | using: | |
691 | ||
692 | tie %daemon_dots, 'daemon'; | |
693 | tied(%daemon_dots)->clobber(1); | |
694 | ||
695 | The clobber method is simply: | |
cb1a09d0 AD |
696 | |
697 | sub clobber { | |
698 | my $self = shift; | |
699 | $self->{CLOBBER} = @_ ? shift : 1; | |
700 | } | |
701 | ||
702 | =item DELETE this, key | |
d74e8afc | 703 | X<DELETE> |
cb1a09d0 AD |
704 | |
705 | This method is triggered when we remove an element from the hash, | |
706 | typically by using the delete() function. Again, we'll | |
707 | be careful to check whether they really want to clobber files. | |
708 | ||
709 | sub DELETE { | |
710 | carp &whowasi if $DEBUG; | |
711 | ||
712 | my $self = shift; | |
713 | my $dot = shift; | |
714 | my $file = $self->{HOME} . "/.$dot"; | |
715 | croak "@{[&whowasi]}: won't remove file $file" | |
716 | unless $self->{CLOBBER}; | |
717 | delete $self->{LIST}->{$dot}; | |
1f57c600 | 718 | my $success = unlink($file); |
719 | carp "@{[&whowasi]}: can't unlink $file: $!" unless $success; | |
720 | $success; | |
cb1a09d0 AD |
721 | } |
722 | ||
1f57c600 | 723 | The value returned by DELETE becomes the return value of the call |
724 | to delete(). If you want to emulate the normal behavior of delete(), | |
725 | you should return whatever FETCH would have returned for this key. | |
726 | In this example, we have chosen instead to return a value which tells | |
727 | the caller whether the file was successfully deleted. | |
728 | ||
cb1a09d0 | 729 | =item CLEAR this |
d74e8afc | 730 | X<CLEAR> |
cb1a09d0 AD |
731 | |
732 | This method is triggered when the whole hash is to be cleared, usually by | |
733 | assigning the empty list to it. | |
734 | ||
5f05dabc | 735 | In our example, that would remove all the user's dot files! It's such a |
cb1a09d0 AD |
736 | dangerous thing that they'll have to set CLOBBER to something higher than |
737 | 1 to make it happen. | |
738 | ||
739 | sub CLEAR { | |
740 | carp &whowasi if $DEBUG; | |
741 | my $self = shift; | |
5f05dabc | 742 | croak "@{[&whowasi]}: won't remove all dot files for $self->{USER}" |
cb1a09d0 AD |
743 | unless $self->{CLOBBER} > 1; |
744 | my $dot; | |
745 | foreach $dot ( keys %{$self->{LIST}}) { | |
746 | $self->DELETE($dot); | |
747 | } | |
748 | } | |
749 | ||
750 | =item EXISTS this, key | |
d74e8afc | 751 | X<EXISTS> |
cb1a09d0 AD |
752 | |
753 | This method is triggered when the user uses the exists() function | |
754 | on a particular hash. In our example, we'll look at the C<{LIST}> | |
755 | hash element for this: | |
756 | ||
757 | sub EXISTS { | |
758 | carp &whowasi if $DEBUG; | |
759 | my $self = shift; | |
760 | my $dot = shift; | |
761 | return exists $self->{LIST}->{$dot}; | |
762 | } | |
763 | ||
764 | =item FIRSTKEY this | |
d74e8afc | 765 | X<FIRSTKEY> |
cb1a09d0 AD |
766 | |
767 | This method will be triggered when the user is going | |
768 | to iterate through the hash, such as via a keys() or each() | |
769 | call. | |
770 | ||
771 | sub FIRSTKEY { | |
772 | carp &whowasi if $DEBUG; | |
773 | my $self = shift; | |
6fdf61fb | 774 | my $a = keys %{$self->{LIST}}; # reset each() iterator |
cb1a09d0 AD |
775 | each %{$self->{LIST}} |
776 | } | |
777 | ||
778 | =item NEXTKEY this, lastkey | |
d74e8afc | 779 | X<NEXTKEY> |
cb1a09d0 AD |
780 | |
781 | This method gets triggered during a keys() or each() iteration. It has a | |
782 | second argument which is the last key that had been accessed. This is | |
783 | useful if you're carrying about ordering or calling the iterator from more | |
784 | than one sequence, or not really storing things in a hash anywhere. | |
785 | ||
5f05dabc | 786 | For our example, we're using a real hash so we'll do just the simple |
787 | thing, but we'll have to go through the LIST field indirectly. | |
cb1a09d0 AD |
788 | |
789 | sub NEXTKEY { | |
790 | carp &whowasi if $DEBUG; | |
791 | my $self = shift; | |
792 | return each %{ $self->{LIST} } | |
793 | } | |
794 | ||
a3bcc51e | 795 | =item SCALAR this |
d74e8afc | 796 | X<SCALAR> |
a3bcc51e TP |
797 | |
798 | This is called when the hash is evaluated in scalar context. In order | |
799 | to mimic the behaviour of untied hashes, this method should return a | |
800 | false value when the tied hash is considered empty. If this method does | |
159b10bb RGS |
801 | not exist, perl will make some educated guesses and return true when |
802 | the hash is inside an iteration. If this isn't the case, FIRSTKEY is | |
803 | called, and the result will be a false value if FIRSTKEY returns the empty | |
804 | list, true otherwise. | |
a3bcc51e | 805 | |
47b1b33c TP |
806 | However, you should B<not> blindly rely on perl always doing the right |
807 | thing. Particularly, perl will mistakenly return true when you clear the | |
808 | hash by repeatedly calling DELETE until it is empty. You are therefore | |
809 | advised to supply your own SCALAR method when you want to be absolutely | |
810 | sure that your hash behaves nicely in scalar context. | |
811 | ||
a3bcc51e TP |
812 | In our example we can just call C<scalar> on the underlying hash |
813 | referenced by C<$self-E<gt>{LIST}>: | |
814 | ||
815 | sub SCALAR { | |
816 | carp &whowasi if $DEBUG; | |
817 | my $self = shift; | |
818 | return scalar %{ $self->{LIST} } | |
819 | } | |
820 | ||
301e8125 | 821 | =item UNTIE this |
d74e8afc | 822 | X<UNTIE> |
301e8125 | 823 | |
d5582e24 | 824 | This is called when C<untie> occurs. See L<The C<untie> Gotcha> below. |
301e8125 | 825 | |
cb1a09d0 | 826 | =item DESTROY this |
d74e8afc | 827 | X<DESTROY> |
cb1a09d0 AD |
828 | |
829 | This method is triggered when a tied hash is about to go out of | |
830 | scope. You don't really need it unless you're trying to add debugging | |
831 | or have auxiliary state to clean up. Here's a very simple function: | |
832 | ||
833 | sub DESTROY { | |
834 | carp &whowasi if $DEBUG; | |
835 | } | |
836 | ||
837 | =back | |
838 | ||
1d2dff63 GS |
839 | Note that functions such as keys() and values() may return huge lists |
840 | when used on large objects, like DBM files. You may prefer to use the | |
841 | each() function to iterate over such. Example: | |
cb1a09d0 AD |
842 | |
843 | # print out history file offsets | |
844 | use NDBM_File; | |
1f57c600 | 845 | tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0); |
cb1a09d0 AD |
846 | while (($key,$val) = each %HIST) { |
847 | print $key, ' = ', unpack('L',$val), "\n"; | |
848 | } | |
849 | untie(%HIST); | |
850 | ||
851 | =head2 Tying FileHandles | |
d74e8afc | 852 | X<filehandle, tying> |
cb1a09d0 | 853 | |
184e9718 | 854 | This is partially implemented now. |
a7adf1f0 | 855 | |
2ae324a7 | 856 | A class implementing a tied filehandle should define the following |
1d603a67 | 857 | methods: TIEHANDLE, at least one of PRINT, PRINTF, WRITE, READLINE, GETC, |
301e8125 | 858 | READ, and possibly CLOSE, UNTIE and DESTROY. The class can also provide: BINMODE, |
4592e6ca NIS |
859 | OPEN, EOF, FILENO, SEEK, TELL - if the corresponding perl operators are |
860 | used on the handle. | |
a7adf1f0 | 861 | |
7ff03255 SG |
862 | When STDERR is tied, its PRINT method will be called to issue warnings |
863 | and error messages. This feature is temporarily disabled during the call, | |
864 | which means you can use C<warn()> inside PRINT without starting a recursive | |
865 | loop. And just like C<__WARN__> and C<__DIE__> handlers, STDERR's PRINT | |
866 | method may be called to report parser errors, so the caveats mentioned under | |
867 | L<perlvar/%SIG> apply. | |
868 | ||
869 | All of this is especially useful when perl is embedded in some other | |
870 | program, where output to STDOUT and STDERR may have to be redirected | |
871 | in some special way. See nvi and the Apache module for examples. | |
a7adf1f0 | 872 | |
a24ded60 | 873 | When tying a handle, the first argument to C<tie> should begin with an |
4a904372 FC |
874 | asterisk. So, if you are tying STDOUT, use C<*STDOUT>. If you have |
875 | assigned it to a scalar variable, say C<$handle>, use C<*$handle>. | |
876 | C<tie $handle> ties the scalar variable C<$handle>, not the handle inside | |
877 | it. | |
a24ded60 | 878 | |
a7adf1f0 | 879 | In our example we're going to create a shouting handle. |
880 | ||
881 | package Shout; | |
882 | ||
13a2d996 | 883 | =over 4 |
a7adf1f0 | 884 | |
885 | =item TIEHANDLE classname, LIST | |
d74e8afc | 886 | X<TIEHANDLE> |
a7adf1f0 | 887 | |
888 | This is the constructor for the class. That means it is expected to | |
184e9718 | 889 | return a blessed reference of some sort. The reference can be used to |
5f05dabc | 890 | hold some internal information. |
a7adf1f0 | 891 | |
7e1af8bc | 892 | sub TIEHANDLE { print "<shout>\n"; my $i; bless \$i, shift } |
a7adf1f0 | 893 | |
1d603a67 | 894 | =item WRITE this, LIST |
d74e8afc | 895 | X<WRITE> |
1d603a67 GB |
896 | |
897 | This method will be called when the handle is written to via the | |
898 | C<syswrite> function. | |
899 | ||
900 | sub WRITE { | |
901 | $r = shift; | |
902 | my($buf,$len,$offset) = @_; | |
903 | print "WRITE called, \$buf=$buf, \$len=$len, \$offset=$offset"; | |
904 | } | |
905 | ||
a7adf1f0 | 906 | =item PRINT this, LIST |
d74e8afc | 907 | X<PRINT> |
a7adf1f0 | 908 | |
46fc3d4c | 909 | This method will be triggered every time the tied handle is printed to |
3a28f3fb MS |
910 | with the C<print()> or C<say()> functions. Beyond its self reference |
911 | it also expects the list that was passed to the print function. | |
a7adf1f0 | 912 | |
58f51617 SV |
913 | sub PRINT { $r = shift; $$r++; print join($,,map(uc($_),@_)),$\ } |
914 | ||
3a28f3fb MS |
915 | C<say()> acts just like C<print()> except $\ will be localized to C<\n> so |
916 | you need do nothing special to handle C<say()> in C<PRINT()>. | |
917 | ||
46fc3d4c | 918 | =item PRINTF this, LIST |
d74e8afc | 919 | X<PRINTF> |
46fc3d4c | 920 | |
921 | This method will be triggered every time the tied handle is printed to | |
922 | with the C<printf()> function. | |
923 | Beyond its self reference it also expects the format and list that was | |
924 | passed to the printf function. | |
925 | ||
926 | sub PRINTF { | |
927 | shift; | |
928 | my $fmt = shift; | |
7687bb23 | 929 | print sprintf($fmt, @_); |
46fc3d4c | 930 | } |
931 | ||
1d603a67 | 932 | =item READ this, LIST |
d74e8afc | 933 | X<READ> |
2ae324a7 | 934 | |
935 | This method will be called when the handle is read from via the C<read> | |
936 | or C<sysread> functions. | |
937 | ||
938 | sub READ { | |
889a76e8 | 939 | my $self = shift; |
69801a40 | 940 | my $bufref = \$_[0]; |
889a76e8 GS |
941 | my(undef,$len,$offset) = @_; |
942 | print "READ called, \$buf=$bufref, \$len=$len, \$offset=$offset"; | |
943 | # add to $$bufref, set $len to number of characters read | |
944 | $len; | |
2ae324a7 | 945 | } |
946 | ||
58f51617 | 947 | =item READLINE this |
d74e8afc | 948 | X<READLINE> |
58f51617 | 949 | |
2207fa4e KR |
950 | This method is called when the handle is read via C<E<lt>HANDLEE<gt>> |
951 | or C<readline HANDLE>. | |
952 | ||
26f1b91d FC |
953 | As per L<C<readline>|perlfunc/readline>, in scalar context it should return |
954 | the next line, or C<undef> for no more data. In list context it should | |
955 | return all remaining lines, or an empty list for no more data. The strings | |
956 | returned should include the input record separator C<$/> (see L<perlvar>), | |
957 | unless it is C<undef> (which means "slurp" mode). | |
2207fa4e KR |
958 | |
959 | sub READLINE { | |
960 | my $r = shift; | |
961 | if (wantarray) { | |
962 | return ("all remaining\n", | |
963 | "lines up\n", | |
964 | "to eof\n"); | |
965 | } else { | |
26f1b91d | 966 | return "READLINE called " . ++$$r . " times\n"; |
2207fa4e KR |
967 | } |
968 | } | |
a7adf1f0 | 969 | |
2ae324a7 | 970 | =item GETC this |
d74e8afc | 971 | X<GETC> |
2ae324a7 | 972 | |
973 | This method will be called when the C<getc> function is called. | |
974 | ||
975 | sub GETC { print "Don't GETC, Get Perl"; return "a"; } | |
976 | ||
32e65323 CS |
977 | =item EOF this |
978 | X<EOF> | |
979 | ||
980 | This method will be called when the C<eof> function is called. | |
981 | ||
982 | Starting with Perl 5.12, an additional integer parameter will be passed. It | |
983 | will be zero if C<eof> is called without parameter; C<1> if C<eof> is given | |
984 | a filehandle as a parameter, e.g. C<eof(FH)>; and C<2> in the very special | |
985 | case that the tied filehandle is C<ARGV> and C<eof> is called with an empty | |
986 | parameter list, e.g. C<eof()>. | |
987 | ||
988 | sub EOF { not length $stringbuf } | |
989 | ||
1d603a67 | 990 | =item CLOSE this |
d74e8afc | 991 | X<CLOSE> |
1d603a67 GB |
992 | |
993 | This method will be called when the handle is closed via the C<close> | |
994 | function. | |
995 | ||
996 | sub CLOSE { print "CLOSE called.\n" } | |
997 | ||
301e8125 | 998 | =item UNTIE this |
d74e8afc | 999 | X<UNTIE> |
301e8125 NIS |
1000 | |
1001 | As with the other types of ties, this method will be called when C<untie> happens. | |
d5582e24 IZ |
1002 | It may be appropriate to "auto CLOSE" when this occurs. See |
1003 | L<The C<untie> Gotcha> below. | |
301e8125 | 1004 | |
a7adf1f0 | 1005 | =item DESTROY this |
d74e8afc | 1006 | X<DESTROY> |
a7adf1f0 | 1007 | |
1008 | As with the other types of ties, this method will be called when the | |
1009 | tied handle is about to be destroyed. This is useful for debugging and | |
1010 | possibly cleaning up. | |
1011 | ||
1012 | sub DESTROY { print "</shout>\n" } | |
1013 | ||
1014 | =back | |
1015 | ||
1016 | Here's how to use our little example: | |
1017 | ||
1018 | tie(*FOO,'Shout'); | |
1019 | print FOO "hello\n"; | |
1020 | $a = 4; $b = 6; | |
1021 | print FOO $a, " plus ", $b, " equals ", $a + $b, "\n"; | |
58f51617 | 1022 | print <FOO>; |
cb1a09d0 | 1023 | |
d7da42b7 | 1024 | =head2 UNTIE this |
d74e8afc | 1025 | X<UNTIE> |
d7da42b7 JH |
1026 | |
1027 | You can define for all tie types an UNTIE method that will be called | |
d5582e24 | 1028 | at untie(). See L<The C<untie> Gotcha> below. |
d7da42b7 | 1029 | |
2752eb9f | 1030 | =head2 The C<untie> Gotcha |
d74e8afc | 1031 | X<untie> |
2752eb9f PM |
1032 | |
1033 | If you intend making use of the object returned from either tie() or | |
1034 | tied(), and if the tie's target class defines a destructor, there is a | |
1035 | subtle gotcha you I<must> guard against. | |
1036 | ||
1037 | As setup, consider this (admittedly rather contrived) example of a | |
1038 | tie; all it does is use a file to keep a log of the values assigned to | |
1039 | a scalar. | |
1040 | ||
1041 | package Remember; | |
1042 | ||
1043 | use strict; | |
9f1b1f2d | 1044 | use warnings; |
2752eb9f PM |
1045 | use IO::File; |
1046 | ||
1047 | sub TIESCALAR { | |
1048 | my $class = shift; | |
1049 | my $filename = shift; | |
63acfd00 | 1050 | my $handle = IO::File->new( "> $filename" ) |
2752eb9f PM |
1051 | or die "Cannot open $filename: $!\n"; |
1052 | ||
1053 | print $handle "The Start\n"; | |
1054 | bless {FH => $handle, Value => 0}, $class; | |
1055 | } | |
1056 | ||
1057 | sub FETCH { | |
1058 | my $self = shift; | |
1059 | return $self->{Value}; | |
1060 | } | |
1061 | ||
1062 | sub STORE { | |
1063 | my $self = shift; | |
1064 | my $value = shift; | |
1065 | my $handle = $self->{FH}; | |
1066 | print $handle "$value\n"; | |
1067 | $self->{Value} = $value; | |
1068 | } | |
1069 | ||
1070 | sub DESTROY { | |
1071 | my $self = shift; | |
1072 | my $handle = $self->{FH}; | |
1073 | print $handle "The End\n"; | |
1074 | close $handle; | |
1075 | } | |
1076 | ||
1077 | 1; | |
1078 | ||
1079 | Here is an example that makes use of this tie: | |
1080 | ||
1081 | use strict; | |
1082 | use Remember; | |
1083 | ||
1084 | my $fred; | |
1085 | tie $fred, 'Remember', 'myfile.txt'; | |
1086 | $fred = 1; | |
1087 | $fred = 4; | |
1088 | $fred = 5; | |
1089 | untie $fred; | |
1090 | system "cat myfile.txt"; | |
1091 | ||
1092 | This is the output when it is executed: | |
1093 | ||
1094 | The Start | |
1095 | 1 | |
1096 | 4 | |
1097 | 5 | |
1098 | The End | |
1099 | ||
1100 | So far so good. Those of you who have been paying attention will have | |
1101 | spotted that the tied object hasn't been used so far. So lets add an | |
1102 | extra method to the Remember class to allow comments to be included in | |
ac036724 | 1103 | the file; say, something like this: |
2752eb9f PM |
1104 | |
1105 | sub comment { | |
1106 | my $self = shift; | |
1107 | my $text = shift; | |
1108 | my $handle = $self->{FH}; | |
1109 | print $handle $text, "\n"; | |
1110 | } | |
1111 | ||
1112 | And here is the previous example modified to use the C<comment> method | |
1113 | (which requires the tied object): | |
1114 | ||
1115 | use strict; | |
1116 | use Remember; | |
1117 | ||
1118 | my ($fred, $x); | |
1119 | $x = tie $fred, 'Remember', 'myfile.txt'; | |
1120 | $fred = 1; | |
1121 | $fred = 4; | |
1122 | comment $x "changing..."; | |
1123 | $fred = 5; | |
1124 | untie $fred; | |
1125 | system "cat myfile.txt"; | |
1126 | ||
1127 | When this code is executed there is no output. Here's why: | |
1128 | ||
1129 | When a variable is tied, it is associated with the object which is the | |
1130 | return value of the TIESCALAR, TIEARRAY, or TIEHASH function. This | |
1131 | object normally has only one reference, namely, the implicit reference | |
1132 | from the tied variable. When untie() is called, that reference is | |
1133 | destroyed. Then, as in the first example above, the object's | |
1134 | destructor (DESTROY) is called, which is normal for objects that have | |
1135 | no more valid references; and thus the file is closed. | |
1136 | ||
1137 | In the second example, however, we have stored another reference to | |
19799a22 | 1138 | the tied object in $x. That means that when untie() gets called |
2752eb9f PM |
1139 | there will still be a valid reference to the object in existence, so |
1140 | the destructor is not called at that time, and thus the file is not | |
1141 | closed. The reason there is no output is because the file buffers | |
1142 | have not been flushed to disk. | |
1143 | ||
1144 | Now that you know what the problem is, what can you do to avoid it? | |
301e8125 NIS |
1145 | Prior to the introduction of the optional UNTIE method the only way |
1146 | was the good old C<-w> flag. Which will spot any instances where you call | |
2752eb9f | 1147 | untie() and there are still valid references to the tied object. If |
9f1b1f2d GS |
1148 | the second script above this near the top C<use warnings 'untie'> |
1149 | or was run with the C<-w> flag, Perl prints this | |
2752eb9f PM |
1150 | warning message: |
1151 | ||
1152 | untie attempted while 1 inner references still exist | |
1153 | ||
1154 | To get the script to work properly and silence the warning make sure | |
1155 | there are no valid references to the tied object I<before> untie() is | |
1156 | called: | |
1157 | ||
1158 | undef $x; | |
1159 | untie $fred; | |
1160 | ||
301e8125 NIS |
1161 | Now that UNTIE exists the class designer can decide which parts of the |
1162 | class functionality are really associated with C<untie> and which with | |
1163 | the object being destroyed. What makes sense for a given class depends | |
1164 | on whether the inner references are being kept so that non-tie-related | |
1165 | methods can be called on the object. But in most cases it probably makes | |
1166 | sense to move the functionality that would have been in DESTROY to the UNTIE | |
1167 | method. | |
1168 | ||
1169 | If the UNTIE method exists then the warning above does not occur. Instead the | |
1170 | UNTIE method is passed the count of "extra" references and can issue its own | |
1171 | warning if appropriate. e.g. to replicate the no UNTIE case this method can | |
1172 | be used: | |
1173 | ||
1174 | sub UNTIE | |
1175 | { | |
1176 | my ($obj,$count) = @_; | |
1177 | carp "untie attempted while $count inner references still exist" if $count; | |
1178 | } | |
1179 | ||
cb1a09d0 AD |
1180 | =head1 SEE ALSO |
1181 | ||
1182 | See L<DB_File> or L<Config> for some interesting tie() implementations. | |
3d0ae7ba GS |
1183 | A good starting point for many tie() implementations is with one of the |
1184 | modules L<Tie::Scalar>, L<Tie::Array>, L<Tie::Hash>, or L<Tie::Handle>. | |
cb1a09d0 AD |
1185 | |
1186 | =head1 BUGS | |
1187 | ||
029149a3 JH |
1188 | The bucket usage information provided by C<scalar(%hash)> is not |
1189 | available. What this means is that using %tied_hash in boolean | |
1190 | context doesn't work right (currently this always tests false, | |
1191 | regardless of whether the hash is empty or hash elements). | |
1192 | ||
1193 | Localizing tied arrays or hashes does not work. After exiting the | |
1194 | scope the arrays or the hashes are not restored. | |
1195 | ||
e77edca3 JH |
1196 | Counting the number of entries in a hash via C<scalar(keys(%hash))> |
1197 | or C<scalar(values(%hash)>) is inefficient since it needs to iterate | |
1198 | through all the entries with FIRSTKEY/NEXTKEY. | |
1199 | ||
1200 | Tied hash/array slices cause multiple FETCH/STORE pairs, there are no | |
1201 | tie methods for slice operations. | |
1202 | ||
c07a80fd | 1203 | You cannot easily tie a multilevel data structure (such as a hash of |
1204 | hashes) to a dbm file. The first problem is that all but GDBM and | |
1205 | Berkeley DB have size limitations, but beyond that, you also have problems | |
bebf870e | 1206 | with how references are to be represented on disk. One |
15c110d5 DC |
1207 | module that does attempt to address this need is DBM::Deep. Check your |
1208 | nearest CPAN site as described in L<perlmodlib> for source code. Note | |
1209 | that despite its name, DBM::Deep does not use dbm. Another earlier attempt | |
1210 | at solving the problem is MLDBM, which is also available on the CPAN, but | |
1211 | which has some fairly serious limitations. | |
c07a80fd | 1212 | |
e08f2115 GA |
1213 | Tied filehandles are still incomplete. sysopen(), truncate(), |
1214 | flock(), fcntl(), stat() and -X can't currently be trapped. | |
1215 | ||
cb1a09d0 AD |
1216 | =head1 AUTHOR |
1217 | ||
1218 | Tom Christiansen | |
a7adf1f0 | 1219 | |
46fc3d4c | 1220 | TIEHANDLE by Sven Verdoolaege <F<skimo@dns.ufsia.ac.be>> and Doug MacEachern <F<dougm@osf.org>> |
301e8125 NIS |
1221 | |
1222 | UNTIE by Nick Ing-Simmons <F<nick@ing-simmons.net>> | |
1223 | ||
a3bcc51e TP |
1224 | SCALAR by Tassilo von Parseval <F<tassilo.von.parseval@rwth-aachen.de>> |
1225 | ||
e1e60e72 | 1226 | Tying Arrays by Casey West <F<casey@geeknest.com>> |