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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 | |
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9 | $object = tied VARIABLE |
10 | |
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11 | untie VARIABLE |
12 | |
13 | =head1 DESCRIPTION |
14 | |
15 | Prior to release 5.0 of Perl, a programmer could use dbmopen() |
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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 |
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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 |
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26 | method calls in the proper class. The complexity of the class is |
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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(), |
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36 | TIEARRAY(), TIEHASH(), or TIEHANDLE(). (Typically these are arguments |
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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 |
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41 | "type" (e.g., HASH or C<CLASSNAME>) so long as it's a properly blessed |
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42 | object.) You can also retrieve a reference to the underlying object |
43 | using the tied() function. |
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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: |
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51 | TIESCALAR, FETCH, STORE, and possibly UNTIE and/or DESTROY. |
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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', $$; |
58 | |
59 | And now whenever either of those variables is accessed, its current |
60 | system priority is retrieved and returned. If those variables are set, |
61 | then the process's priority is changed! |
62 | |
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63 | We'll use Jarkko Hietaniemi <F<jhi@iki.fi>>'s BSD::Resource class (not |
64 | included) to access the PRIO_PROCESS, PRIO_MIN, and PRIO_MAX constants |
65 | from your system, as well as the getpriority() and setpriority() system |
66 | calls. Here's the preamble of the class. |
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67 | |
68 | package Nice; |
69 | use Carp; |
70 | use BSD::Resource; |
71 | use strict; |
72 | $Nice::DEBUG = 0 unless defined $Nice::DEBUG; |
73 | |
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74 | =over 4 |
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75 | |
76 | =item TIESCALAR classname, LIST |
77 | |
78 | This is the constructor for the class. That means it is |
79 | expected to return a blessed reference to a new scalar |
80 | (probably anonymous) that it's creating. For example: |
81 | |
82 | sub TIESCALAR { |
83 | my $class = shift; |
84 | my $pid = shift || $$; # 0 means me |
85 | |
86 | if ($pid !~ /^\d+$/) { |
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87 | carp "Nice::Tie::Scalar got non-numeric pid $pid" if $^W; |
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88 | return undef; |
89 | } |
90 | |
91 | unless (kill 0, $pid) { # EPERM or ERSCH, no doubt |
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92 | carp "Nice::Tie::Scalar got bad pid $pid: $!" if $^W; |
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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 |
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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 $$self |
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110 | allows the method to get at the real value stored there. In our example |
111 | below, that real value is the process ID to which we've tied our variable. |
112 | |
113 | sub FETCH { |
114 | my $self = shift; |
115 | confess "wrong type" unless ref $self; |
116 | croak "usage error" if @_; |
117 | my $nicety; |
118 | local($!) = 0; |
119 | $nicety = getpriority(PRIO_PROCESS, $$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, $$self, $new_nicety)) { |
155 | confess "setpriority failed: $!"; |
156 | } |
157 | return $new_nicety; |
158 | } |
159 | |
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160 | =item UNTIE this |
161 | |
162 | This method will be triggered when the C<untie> occurs. This can be useful |
163 | if the class needs to know when no further calls will be made. (Except DESTROY |
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164 | of course.) See L<The C<untie> Gotcha> below for more details. |
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165 | |
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166 | =item DESTROY this |
167 | |
168 | This method will be triggered when the tied variable needs to be destructed. |
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169 | As with other object classes, such a method is seldom necessary, because Perl |
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170 | deallocates its moribund object's memory for you automatically--this isn't |
171 | C++, you know. We'll use a DESTROY method here for debugging purposes only. |
172 | |
173 | sub DESTROY { |
174 | my $self = shift; |
175 | confess "wrong type" unless ref $self; |
176 | carp "[ Nice::DESTROY pid $$self ]" if $Nice::DEBUG; |
177 | } |
178 | |
179 | =back |
180 | |
181 | That's about all there is to it. Actually, it's more than all there |
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182 | is to it, because we've done a few nice things here for the sake |
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183 | of completeness, robustness, and general aesthetics. Simpler |
184 | TIESCALAR classes are certainly possible. |
185 | |
186 | =head2 Tying Arrays |
187 | |
188 | A class implementing a tied ordinary array should define the following |
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189 | methods: TIEARRAY, FETCH, STORE, FETCHSIZE, STORESIZE and perhaps UNTIE and/or DESTROY. |
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190 | |
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191 | FETCHSIZE and STORESIZE are used to provide C<$#array> and |
192 | equivalent C<scalar(@array)> access. |
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193 | |
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194 | The methods POP, PUSH, SHIFT, UNSHIFT, SPLICE, DELETE, and EXISTS are |
195 | required if the perl operator with the corresponding (but lowercase) name |
196 | is to operate on the tied array. The B<Tie::Array> class can be used as a |
197 | base class to implement the first five of these in terms of the basic |
198 | methods above. The default implementations of DELETE and EXISTS in |
199 | B<Tie::Array> simply C<croak>. |
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200 | |
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201 | In addition EXTEND will be called when perl would have pre-extended |
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202 | allocation in a real array. |
203 | |
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204 | For this discussion, we'll implement an array whose elements are a fixed |
205 | size at creation. If you try to create an element larger than the fixed |
206 | size, you'll take an exception. For example: |
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207 | |
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208 | use FixedElem_Array; |
209 | tie @array, 'FixedElem_Array', 3; |
210 | $array[0] = 'cat'; # ok. |
211 | $array[1] = 'dogs'; # exception, length('dogs') > 3. |
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212 | |
213 | The preamble code for the class is as follows: |
214 | |
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215 | package FixedElem_Array; |
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216 | use Carp; |
217 | use strict; |
218 | |
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219 | =over 4 |
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220 | |
221 | =item TIEARRAY classname, LIST |
222 | |
223 | This is the constructor for the class. That means it is expected to |
224 | return a blessed reference through which the new array (probably an |
225 | anonymous ARRAY ref) will be accessed. |
226 | |
227 | In our example, just to show you that you don't I<really> have to return an |
228 | ARRAY reference, we'll choose a HASH reference to represent our object. |
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229 | A HASH works out well as a generic record type: the C<{ELEMSIZE}> field will |
230 | store the maximum element size allowed, and the C<{ARRAY}> field will hold the |
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231 | true ARRAY ref. If someone outside the class tries to dereference the |
232 | object returned (doubtless thinking it an ARRAY ref), they'll blow up. |
233 | This just goes to show you that you should respect an object's privacy. |
234 | |
235 | sub TIEARRAY { |
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236 | my $class = shift; |
237 | my $elemsize = shift; |
238 | if ( @_ || $elemsize =~ /\D/ ) { |
239 | croak "usage: tie ARRAY, '" . __PACKAGE__ . "', elem_size"; |
240 | } |
241 | return bless { |
242 | ELEMSIZE => $elemsize, |
243 | ARRAY => [], |
244 | }, $class; |
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245 | } |
246 | |
247 | =item FETCH this, index |
248 | |
249 | This method will be triggered every time an individual element the tied array |
250 | is accessed (read). It takes one argument beyond its self reference: the |
251 | index whose value we're trying to fetch. |
252 | |
253 | sub FETCH { |
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254 | my $self = shift; |
255 | my $index = shift; |
256 | return $self->{ARRAY}->[$index]; |
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257 | } |
258 | |
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259 | If a negative array index is used to read from an array, the index |
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260 | will be translated to a positive one internally by calling FETCHSIZE |
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261 | before being passed to FETCH. You may disable this feature by |
262 | assigning a true value to the variable C<$NEGATIVE_INDICES> in the |
263 | tied array class. |
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264 | |
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265 | As you may have noticed, the name of the FETCH method (et al.) is the same |
266 | for all accesses, even though the constructors differ in names (TIESCALAR |
267 | vs TIEARRAY). While in theory you could have the same class servicing |
268 | several tied types, in practice this becomes cumbersome, and it's easiest |
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269 | to keep them at simply one tie type per class. |
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270 | |
271 | =item STORE this, index, value |
272 | |
273 | This method will be triggered every time an element in the tied array is set |
274 | (written). It takes two arguments beyond its self reference: the index at |
275 | which we're trying to store something and the value we're trying to put |
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276 | there. |
277 | |
278 | In our example, C<undef> is really C<$self-E<gt>{ELEMSIZE}> number of |
279 | spaces so we have a little more work to do here: |
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280 | |
281 | sub STORE { |
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282 | my $self = shift; |
283 | my( $index, $value ) = @_; |
284 | if ( length $value > $self->{ELEMSIZE} ) { |
285 | croak "length of $value is greater than $self->{ELEMSIZE}"; |
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286 | } |
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287 | # fill in the blanks |
288 | $self->EXTEND( $index ) if $index > $self->FETCHSIZE(); |
289 | # right justify to keep element size for smaller elements |
290 | $self->{ARRAY}->[$index] = sprintf "%$self->{ELEMSIZE}s", $value; |
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291 | } |
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292 | |
293 | Negative indexes are treated the same as with FETCH. |
294 | |
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295 | =item FETCHSIZE this |
296 | |
297 | Returns the total number of items in the tied array associated with |
298 | object I<this>. (Equivalent to C<scalar(@array)>). For example: |
299 | |
300 | sub FETCHSIZE { |
301 | my $self = shift; |
302 | return scalar @{$self->{ARRAY}}; |
303 | } |
304 | |
305 | =item STORESIZE this, count |
306 | |
307 | Sets the total number of items in the tied array associated with |
308 | object I<this> to be I<count>. If this makes the array larger then |
309 | class's mapping of C<undef> should be returned for new positions. |
310 | If the array becomes smaller then entries beyond count should be |
311 | deleted. |
312 | |
313 | In our example, 'undef' is really an element containing |
314 | C<$self-E<gt>{ELEMSIZE}> number of spaces. Observe: |
315 | |
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316 | sub STORESIZE { |
317 | my $self = shift; |
318 | my $count = shift; |
319 | if ( $count > $self->FETCHSIZE() ) { |
320 | foreach ( $count - $self->FETCHSIZE() .. $count ) { |
321 | $self->STORE( $_, '' ); |
322 | } |
323 | } elsif ( $count < $self->FETCHSIZE() ) { |
324 | foreach ( 0 .. $self->FETCHSIZE() - $count - 2 ) { |
325 | $self->POP(); |
326 | } |
327 | } |
328 | } |
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329 | |
330 | =item EXTEND this, count |
331 | |
332 | Informative call that array is likely to grow to have I<count> entries. |
333 | Can be used to optimize allocation. This method need do nothing. |
334 | |
335 | In our example, we want to make sure there are no blank (C<undef>) |
336 | entries, so C<EXTEND> will make use of C<STORESIZE> to fill elements |
337 | as needed: |
338 | |
339 | sub EXTEND { |
340 | my $self = shift; |
341 | my $count = shift; |
342 | $self->STORESIZE( $count ); |
343 | } |
344 | |
345 | =item EXISTS this, key |
346 | |
347 | Verify that the element at index I<key> exists in the tied array I<this>. |
348 | |
349 | In our example, we will determine that if an element consists of |
350 | C<$self-E<gt>{ELEMSIZE}> spaces only, it does not exist: |
351 | |
352 | sub EXISTS { |
353 | my $self = shift; |
354 | my $index = shift; |
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355 | return 0 if ! defined $self->{ARRAY}->[$index] || |
356 | $self->{ARRAY}->[$index] eq ' ' x $self->{ELEMSIZE}; |
357 | return 1; |
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358 | } |
359 | |
360 | =item DELETE this, key |
361 | |
362 | Delete the element at index I<key> from the tied array I<this>. |
363 | |
364 | In our example, a deleted item is C<$self->{ELEMSIZE}> spaces: |
365 | |
366 | sub DELETE { |
367 | my $self = shift; |
368 | my $index = shift; |
369 | return $self->STORE( $index, '' ); |
370 | } |
371 | |
372 | =item CLEAR this |
373 | |
374 | Clear (remove, delete, ...) all values from the tied array associated with |
375 | object I<this>. For example: |
376 | |
377 | sub CLEAR { |
378 | my $self = shift; |
379 | return $self->{ARRAY} = []; |
380 | } |
381 | |
382 | =item PUSH this, LIST |
383 | |
384 | Append elements of I<LIST> to the array. For example: |
385 | |
386 | sub PUSH { |
387 | my $self = shift; |
388 | my @list = @_; |
389 | my $last = $self->FETCHSIZE(); |
390 | $self->STORE( $last + $_, $list[$_] ) foreach 0 .. $#list; |
391 | return $self->FETCHSIZE(); |
392 | } |
393 | |
394 | =item POP this |
395 | |
396 | Remove last element of the array and return it. For example: |
397 | |
398 | sub POP { |
399 | my $self = shift; |
400 | return pop @{$self->{ARRAY}}; |
401 | } |
402 | |
403 | =item SHIFT this |
404 | |
405 | Remove the first element of the array (shifting other elements down) |
406 | and return it. For example: |
407 | |
408 | sub SHIFT { |
409 | my $self = shift; |
410 | return shift @{$self->{ARRAY}}; |
411 | } |
412 | |
413 | =item UNSHIFT this, LIST |
414 | |
415 | Insert LIST elements at the beginning of the array, moving existing elements |
416 | up to make room. For example: |
417 | |
418 | sub UNSHIFT { |
419 | my $self = shift; |
420 | my @list = @_; |
421 | my $size = scalar( @list ); |
422 | # make room for our list |
423 | @{$self->{ARRAY}}[ $size .. $#{$self->{ARRAY}} + $size ] |
424 | = @{$self->{ARRAY}}; |
425 | $self->STORE( $_, $list[$_] ) foreach 0 .. $#list; |
426 | } |
427 | |
428 | =item SPLICE this, offset, length, LIST |
429 | |
430 | Perform the equivalent of C<splice> on the array. |
431 | |
432 | I<offset> is optional and defaults to zero, negative values count back |
433 | from the end of the array. |
434 | |
435 | I<length> is optional and defaults to rest of the array. |
436 | |
437 | I<LIST> may be empty. |
438 | |
439 | Returns a list of the original I<length> elements at I<offset>. |
440 | |
441 | In our example, we'll use a little shortcut if there is a I<LIST>: |
442 | |
443 | sub SPLICE { |
444 | my $self = shift; |
445 | my $offset = shift || 0; |
446 | my $length = shift || $self->FETCHSIZE() - $offset; |
447 | my @list = (); |
448 | if ( @_ ) { |
449 | tie @list, __PACKAGE__, $self->{ELEMSIZE}; |
450 | @list = @_; |
451 | } |
452 | return splice @{$self->{ARRAY}}, $offset, $length, @list; |
453 | } |
454 | |
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455 | =item UNTIE this |
456 | |
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457 | Will be called when C<untie> happens. (See L<The C<untie> Gotcha> below.) |
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458 | |
459 | =item DESTROY this |
460 | |
461 | This method will be triggered when the tied variable needs to be destructed. |
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462 | As with the scalar tie class, this is almost never needed in a |
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463 | language that does its own garbage collection, so this time we'll |
464 | just leave it out. |
465 | |
466 | =back |
467 | |
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468 | =head2 Tying Hashes |
469 | |
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470 | Hashes were the first Perl data type to be tied (see dbmopen()). A class |
471 | implementing a tied hash should define the following methods: TIEHASH is |
472 | the constructor. FETCH and STORE access the key and value pairs. EXISTS |
473 | reports whether a key is present in the hash, and DELETE deletes one. |
474 | CLEAR empties the hash by deleting all the key and value pairs. FIRSTKEY |
475 | and NEXTKEY implement the keys() and each() functions to iterate over all |
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476 | the keys. UNTIE is called when C<untie> happens, and DESTROY is called when |
477 | the tied variable is garbage collected. |
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478 | |
479 | If this seems like a lot, then feel free to inherit from merely the |
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480 | standard Tie::StdHash module for most of your methods, redefining only the |
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481 | interesting ones. See L<Tie::Hash> for details. |
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482 | |
483 | Remember that Perl distinguishes between a key not existing in the hash, |
484 | and the key existing in the hash but having a corresponding value of |
485 | C<undef>. The two possibilities can be tested with the C<exists()> and |
486 | C<defined()> functions. |
487 | |
488 | Here's an example of a somewhat interesting tied hash class: it gives you |
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489 | a hash representing a particular user's dot files. You index into the hash |
490 | with the name of the file (minus the dot) and you get back that dot file's |
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491 | contents. For example: |
492 | |
493 | use DotFiles; |
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494 | tie %dot, 'DotFiles'; |
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495 | if ( $dot{profile} =~ /MANPATH/ || |
496 | $dot{login} =~ /MANPATH/ || |
497 | $dot{cshrc} =~ /MANPATH/ ) |
498 | { |
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499 | print "you seem to set your MANPATH\n"; |
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500 | } |
501 | |
502 | Or here's another sample of using our tied class: |
503 | |
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504 | tie %him, 'DotFiles', 'daemon'; |
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505 | foreach $f ( keys %him ) { |
506 | printf "daemon dot file %s is size %d\n", |
507 | $f, length $him{$f}; |
508 | } |
509 | |
510 | In our tied hash DotFiles example, we use a regular |
511 | hash for the object containing several important |
512 | fields, of which only the C<{LIST}> field will be what the |
513 | user thinks of as the real hash. |
514 | |
515 | =over 5 |
516 | |
517 | =item USER |
518 | |
519 | whose dot files this object represents |
520 | |
521 | =item HOME |
522 | |
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523 | where those dot files live |
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524 | |
525 | =item CLOBBER |
526 | |
527 | whether we should try to change or remove those dot files |
528 | |
529 | =item LIST |
530 | |
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531 | the hash of dot file names and content mappings |
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532 | |
533 | =back |
534 | |
535 | Here's the start of F<Dotfiles.pm>: |
536 | |
537 | package DotFiles; |
538 | use Carp; |
539 | sub whowasi { (caller(1))[3] . '()' } |
540 | my $DEBUG = 0; |
541 | sub debug { $DEBUG = @_ ? shift : 1 } |
542 | |
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543 | For our example, we want to be able to emit debugging info to help in tracing |
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544 | during development. We keep also one convenience function around |
545 | internally to help print out warnings; whowasi() returns the function name |
546 | that calls it. |
547 | |
548 | Here are the methods for the DotFiles tied hash. |
549 | |
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550 | =over 4 |
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551 | |
552 | =item TIEHASH classname, LIST |
553 | |
554 | This is the constructor for the class. That means it is expected to |
555 | return a blessed reference through which the new object (probably but not |
556 | necessarily an anonymous hash) will be accessed. |
557 | |
558 | Here's the constructor: |
559 | |
560 | sub TIEHASH { |
561 | my $self = shift; |
562 | my $user = shift || $>; |
563 | my $dotdir = shift || ''; |
564 | croak "usage: @{[&whowasi]} [USER [DOTDIR]]" if @_; |
565 | $user = getpwuid($user) if $user =~ /^\d+$/; |
566 | my $dir = (getpwnam($user))[7] |
567 | || croak "@{[&whowasi]}: no user $user"; |
568 | $dir .= "/$dotdir" if $dotdir; |
569 | |
570 | my $node = { |
571 | USER => $user, |
572 | HOME => $dir, |
573 | LIST => {}, |
574 | CLOBBER => 0, |
575 | }; |
576 | |
577 | opendir(DIR, $dir) |
578 | || croak "@{[&whowasi]}: can't opendir $dir: $!"; |
579 | foreach $dot ( grep /^\./ && -f "$dir/$_", readdir(DIR)) { |
580 | $dot =~ s/^\.//; |
581 | $node->{LIST}{$dot} = undef; |
582 | } |
583 | closedir DIR; |
584 | return bless $node, $self; |
585 | } |
586 | |
587 | It's probably worth mentioning that if you're going to filetest the |
588 | return values out of a readdir, you'd better prepend the directory |
5f05dabc |
589 | in question. Otherwise, because we didn't chdir() there, it would |
2ae324a7 |
590 | have been testing the wrong file. |
cb1a09d0 |
591 | |
592 | =item FETCH this, key |
593 | |
594 | This method will be triggered every time an element in the tied hash is |
595 | accessed (read). It takes one argument beyond its self reference: the key |
596 | whose value we're trying to fetch. |
597 | |
598 | Here's the fetch for our DotFiles example. |
599 | |
600 | sub FETCH { |
601 | carp &whowasi if $DEBUG; |
602 | my $self = shift; |
603 | my $dot = shift; |
604 | my $dir = $self->{HOME}; |
605 | my $file = "$dir/.$dot"; |
606 | |
607 | unless (exists $self->{LIST}->{$dot} || -f $file) { |
608 | carp "@{[&whowasi]}: no $dot file" if $DEBUG; |
609 | return undef; |
610 | } |
611 | |
612 | if (defined $self->{LIST}->{$dot}) { |
613 | return $self->{LIST}->{$dot}; |
614 | } else { |
615 | return $self->{LIST}->{$dot} = `cat $dir/.$dot`; |
616 | } |
617 | } |
618 | |
619 | It was easy to write by having it call the Unix cat(1) command, but it |
620 | would probably be more portable to open the file manually (and somewhat |
5f05dabc |
621 | more efficient). Of course, because dot files are a Unixy concept, we're |
cb1a09d0 |
622 | not that concerned. |
623 | |
624 | =item STORE this, key, value |
625 | |
626 | This method will be triggered every time an element in the tied hash is set |
627 | (written). It takes two arguments beyond its self reference: the index at |
628 | which we're trying to store something, and the value we're trying to put |
629 | there. |
630 | |
631 | Here in our DotFiles example, we'll be careful not to let |
632 | them try to overwrite the file unless they've called the clobber() |
633 | method on the original object reference returned by tie(). |
634 | |
635 | sub STORE { |
636 | carp &whowasi if $DEBUG; |
637 | my $self = shift; |
638 | my $dot = shift; |
639 | my $value = shift; |
640 | my $file = $self->{HOME} . "/.$dot"; |
641 | my $user = $self->{USER}; |
642 | |
643 | croak "@{[&whowasi]}: $file not clobberable" |
644 | unless $self->{CLOBBER}; |
645 | |
646 | open(F, "> $file") || croak "can't open $file: $!"; |
647 | print F $value; |
648 | close(F); |
649 | } |
650 | |
651 | If they wanted to clobber something, they might say: |
652 | |
653 | $ob = tie %daemon_dots, 'daemon'; |
654 | $ob->clobber(1); |
655 | $daemon_dots{signature} = "A true daemon\n"; |
656 | |
6fdf61fb |
657 | Another way to lay hands on a reference to the underlying object is to |
658 | use the tied() function, so they might alternately have set clobber |
659 | using: |
660 | |
661 | tie %daemon_dots, 'daemon'; |
662 | tied(%daemon_dots)->clobber(1); |
663 | |
664 | The clobber method is simply: |
cb1a09d0 |
665 | |
666 | sub clobber { |
667 | my $self = shift; |
668 | $self->{CLOBBER} = @_ ? shift : 1; |
669 | } |
670 | |
671 | =item DELETE this, key |
672 | |
673 | This method is triggered when we remove an element from the hash, |
674 | typically by using the delete() function. Again, we'll |
675 | be careful to check whether they really want to clobber files. |
676 | |
677 | sub DELETE { |
678 | carp &whowasi if $DEBUG; |
679 | |
680 | my $self = shift; |
681 | my $dot = shift; |
682 | my $file = $self->{HOME} . "/.$dot"; |
683 | croak "@{[&whowasi]}: won't remove file $file" |
684 | unless $self->{CLOBBER}; |
685 | delete $self->{LIST}->{$dot}; |
1f57c600 |
686 | my $success = unlink($file); |
687 | carp "@{[&whowasi]}: can't unlink $file: $!" unless $success; |
688 | $success; |
cb1a09d0 |
689 | } |
690 | |
1f57c600 |
691 | The value returned by DELETE becomes the return value of the call |
692 | to delete(). If you want to emulate the normal behavior of delete(), |
693 | you should return whatever FETCH would have returned for this key. |
694 | In this example, we have chosen instead to return a value which tells |
695 | the caller whether the file was successfully deleted. |
696 | |
cb1a09d0 |
697 | =item CLEAR this |
698 | |
699 | This method is triggered when the whole hash is to be cleared, usually by |
700 | assigning the empty list to it. |
701 | |
5f05dabc |
702 | In our example, that would remove all the user's dot files! It's such a |
cb1a09d0 |
703 | dangerous thing that they'll have to set CLOBBER to something higher than |
704 | 1 to make it happen. |
705 | |
706 | sub CLEAR { |
707 | carp &whowasi if $DEBUG; |
708 | my $self = shift; |
5f05dabc |
709 | croak "@{[&whowasi]}: won't remove all dot files for $self->{USER}" |
cb1a09d0 |
710 | unless $self->{CLOBBER} > 1; |
711 | my $dot; |
712 | foreach $dot ( keys %{$self->{LIST}}) { |
713 | $self->DELETE($dot); |
714 | } |
715 | } |
716 | |
717 | =item EXISTS this, key |
718 | |
719 | This method is triggered when the user uses the exists() function |
720 | on a particular hash. In our example, we'll look at the C<{LIST}> |
721 | hash element for this: |
722 | |
723 | sub EXISTS { |
724 | carp &whowasi if $DEBUG; |
725 | my $self = shift; |
726 | my $dot = shift; |
727 | return exists $self->{LIST}->{$dot}; |
728 | } |
729 | |
730 | =item FIRSTKEY this |
731 | |
732 | This method will be triggered when the user is going |
733 | to iterate through the hash, such as via a keys() or each() |
734 | call. |
735 | |
736 | sub FIRSTKEY { |
737 | carp &whowasi if $DEBUG; |
738 | my $self = shift; |
6fdf61fb |
739 | my $a = keys %{$self->{LIST}}; # reset each() iterator |
cb1a09d0 |
740 | each %{$self->{LIST}} |
741 | } |
742 | |
743 | =item NEXTKEY this, lastkey |
744 | |
745 | This method gets triggered during a keys() or each() iteration. It has a |
746 | second argument which is the last key that had been accessed. This is |
747 | useful if you're carrying about ordering or calling the iterator from more |
748 | than one sequence, or not really storing things in a hash anywhere. |
749 | |
5f05dabc |
750 | For our example, we're using a real hash so we'll do just the simple |
751 | thing, but we'll have to go through the LIST field indirectly. |
cb1a09d0 |
752 | |
753 | sub NEXTKEY { |
754 | carp &whowasi if $DEBUG; |
755 | my $self = shift; |
756 | return each %{ $self->{LIST} } |
757 | } |
758 | |
301e8125 |
759 | =item UNTIE this |
760 | |
d5582e24 |
761 | This is called when C<untie> occurs. See L<The C<untie> Gotcha> below. |
301e8125 |
762 | |
cb1a09d0 |
763 | =item DESTROY this |
764 | |
765 | This method is triggered when a tied hash is about to go out of |
766 | scope. You don't really need it unless you're trying to add debugging |
767 | or have auxiliary state to clean up. Here's a very simple function: |
768 | |
769 | sub DESTROY { |
770 | carp &whowasi if $DEBUG; |
771 | } |
772 | |
773 | =back |
774 | |
1d2dff63 |
775 | Note that functions such as keys() and values() may return huge lists |
776 | when used on large objects, like DBM files. You may prefer to use the |
777 | each() function to iterate over such. Example: |
cb1a09d0 |
778 | |
779 | # print out history file offsets |
780 | use NDBM_File; |
1f57c600 |
781 | tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0); |
cb1a09d0 |
782 | while (($key,$val) = each %HIST) { |
783 | print $key, ' = ', unpack('L',$val), "\n"; |
784 | } |
785 | untie(%HIST); |
786 | |
787 | =head2 Tying FileHandles |
788 | |
184e9718 |
789 | This is partially implemented now. |
a7adf1f0 |
790 | |
2ae324a7 |
791 | A class implementing a tied filehandle should define the following |
1d603a67 |
792 | methods: TIEHANDLE, at least one of PRINT, PRINTF, WRITE, READLINE, GETC, |
301e8125 |
793 | READ, and possibly CLOSE, UNTIE and DESTROY. The class can also provide: BINMODE, |
4592e6ca |
794 | OPEN, EOF, FILENO, SEEK, TELL - if the corresponding perl operators are |
795 | used on the handle. |
a7adf1f0 |
796 | |
797 | It is especially useful when perl is embedded in some other program, |
798 | where output to STDOUT and STDERR may have to be redirected in some |
799 | special way. See nvi and the Apache module for examples. |
800 | |
801 | In our example we're going to create a shouting handle. |
802 | |
803 | package Shout; |
804 | |
13a2d996 |
805 | =over 4 |
a7adf1f0 |
806 | |
807 | =item TIEHANDLE classname, LIST |
808 | |
809 | This is the constructor for the class. That means it is expected to |
184e9718 |
810 | return a blessed reference of some sort. The reference can be used to |
5f05dabc |
811 | hold some internal information. |
a7adf1f0 |
812 | |
7e1af8bc |
813 | sub TIEHANDLE { print "<shout>\n"; my $i; bless \$i, shift } |
a7adf1f0 |
814 | |
1d603a67 |
815 | =item WRITE this, LIST |
816 | |
817 | This method will be called when the handle is written to via the |
818 | C<syswrite> function. |
819 | |
820 | sub WRITE { |
821 | $r = shift; |
822 | my($buf,$len,$offset) = @_; |
823 | print "WRITE called, \$buf=$buf, \$len=$len, \$offset=$offset"; |
824 | } |
825 | |
a7adf1f0 |
826 | =item PRINT this, LIST |
827 | |
46fc3d4c |
828 | This method will be triggered every time the tied handle is printed to |
829 | with the C<print()> function. |
184e9718 |
830 | Beyond its self reference it also expects the list that was passed to |
a7adf1f0 |
831 | the print function. |
832 | |
58f51617 |
833 | sub PRINT { $r = shift; $$r++; print join($,,map(uc($_),@_)),$\ } |
834 | |
46fc3d4c |
835 | =item PRINTF this, LIST |
836 | |
837 | This method will be triggered every time the tied handle is printed to |
838 | with the C<printf()> function. |
839 | Beyond its self reference it also expects the format and list that was |
840 | passed to the printf function. |
841 | |
842 | sub PRINTF { |
843 | shift; |
844 | my $fmt = shift; |
845 | print sprintf($fmt, @_)."\n"; |
846 | } |
847 | |
1d603a67 |
848 | =item READ this, LIST |
2ae324a7 |
849 | |
850 | This method will be called when the handle is read from via the C<read> |
851 | or C<sysread> functions. |
852 | |
853 | sub READ { |
889a76e8 |
854 | my $self = shift; |
69801a40 |
855 | my $bufref = \$_[0]; |
889a76e8 |
856 | my(undef,$len,$offset) = @_; |
857 | print "READ called, \$buf=$bufref, \$len=$len, \$offset=$offset"; |
858 | # add to $$bufref, set $len to number of characters read |
859 | $len; |
2ae324a7 |
860 | } |
861 | |
58f51617 |
862 | =item READLINE this |
863 | |
2ae324a7 |
864 | This method will be called when the handle is read from via <HANDLE>. |
865 | The method should return undef when there is no more data. |
58f51617 |
866 | |
889a76e8 |
867 | sub READLINE { $r = shift; "READLINE called $$r times\n"; } |
a7adf1f0 |
868 | |
2ae324a7 |
869 | =item GETC this |
870 | |
871 | This method will be called when the C<getc> function is called. |
872 | |
873 | sub GETC { print "Don't GETC, Get Perl"; return "a"; } |
874 | |
1d603a67 |
875 | =item CLOSE this |
876 | |
877 | This method will be called when the handle is closed via the C<close> |
878 | function. |
879 | |
880 | sub CLOSE { print "CLOSE called.\n" } |
881 | |
301e8125 |
882 | =item UNTIE this |
883 | |
884 | As with the other types of ties, this method will be called when C<untie> happens. |
d5582e24 |
885 | It may be appropriate to "auto CLOSE" when this occurs. See |
886 | L<The C<untie> Gotcha> below. |
301e8125 |
887 | |
a7adf1f0 |
888 | =item DESTROY this |
889 | |
890 | As with the other types of ties, this method will be called when the |
891 | tied handle is about to be destroyed. This is useful for debugging and |
892 | possibly cleaning up. |
893 | |
894 | sub DESTROY { print "</shout>\n" } |
895 | |
896 | =back |
897 | |
898 | Here's how to use our little example: |
899 | |
900 | tie(*FOO,'Shout'); |
901 | print FOO "hello\n"; |
902 | $a = 4; $b = 6; |
903 | print FOO $a, " plus ", $b, " equals ", $a + $b, "\n"; |
58f51617 |
904 | print <FOO>; |
cb1a09d0 |
905 | |
d7da42b7 |
906 | =head2 UNTIE this |
907 | |
908 | You can define for all tie types an UNTIE method that will be called |
d5582e24 |
909 | at untie(). See L<The C<untie> Gotcha> below. |
d7da42b7 |
910 | |
2752eb9f |
911 | =head2 The C<untie> Gotcha |
912 | |
913 | If you intend making use of the object returned from either tie() or |
914 | tied(), and if the tie's target class defines a destructor, there is a |
915 | subtle gotcha you I<must> guard against. |
916 | |
917 | As setup, consider this (admittedly rather contrived) example of a |
918 | tie; all it does is use a file to keep a log of the values assigned to |
919 | a scalar. |
920 | |
921 | package Remember; |
922 | |
923 | use strict; |
9f1b1f2d |
924 | use warnings; |
2752eb9f |
925 | use IO::File; |
926 | |
927 | sub TIESCALAR { |
928 | my $class = shift; |
929 | my $filename = shift; |
930 | my $handle = new IO::File "> $filename" |
931 | or die "Cannot open $filename: $!\n"; |
932 | |
933 | print $handle "The Start\n"; |
934 | bless {FH => $handle, Value => 0}, $class; |
935 | } |
936 | |
937 | sub FETCH { |
938 | my $self = shift; |
939 | return $self->{Value}; |
940 | } |
941 | |
942 | sub STORE { |
943 | my $self = shift; |
944 | my $value = shift; |
945 | my $handle = $self->{FH}; |
946 | print $handle "$value\n"; |
947 | $self->{Value} = $value; |
948 | } |
949 | |
950 | sub DESTROY { |
951 | my $self = shift; |
952 | my $handle = $self->{FH}; |
953 | print $handle "The End\n"; |
954 | close $handle; |
955 | } |
956 | |
957 | 1; |
958 | |
959 | Here is an example that makes use of this tie: |
960 | |
961 | use strict; |
962 | use Remember; |
963 | |
964 | my $fred; |
965 | tie $fred, 'Remember', 'myfile.txt'; |
966 | $fred = 1; |
967 | $fred = 4; |
968 | $fred = 5; |
969 | untie $fred; |
970 | system "cat myfile.txt"; |
971 | |
972 | This is the output when it is executed: |
973 | |
974 | The Start |
975 | 1 |
976 | 4 |
977 | 5 |
978 | The End |
979 | |
980 | So far so good. Those of you who have been paying attention will have |
981 | spotted that the tied object hasn't been used so far. So lets add an |
982 | extra method to the Remember class to allow comments to be included in |
983 | the file -- say, something like this: |
984 | |
985 | sub comment { |
986 | my $self = shift; |
987 | my $text = shift; |
988 | my $handle = $self->{FH}; |
989 | print $handle $text, "\n"; |
990 | } |
991 | |
992 | And here is the previous example modified to use the C<comment> method |
993 | (which requires the tied object): |
994 | |
995 | use strict; |
996 | use Remember; |
997 | |
998 | my ($fred, $x); |
999 | $x = tie $fred, 'Remember', 'myfile.txt'; |
1000 | $fred = 1; |
1001 | $fred = 4; |
1002 | comment $x "changing..."; |
1003 | $fred = 5; |
1004 | untie $fred; |
1005 | system "cat myfile.txt"; |
1006 | |
1007 | When this code is executed there is no output. Here's why: |
1008 | |
1009 | When a variable is tied, it is associated with the object which is the |
1010 | return value of the TIESCALAR, TIEARRAY, or TIEHASH function. This |
1011 | object normally has only one reference, namely, the implicit reference |
1012 | from the tied variable. When untie() is called, that reference is |
1013 | destroyed. Then, as in the first example above, the object's |
1014 | destructor (DESTROY) is called, which is normal for objects that have |
1015 | no more valid references; and thus the file is closed. |
1016 | |
1017 | In the second example, however, we have stored another reference to |
19799a22 |
1018 | the tied object in $x. That means that when untie() gets called |
2752eb9f |
1019 | there will still be a valid reference to the object in existence, so |
1020 | the destructor is not called at that time, and thus the file is not |
1021 | closed. The reason there is no output is because the file buffers |
1022 | have not been flushed to disk. |
1023 | |
1024 | Now that you know what the problem is, what can you do to avoid it? |
301e8125 |
1025 | Prior to the introduction of the optional UNTIE method the only way |
1026 | was the good old C<-w> flag. Which will spot any instances where you call |
2752eb9f |
1027 | untie() and there are still valid references to the tied object. If |
9f1b1f2d |
1028 | the second script above this near the top C<use warnings 'untie'> |
1029 | or was run with the C<-w> flag, Perl prints this |
2752eb9f |
1030 | warning message: |
1031 | |
1032 | untie attempted while 1 inner references still exist |
1033 | |
1034 | To get the script to work properly and silence the warning make sure |
1035 | there are no valid references to the tied object I<before> untie() is |
1036 | called: |
1037 | |
1038 | undef $x; |
1039 | untie $fred; |
1040 | |
301e8125 |
1041 | Now that UNTIE exists the class designer can decide which parts of the |
1042 | class functionality are really associated with C<untie> and which with |
1043 | the object being destroyed. What makes sense for a given class depends |
1044 | on whether the inner references are being kept so that non-tie-related |
1045 | methods can be called on the object. But in most cases it probably makes |
1046 | sense to move the functionality that would have been in DESTROY to the UNTIE |
1047 | method. |
1048 | |
1049 | If the UNTIE method exists then the warning above does not occur. Instead the |
1050 | UNTIE method is passed the count of "extra" references and can issue its own |
1051 | warning if appropriate. e.g. to replicate the no UNTIE case this method can |
1052 | be used: |
1053 | |
1054 | sub UNTIE |
1055 | { |
1056 | my ($obj,$count) = @_; |
1057 | carp "untie attempted while $count inner references still exist" if $count; |
1058 | } |
1059 | |
cb1a09d0 |
1060 | =head1 SEE ALSO |
1061 | |
1062 | See L<DB_File> or L<Config> for some interesting tie() implementations. |
3d0ae7ba |
1063 | A good starting point for many tie() implementations is with one of the |
1064 | modules L<Tie::Scalar>, L<Tie::Array>, L<Tie::Hash>, or L<Tie::Handle>. |
cb1a09d0 |
1065 | |
1066 | =head1 BUGS |
1067 | |
c07a80fd |
1068 | You cannot easily tie a multilevel data structure (such as a hash of |
1069 | hashes) to a dbm file. The first problem is that all but GDBM and |
1070 | Berkeley DB have size limitations, but beyond that, you also have problems |
1071 | with how references are to be represented on disk. One experimental |
5f05dabc |
1072 | module that does attempt to address this need partially is the MLDBM |
f102b883 |
1073 | module. Check your nearest CPAN site as described in L<perlmodlib> for |
c07a80fd |
1074 | source code to MLDBM. |
1075 | |
e08f2115 |
1076 | Tied filehandles are still incomplete. sysopen(), truncate(), |
1077 | flock(), fcntl(), stat() and -X can't currently be trapped. |
1078 | |
cb1a09d0 |
1079 | =head1 AUTHOR |
1080 | |
1081 | Tom Christiansen |
a7adf1f0 |
1082 | |
46fc3d4c |
1083 | TIEHANDLE by Sven Verdoolaege <F<skimo@dns.ufsia.ac.be>> and Doug MacEachern <F<dougm@osf.org>> |
301e8125 |
1084 | |
1085 | UNTIE by Nick Ing-Simmons <F<nick@ing-simmons.net>> |
1086 | |
e1e60e72 |
1087 | Tying Arrays by Casey West <F<casey@geeknest.com>> |