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Negative subscripts optionally passed to tied array methods
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1=head1 NAME
2
3perltie - 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
15Prior to release 5.0 of Perl, a programmer could use dbmopen()
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16to connect an on-disk database in the standard Unix dbm(3x)
17format magically to a %HASH in their program. However, their Perl was either
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18built with one particular dbm library or another, but not both, and
19you couldn't extend this mechanism to other packages or types of variables.
20
21Now you can.
22
23The tie() function binds a variable to a class (package) that will provide
24the implementation for access methods for that variable. Once this magic
25has been performed, accessing a tied variable automatically triggers
5a964f20 26method calls in the proper class. The complexity of the class is
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27hidden behind magic methods calls. The method names are in ALL CAPS,
28which is a convention that Perl uses to indicate that they're called
29implicitly rather than explicitly--just like the BEGIN() and END()
30functions.
31
32In the tie() call, C<VARIABLE> is the name of the variable to be
33enchanted. C<CLASSNAME> is the name of a class implementing objects of
34the correct type. Any additional arguments in the C<LIST> are passed to
35the appropriate constructor method for that class--meaning TIESCALAR(),
5f05dabc 36TIEARRAY(), TIEHASH(), or TIEHANDLE(). (Typically these are arguments
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37such as might be passed to the dbminit() function of C.) The object
38returned by the "new" method is also returned by the tie() function,
39which would be useful if you wanted to access other methods in
40C<CLASSNAME>. (You don't actually have to return a reference to a right
5f05dabc 41"type" (e.g., HASH or C<CLASSNAME>) so long as it's a properly blessed
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42object.) You can also retrieve a reference to the underlying object
43using the tied() function.
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44
45Unlike dbmopen(), the tie() function will not C<use> or C<require> a module
46for you--you need to do that explicitly yourself.
47
48=head2 Tying Scalars
49
50A class implementing a tied scalar should define the following methods:
301e8125 51TIESCALAR, FETCH, STORE, and possibly UNTIE and/or DESTROY.
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52
53Let's look at each in turn, using as an example a tie class for
54scalars that allows the user to do something like:
55
56 tie $his_speed, 'Nice', getppid();
57 tie $my_speed, 'Nice', $$;
58
59And now whenever either of those variables is accessed, its current
60system priority is retrieved and returned. If those variables are set,
61then the process's priority is changed!
62
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63We'll use Jarkko Hietaniemi <F<jhi@iki.fi>>'s BSD::Resource class (not
64included) to access the PRIO_PROCESS, PRIO_MIN, and PRIO_MAX constants
65from your system, as well as the getpriority() and setpriority() system
66calls. Here's the preamble of the class.
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67
68 package Nice;
69 use Carp;
70 use BSD::Resource;
71 use strict;
72 $Nice::DEBUG = 0 unless defined $Nice::DEBUG;
73
13a2d996 74=over 4
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75
76=item TIESCALAR classname, LIST
77
78This is the constructor for the class. That means it is
79expected to return a blessed reference to a new scalar
80(probably anonymous) that it's creating. For example:
81
82 sub TIESCALAR {
83 my $class = shift;
84 my $pid = shift || $$; # 0 means me
85
86 if ($pid !~ /^\d+$/) {
6fdf61fb 87 carp "Nice::Tie::Scalar got non-numeric pid $pid" if $^W;
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88 return undef;
89 }
90
91 unless (kill 0, $pid) { # EPERM or ERSCH, no doubt
6fdf61fb 92 carp "Nice::Tie::Scalar got bad pid $pid: $!" if $^W;
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93 return undef;
94 }
95
96 return bless \$pid, $class;
97 }
98
99This tie class has chosen to return an error rather than raising an
100exception if its constructor should fail. While this is how dbmopen() works,
101other classes may well not wish to be so forgiving. It checks the global
102variable C<$^W> to see whether to emit a bit of noise anyway.
103
104=item FETCH this
105
106This 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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108object representing the scalar we're dealing with. Because in this case
109we're using just a SCALAR ref for the tied scalar object, a simple $$self
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110allows the method to get at the real value stored there. In our example
111below, that real value is the process ID to which we've tied our variable.
112
113 sub FETCH {
114 my $self = shift;
115 confess "wrong type" unless ref $self;
116 croak "usage error" if @_;
117 my $nicety;
118 local($!) = 0;
119 $nicety = getpriority(PRIO_PROCESS, $$self);
120 if ($!) { croak "getpriority failed: $!" }
121 return $nicety;
122 }
123
124This time we've decided to blow up (raise an exception) if the renice
125fails--there's no place for us to return an error otherwise, and it's
126probably the right thing to do.
127
128=item STORE this, value
129
130This method will be triggered every time the tied variable is set
131(assigned). Beyond its self reference, it also expects one (and only one)
132argument--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, $$self, $new_nicety)) {
155 confess "setpriority failed: $!";
156 }
157 return $new_nicety;
158 }
159
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160=item UNTIE this
161
162This method will be triggered when the C<untie> occurs. This can be useful
163if the class needs to know when no further calls will be made. (Except DESTROY
d5582e24 164of course.) See L<The C<untie> Gotcha> below for more details.
301e8125 165
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166=item DESTROY this
167
168This method will be triggered when the tied variable needs to be destructed.
5f05dabc 169As with other object classes, such a method is seldom necessary, because Perl
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170deallocates its moribund object's memory for you automatically--this isn't
171C++, you know. We'll use a DESTROY method here for debugging purposes only.
172
173 sub DESTROY {
174 my $self = shift;
175 confess "wrong type" unless ref $self;
176 carp "[ Nice::DESTROY pid $$self ]" if $Nice::DEBUG;
177 }
178
179=back
180
181That's about all there is to it. Actually, it's more than all there
5f05dabc 182is to it, because we've done a few nice things here for the sake
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183of completeness, robustness, and general aesthetics. Simpler
184TIESCALAR classes are certainly possible.
185
186=head2 Tying Arrays
187
188A class implementing a tied ordinary array should define the following
301e8125 189methods: TIEARRAY, FETCH, STORE, FETCHSIZE, STORESIZE and perhaps UNTIE and/or DESTROY.
cb1a09d0 190
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191FETCHSIZE and STORESIZE are used to provide C<$#array> and
192equivalent C<scalar(@array)> access.
c47ff5f1 193
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194The methods POP, PUSH, SHIFT, UNSHIFT, SPLICE, DELETE, and EXISTS are
195required if the perl operator with the corresponding (but lowercase) name
196is to operate on the tied array. The B<Tie::Array> class can be used as a
197base class to implement the first five of these in terms of the basic
198methods above. The default implementations of DELETE and EXISTS in
199B<Tie::Array> simply C<croak>.
a60c0954 200
301e8125 201In addition EXTEND will be called when perl would have pre-extended
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202allocation in a real array.
203
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204For this discussion, we'll implement an array whose elements are a fixed
205size at creation. If you try to create an element larger than the fixed
206size, you'll take an exception. For example:
cb1a09d0 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
213The preamble code for the class is as follows:
214
4ae85618 215 package FixedElem_Array;
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216 use Carp;
217 use strict;
218
13a2d996 219=over 4
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220
221=item TIEARRAY classname, LIST
222
223This is the constructor for the class. That means it is expected to
224return a blessed reference through which the new array (probably an
225anonymous ARRAY ref) will be accessed.
226
227In our example, just to show you that you don't I<really> have to return an
228ARRAY reference, we'll choose a HASH reference to represent our object.
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229A HASH works out well as a generic record type: the C<{ELEMSIZE}> field will
230store the maximum element size allowed, and the C<{ARRAY}> field will hold the
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231true ARRAY ref. If someone outside the class tries to dereference the
232object returned (doubtless thinking it an ARRAY ref), they'll blow up.
233This 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
249This method will be triggered every time an individual element the tied array
250is accessed (read). It takes one argument beyond its self reference: the
251index 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
301e8125 259If a negative array index is used to read from an array, the index
0b931be4 260will be translated to a positive one internally by calling FETCHSIZE
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261before being passed to FETCH. You may disable this feature by
262assigning a true value to the variable C<$NEGATIVE_INDICES> in the
263tied array class.
301e8125 264
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265As you may have noticed, the name of the FETCH method (et al.) is the same
266for all accesses, even though the constructors differ in names (TIESCALAR
267vs TIEARRAY). While in theory you could have the same class servicing
268several tied types, in practice this becomes cumbersome, and it's easiest
5f05dabc 269to keep them at simply one tie type per class.
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270
271=item STORE this, index, value
272
273This method will be triggered every time an element in the tied array is set
274(written). It takes two arguments beyond its self reference: the index at
275which we're trying to store something and the value we're trying to put
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276there.
277
278In our example, C<undef> is really C<$self-E<gt>{ELEMSIZE}> number of
279spaces 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}";
cb1a09d0 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;
cb1a09d0 291 }
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292
293Negative indexes are treated the same as with FETCH.
294
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295=item FETCHSIZE this
296
297Returns the total number of items in the tied array associated with
298object 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
307Sets the total number of items in the tied array associated with
308object I<this> to be I<count>. If this makes the array larger then
309class's mapping of C<undef> should be returned for new positions.
310If the array becomes smaller then entries beyond count should be
311deleted.
312
313In our example, 'undef' is really an element containing
314C<$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
332Informative call that array is likely to grow to have I<count> entries.
333Can be used to optimize allocation. This method need do nothing.
334
335In our example, we want to make sure there are no blank (C<undef>)
336entries, so C<EXTEND> will make use of C<STORESIZE> to fill elements
337as needed:
338
339 sub EXTEND {
340 my $self = shift;
341 my $count = shift;
342 $self->STORESIZE( $count );
343 }
344
345=item EXISTS this, key
346
347Verify that the element at index I<key> exists in the tied array I<this>.
348
349In our example, we will determine that if an element consists of
350C<$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 $self->{ELEMSIZE};
357 return 1;
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358 }
359
360=item DELETE this, key
361
362Delete the element at index I<key> from the tied array I<this>.
363
364In our example, a deleted item is C<$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
374Clear (remove, delete, ...) all values from the tied array associated with
375object I<this>. For example:
376
377 sub CLEAR {
378 my $self = shift;
379 return $self->{ARRAY} = [];
380 }
381
382=item PUSH this, LIST
383
384Append 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
396Remove 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
405Remove the first element of the array (shifting other elements down)
406and 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
415Insert LIST elements at the beginning of the array, moving existing elements
416up 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
430Perform the equivalent of C<splice> on the array.
431
432I<offset> is optional and defaults to zero, negative values count back
433from the end of the array.
434
435I<length> is optional and defaults to rest of the array.
436
437I<LIST> may be empty.
438
439Returns a list of the original I<length> elements at I<offset>.
440
441In 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
d5582e24 457Will be called when C<untie> happens. (See L<The C<untie> Gotcha> below.)
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458
459=item DESTROY this
460
461This method will be triggered when the tied variable needs to be destructed.
184e9718 462As with the scalar tie class, this is almost never needed in a
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463language that does its own garbage collection, so this time we'll
464just leave it out.
465
466=back
467
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468=head2 Tying Hashes
469
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470Hashes were the first Perl data type to be tied (see dbmopen()). A class
471implementing a tied hash should define the following methods: TIEHASH is
472the constructor. FETCH and STORE access the key and value pairs. EXISTS
473reports whether a key is present in the hash, and DELETE deletes one.
474CLEAR empties the hash by deleting all the key and value pairs. FIRSTKEY
475and NEXTKEY implement the keys() and each() functions to iterate over all
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476the keys. UNTIE is called when C<untie> happens, and DESTROY is called when
477the tied variable is garbage collected.
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478
479If this seems like a lot, then feel free to inherit from merely the
d5582e24 480standard Tie::StdHash module for most of your methods, redefining only the
aa689395 481interesting ones. See L<Tie::Hash> for details.
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482
483Remember that Perl distinguishes between a key not existing in the hash,
484and the key existing in the hash but having a corresponding value of
485C<undef>. The two possibilities can be tested with the C<exists()> and
486C<defined()> functions.
487
488Here's an example of a somewhat interesting tied hash class: it gives you
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489a hash representing a particular user's dot files. You index into the hash
490with the name of the file (minus the dot) and you get back that dot file's
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491contents. For example:
492
493 use DotFiles;
1f57c600 494 tie %dot, 'DotFiles';
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495 if ( $dot{profile} =~ /MANPATH/ ||
496 $dot{login} =~ /MANPATH/ ||
497 $dot{cshrc} =~ /MANPATH/ )
498 {
5f05dabc 499 print "you seem to set your MANPATH\n";
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500 }
501
502Or here's another sample of using our tied class:
503
1f57c600 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
510In our tied hash DotFiles example, we use a regular
511hash for the object containing several important
512fields, of which only the C<{LIST}> field will be what the
513user thinks of as the real hash.
514
515=over 5
516
517=item USER
518
519whose dot files this object represents
520
521=item HOME
522
5f05dabc 523where those dot files live
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524
525=item CLOBBER
526
527whether we should try to change or remove those dot files
528
529=item LIST
530
5f05dabc 531the hash of dot file names and content mappings
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532
533=back
534
535Here'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
5f05dabc 543For our example, we want to be able to emit debugging info to help in tracing
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544during development. We keep also one convenience function around
545internally to help print out warnings; whowasi() returns the function name
546that calls it.
547
548Here are the methods for the DotFiles tied hash.
549
13a2d996 550=over 4
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551
552=item TIEHASH classname, LIST
553
554This is the constructor for the class. That means it is expected to
555return a blessed reference through which the new object (probably but not
556necessarily an anonymous hash) will be accessed.
557
558Here'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
587It's probably worth mentioning that if you're going to filetest the
588return values out of a readdir, you'd better prepend the directory
5f05dabc 589in question. Otherwise, because we didn't chdir() there, it would
2ae324a7 590have been testing the wrong file.
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591
592=item FETCH this, key
593
594This method will be triggered every time an element in the tied hash is
595accessed (read). It takes one argument beyond its self reference: the key
596whose value we're trying to fetch.
597
598Here'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
619It was easy to write by having it call the Unix cat(1) command, but it
620would probably be more portable to open the file manually (and somewhat
5f05dabc 621more efficient). Of course, because dot files are a Unixy concept, we're
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622not that concerned.
623
624=item STORE this, key, value
625
626This 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
628which we're trying to store something, and the value we're trying to put
629there.
630
631Here in our DotFiles example, we'll be careful not to let
632them try to overwrite the file unless they've called the clobber()
633method 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
651If 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
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657Another way to lay hands on a reference to the underlying object is to
658use the tied() function, so they might alternately have set clobber
659using:
660
661 tie %daemon_dots, 'daemon';
662 tied(%daemon_dots)->clobber(1);
663
664The clobber method is simply:
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665
666 sub clobber {
667 my $self = shift;
668 $self->{CLOBBER} = @_ ? shift : 1;
669 }
670
671=item DELETE this, key
672
673This method is triggered when we remove an element from the hash,
674typically by using the delete() function. Again, we'll
675be 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};
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686 my $success = unlink($file);
687 carp "@{[&whowasi]}: can't unlink $file: $!" unless $success;
688 $success;
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689 }
690
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691The value returned by DELETE becomes the return value of the call
692to delete(). If you want to emulate the normal behavior of delete(),
693you should return whatever FETCH would have returned for this key.
694In this example, we have chosen instead to return a value which tells
695the caller whether the file was successfully deleted.
696
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697=item CLEAR this
698
699This method is triggered when the whole hash is to be cleared, usually by
700assigning the empty list to it.
701
5f05dabc 702In our example, that would remove all the user's dot files! It's such a
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703dangerous thing that they'll have to set CLOBBER to something higher than
7041 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}"
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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
719This method is triggered when the user uses the exists() function
720on a particular hash. In our example, we'll look at the C<{LIST}>
721hash 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
732This method will be triggered when the user is going
733to iterate through the hash, such as via a keys() or each()
734call.
735
736 sub FIRSTKEY {
737 carp &whowasi if $DEBUG;
738 my $self = shift;
6fdf61fb 739 my $a = keys %{$self->{LIST}}; # reset each() iterator
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740 each %{$self->{LIST}}
741 }
742
743=item NEXTKEY this, lastkey
744
745This method gets triggered during a keys() or each() iteration. It has a
746second argument which is the last key that had been accessed. This is
747useful if you're carrying about ordering or calling the iterator from more
748than one sequence, or not really storing things in a hash anywhere.
749
5f05dabc
PP
750For our example, we're using a real hash so we'll do just the simple
751thing, but we'll have to go through the LIST field indirectly.
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752
753 sub NEXTKEY {
754 carp &whowasi if $DEBUG;
755 my $self = shift;
756 return each %{ $self->{LIST} }
757 }
758
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759=item UNTIE this
760
d5582e24 761This is called when C<untie> occurs. See L<The C<untie> Gotcha> below.
301e8125 762
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763=item DESTROY this
764
765This method is triggered when a tied hash is about to go out of
766scope. You don't really need it unless you're trying to add debugging
767or have auxiliary state to clean up. Here's a very simple function:
768
769 sub DESTROY {
770 carp &whowasi if $DEBUG;
771 }
772
773=back
774
1d2dff63
GS
775Note that functions such as keys() and values() may return huge lists
776when used on large objects, like DBM files. You may prefer to use the
777each() function to iterate over such. Example:
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778
779 # print out history file offsets
780 use NDBM_File;
1f57c600 781 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
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782 while (($key,$val) = each %HIST) {
783 print $key, ' = ', unpack('L',$val), "\n";
784 }
785 untie(%HIST);
786
787=head2 Tying FileHandles
788
184e9718 789This is partially implemented now.
a7adf1f0 790
2ae324a7 791A class implementing a tied filehandle should define the following
1d603a67 792methods: TIEHANDLE, at least one of PRINT, PRINTF, WRITE, READLINE, GETC,
301e8125 793READ, and possibly CLOSE, UNTIE and DESTROY. The class can also provide: BINMODE,
4592e6ca
NIS
794OPEN, EOF, FILENO, SEEK, TELL - if the corresponding perl operators are
795used on the handle.
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PP
796
797It is especially useful when perl is embedded in some other program,
798where output to STDOUT and STDERR may have to be redirected in some
799special way. See nvi and the Apache module for examples.
800
801In our example we're going to create a shouting handle.
802
803 package Shout;
804
13a2d996 805=over 4
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806
807=item TIEHANDLE classname, LIST
808
809This is the constructor for the class. That means it is expected to
184e9718 810return a blessed reference of some sort. The reference can be used to
5f05dabc 811hold some internal information.
a7adf1f0 812
7e1af8bc 813 sub TIEHANDLE { print "<shout>\n"; my $i; bless \$i, shift }
a7adf1f0 814
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815=item WRITE this, LIST
816
817This method will be called when the handle is written to via the
818C<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
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826=item PRINT this, LIST
827
46fc3d4c
PP
828This method will be triggered every time the tied handle is printed to
829with the C<print()> function.
184e9718 830Beyond its self reference it also expects the list that was passed to
a7adf1f0
PP
831the print function.
832
58f51617
SV
833 sub PRINT { $r = shift; $$r++; print join($,,map(uc($_),@_)),$\ }
834
46fc3d4c
PP
835=item PRINTF this, LIST
836
837This method will be triggered every time the tied handle is printed to
838with the C<printf()> function.
839Beyond its self reference it also expects the format and list that was
840passed to the printf function.
841
842 sub PRINTF {
843 shift;
844 my $fmt = shift;
845 print sprintf($fmt, @_)."\n";
846 }
847
1d603a67 848=item READ this, LIST
2ae324a7
PP
849
850This method will be called when the handle is read from via the C<read>
851or C<sysread> functions.
852
853 sub READ {
889a76e8 854 my $self = shift;
69801a40 855 my $bufref = \$_[0];
889a76e8
GS
856 my(undef,$len,$offset) = @_;
857 print "READ called, \$buf=$bufref, \$len=$len, \$offset=$offset";
858 # add to $$bufref, set $len to number of characters read
859 $len;
2ae324a7
PP
860 }
861
58f51617
SV
862=item READLINE this
863
2ae324a7
PP
864This method will be called when the handle is read from via <HANDLE>.
865The 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
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869=item GETC this
870
871This 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
GB
875=item CLOSE this
876
877This method will be called when the handle is closed via the C<close>
878function.
879
880 sub CLOSE { print "CLOSE called.\n" }
881
301e8125
NIS
882=item UNTIE this
883
884As with the other types of ties, this method will be called when C<untie> happens.
d5582e24
IZ
885It may be appropriate to "auto CLOSE" when this occurs. See
886L<The C<untie> Gotcha> below.
301e8125 887
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888=item DESTROY this
889
890As with the other types of ties, this method will be called when the
891tied handle is about to be destroyed. This is useful for debugging and
892possibly cleaning up.
893
894 sub DESTROY { print "</shout>\n" }
895
896=back
897
898Here'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
JH
906=head2 UNTIE this
907
908You can define for all tie types an UNTIE method that will be called
d5582e24 909at untie(). See L<The C<untie> Gotcha> below.
d7da42b7 910
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911=head2 The C<untie> Gotcha
912
913If you intend making use of the object returned from either tie() or
914tied(), and if the tie's target class defines a destructor, there is a
915subtle gotcha you I<must> guard against.
916
917As setup, consider this (admittedly rather contrived) example of a
918tie; all it does is use a file to keep a log of the values assigned to
919a scalar.
920
921 package Remember;
922
923 use strict;
9f1b1f2d 924 use warnings;
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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
959Here 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
972This is the output when it is executed:
973
974 The Start
975 1
976 4
977 5
978 The End
979
980So far so good. Those of you who have been paying attention will have
981spotted that the tied object hasn't been used so far. So lets add an
982extra method to the Remember class to allow comments to be included in
983the 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 $text, "\n";
990 }
991
992And here is the previous example modified to use the C<comment> method
993(which requires the tied object):
994
995 use strict;
996 use Remember;
997
998 my ($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
1007When this code is executed there is no output. Here's why:
1008
1009When a variable is tied, it is associated with the object which is the
1010return value of the TIESCALAR, TIEARRAY, or TIEHASH function. This
1011object normally has only one reference, namely, the implicit reference
1012from the tied variable. When untie() is called, that reference is
1013destroyed. Then, as in the first example above, the object's
1014destructor (DESTROY) is called, which is normal for objects that have
1015no more valid references; and thus the file is closed.
1016
1017In the second example, however, we have stored another reference to
19799a22 1018the tied object in $x. That means that when untie() gets called
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1019there will still be a valid reference to the object in existence, so
1020the destructor is not called at that time, and thus the file is not
1021closed. The reason there is no output is because the file buffers
1022have not been flushed to disk.
1023
1024Now that you know what the problem is, what can you do to avoid it?
301e8125
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1025Prior to the introduction of the optional UNTIE method the only way
1026was the good old C<-w> flag. Which will spot any instances where you call
2752eb9f 1027untie() and there are still valid references to the tied object. If
9f1b1f2d
GS
1028the second script above this near the top C<use warnings 'untie'>
1029or was run with the C<-w> flag, Perl prints this
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1030warning message:
1031
1032 untie attempted while 1 inner references still exist
1033
1034To get the script to work properly and silence the warning make sure
1035there are no valid references to the tied object I<before> untie() is
1036called:
1037
1038 undef $x;
1039 untie $fred;
1040
301e8125
NIS
1041Now that UNTIE exists the class designer can decide which parts of the
1042class functionality are really associated with C<untie> and which with
1043the object being destroyed. What makes sense for a given class depends
1044on whether the inner references are being kept so that non-tie-related
1045methods can be called on the object. But in most cases it probably makes
1046sense to move the functionality that would have been in DESTROY to the UNTIE
1047method.
1048
1049If the UNTIE method exists then the warning above does not occur. Instead the
1050UNTIE method is passed the count of "extra" references and can issue its own
1051warning if appropriate. e.g. to replicate the no UNTIE case this method can
1052be used:
1053
1054 sub UNTIE
1055 {
1056 my ($obj,$count) = @_;
1057 carp "untie attempted while $count inner references still exist" if $count;
1058 }
1059
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1060=head1 SEE ALSO
1061
1062See L<DB_File> or L<Config> for some interesting tie() implementations.
3d0ae7ba
GS
1063A good starting point for many tie() implementations is with one of the
1064modules L<Tie::Scalar>, L<Tie::Array>, L<Tie::Hash>, or L<Tie::Handle>.
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1065
1066=head1 BUGS
1067
c07a80fd
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1068You cannot easily tie a multilevel data structure (such as a hash of
1069hashes) to a dbm file. The first problem is that all but GDBM and
1070Berkeley DB have size limitations, but beyond that, you also have problems
1071with how references are to be represented on disk. One experimental
5f05dabc 1072module that does attempt to address this need partially is the MLDBM
f102b883 1073module. Check your nearest CPAN site as described in L<perlmodlib> for
c07a80fd
PP
1074source code to MLDBM.
1075
e08f2115
GA
1076Tied filehandles are still incomplete. sysopen(), truncate(),
1077flock(), fcntl(), stat() and -X can't currently be trapped.
1078
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1079=head1 AUTHOR
1080
1081Tom Christiansen
a7adf1f0 1082
46fc3d4c 1083TIEHANDLE by Sven Verdoolaege <F<skimo@dns.ufsia.ac.be>> and Doug MacEachern <F<dougm@osf.org>>
301e8125
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1084
1085UNTIE by Nick Ing-Simmons <F<nick@ing-simmons.net>>
1086
e1e60e72 1087Tying Arrays by Casey West <F<casey@geeknest.com>>