3 perltie - how to hide an object class in a simple variable
7 tie VARIABLE, CLASSNAME, LIST
9 $object = tied VARIABLE
15 Prior to release 5.0 of Perl, a programmer could use dbmopen()
16 to magically connect an on-disk database in the standard Unix dbm(3x)
17 format to a %HASH in their program. However, their Perl was either
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.
23 The tie() function binds a variable to a class (package) that will provide
24 the implementation for access methods for that variable. Once this magic
25 has been performed, accessing a tied variable automatically triggers
26 method calls in the proper class. All of the complexity of the class is
27 hidden behind magic methods calls. The method names are in ALL CAPS,
28 which is a convention that Perl uses to indicate that they're called
29 implicitly rather than explicitly--just like the BEGIN() and END()
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(),
36 TIEARRAY(), or TIEHASH(). (Typically these are arguments such as might be
37 passed to the dbminit() function of C.) The object returned by the "new"
38 method is also returned by the tie() function, which would be useful if
39 you wanted to access other methods in C<CLASSNAME>. (You don't actually
40 have to return a reference to a right "type" (e.g. HASH or C<CLASSNAME>)
41 so long as it's a properly blessed object.) You can also retrieve
42 a reference to the underlying object using the tied() function.
44 Unlike dbmopen(), the tie() function will not C<use> or C<require> a module
45 for you--you need to do that explicitly yourself.
49 A class implementing a tied scalar should define the following methods:
50 TIESCALAR, FETCH, STORE, and possibly DESTROY.
52 Let's look at each in turn, using as an example a tie class for
53 scalars that allows the user to do something like:
55 tie $his_speed, 'Nice', getppid();
56 tie $my_speed, 'Nice', $$;
58 And now whenever either of those variables is accessed, its current
59 system priority is retrieved and returned. If those variables are set,
60 then the process's priority is changed!
62 We'll use Jarkko Hietaniemi F<E<lt>Jarkko.Hietaniemi@hut.fiE<gt>>'s
63 BSD::Resource class (not included) to access the PRIO_PROCESS, PRIO_MIN,
64 and PRIO_MAX constants from your system, as well as the getpriority() and
65 setpriority() system calls. Here's the preamble of the class.
71 $Nice::DEBUG = 0 unless defined $Nice::DEBUG;
75 =item TIESCALAR classname, LIST
77 This is the constructor for the class. That means it is
78 expected to return a blessed reference to a new scalar
79 (probably anonymous) that it's creating. For example:
83 my $pid = shift || $$; # 0 means me
85 if ($pid !~ /^\d+$/) {
86 carp "Nice::Tie::Scalar got non-numeric pid $pid" if $^W;
90 unless (kill 0, $pid) { # EPERM or ERSCH, no doubt
91 carp "Nice::Tie::Scalar got bad pid $pid: $!" if $^W;
95 return bless \$pid, $class;
98 This tie class has chosen to return an error rather than raising an
99 exception if its constructor should fail. While this is how dbmopen() works,
100 other classes may well not wish to be so forgiving. It checks the global
101 variable C<$^W> to see whether to emit a bit of noise anyway.
105 This method will be triggered every time the tied variable is accessed
106 (read). It takes no arguments beyond its self reference, which is the
107 object representing the scalar we're dealing with. Since in this case
108 we're just using a SCALAR ref for the tied scalar object, a simple $$self
109 allows the method to get at the real value stored there. In our example
110 below, that real value is the process ID to which we've tied our variable.
114 confess "wrong type" unless ref $self;
115 croak "usage error" if @_;
118 $nicety = getpriority(PRIO_PROCESS, $$self);
119 if ($!) { croak "getpriority failed: $!" }
123 This time we've decided to blow up (raise an exception) if the renice
124 fails--there's no place for us to return an error otherwise, and it's
125 probably the right thing to do.
127 =item STORE this, value
129 This method will be triggered every time the tied variable is set
130 (assigned). Beyond its self reference, it also expects one (and only one)
131 argument--the new value the user is trying to assign.
135 confess "wrong type" unless ref $self;
136 my $new_nicety = shift;
137 croak "usage error" if @_;
139 if ($new_nicety < PRIO_MIN) {
141 "WARNING: priority %d less than minimum system priority %d",
142 $new_nicety, PRIO_MIN if $^W;
143 $new_nicety = PRIO_MIN;
146 if ($new_nicety > PRIO_MAX) {
148 "WARNING: priority %d greater than maximum system priority %d",
149 $new_nicety, PRIO_MAX if $^W;
150 $new_nicety = PRIO_MAX;
153 unless (defined setpriority(PRIO_PROCESS, $$self, $new_nicety)) {
154 confess "setpriority failed: $!";
161 This method will be triggered when the tied variable needs to be destructed.
162 As with other object classes, such a method is seldom ncessary, since Perl
163 deallocates its moribund object's memory for you automatically--this isn't
164 C++, you know. We'll use a DESTROY method here for debugging purposes only.
168 confess "wrong type" unless ref $self;
169 carp "[ Nice::DESTROY pid $$self ]" if $Nice::DEBUG;
174 That's about all there is to it. Actually, it's more than all there
175 is to it, since we've done a few nice things here for the sake
176 of completeness, robustness, and general aesthetics. Simpler
177 TIESCALAR classes are certainly possible.
181 A class implementing a tied ordinary array should define the following
182 methods: TIEARRAY, FETCH, STORE, and perhaps DESTROY.
184 B<WARNING>: Tied arrays are I<incomplete>. They are also distinctly lacking
185 something for the C<$#ARRAY> access (which is hard, as it's an lvalue), as
186 well as the other obvious array functions, like push(), pop(), shift(),
187 unshift(), and splice().
189 For this discussion, we'll implement an array whose indices are fixed at
190 its creation. If you try to access anything beyond those bounds, you'll
191 take an exception. (Well, if you access an individual element; an
192 aggregate assignment would be missed.) For example:
194 require Bounded_Array;
195 tie @ary, Bounded_Array, 2;
198 print "setting index $i: ";
201 print "value of elt $i now $ary[$i]\n";
204 The preamble code for the class is as follows:
206 package Bounded_Array;
212 =item TIEARRAY classname, LIST
214 This is the constructor for the class. That means it is expected to
215 return a blessed reference through which the new array (probably an
216 anonymous ARRAY ref) will be accessed.
218 In our example, just to show you that you don't I<really> have to return an
219 ARRAY reference, we'll choose a HASH reference to represent our object.
220 A HASH works out well as a generic record type: the C<{BOUND}> field will
221 store the maximum bound allowed, and the C<{ARRAY} field will hold the
222 true ARRAY ref. If someone outside the class tries to dereference the
223 object returned (doubtless thinking it an ARRAY ref), they'll blow up.
224 This just goes to show you that you should respect an object's privacy.
229 confess "usage: tie(\@ary, 'Bounded_Array', max_subscript)"
230 if @_ || $bound =~ /\D/;
237 =item FETCH this, index
239 This method will be triggered every time an individual element the tied array
240 is accessed (read). It takes one argument beyond its self reference: the
241 index whose value we're trying to fetch.
245 if ($idx > $self->{BOUND}) {
246 confess "Array OOB: $idx > $self->{BOUND}";
248 return $self->{ARRAY}[$idx];
251 As you may have noticed, the name of the FETCH method (et al.) is the same
252 for all accesses, even though the constructors differ in names (TIESCALAR
253 vs TIEARRAY). While in theory you could have the same class servicing
254 several tied types, in practice this becomes cumbersome, and it's easiest
255 to simply keep them at one tie type per class.
257 =item STORE this, index, value
259 This method will be triggered every time an element in the tied array is set
260 (written). It takes two arguments beyond its self reference: the index at
261 which we're trying to store something and the value we're trying to put
265 my($self, $idx, $value) = @_;
266 print "[STORE $value at $idx]\n" if _debug;
267 if ($idx > $self->{BOUND} ) {
268 confess "Array OOB: $idx > $self->{BOUND}";
270 return $self->{ARRAY}[$idx] = $value;
275 This method will be triggered when the tied variable needs to be destructed.
276 As with the sclar tie class, this is almost never needed in a
277 language that does its own garbage collection, so this time we'll
282 The code we presented at the top of the tied array class accesses many
283 elements of the array, far more than we've set the bounds to. Therefore,
284 it will blow up once they try to access beyond the 2nd element of @ary, as
285 the following output demonstrates:
287 setting index 0: value of elt 0 now 0
288 setting index 1: value of elt 1 now 10
289 setting index 2: value of elt 2 now 20
290 setting index 3: Array OOB: 3 > 2 at Bounded_Array.pm line 39
291 Bounded_Array::FETCH called at testba line 12
295 As the first Perl data type to be tied (see dbmopen()), associative arrays
296 have the most complete and useful tie() implementation. A class
297 implementing a tied associative array should define the following
298 methods: TIEHASH is the constructor. FETCH and STORE access the key and
299 value pairs. EXISTS reports whether a key is present in the hash, and
300 DELETE deletes one. CLEAR empties the hash by deleting all the key and
301 value pairs. FIRSTKEY and NEXTKEY implement the keys() and each()
302 functions to iterate over all the keys. And DESTROY is called when the
303 tied variable is garbage collected.
305 If this seems like a lot, then feel free to merely inherit
306 from the standard Tie::Hash module for most of your methods, redefining only
307 the interesting ones. See L<Tie::Hash> for details.
309 Remember that Perl distinguishes between a key not existing in the hash,
310 and the key existing in the hash but having a corresponding value of
311 C<undef>. The two possibilities can be tested with the C<exists()> and
312 C<defined()> functions.
314 Here's an example of a somewhat interesting tied hash class: it gives you
315 a hash representing a particular user's dotfiles. You index into the hash
316 with the name of the file (minus the dot) and you get back that dotfile's
317 contents. For example:
321 if ( $dot{profile} =~ /MANPATH/ ||
322 $dot{login} =~ /MANPATH/ ||
323 $dot{cshrc} =~ /MANPATH/ )
325 print "you seem to set your manpath\n";
328 Or here's another sample of using our tied class:
330 tie %him, DotFiles, 'daemon';
331 foreach $f ( keys %him ) {
332 printf "daemon dot file %s is size %d\n",
336 In our tied hash DotFiles example, we use a regular
337 hash for the object containing several important
338 fields, of which only the C<{LIST}> field will be what the
339 user thinks of as the real hash.
345 whose dot files this object represents
349 where those dotfiles live
353 whether we should try to change or remove those dot files
357 the hash of dotfile names and content mappings
361 Here's the start of F<Dotfiles.pm>:
365 sub whowasi { (caller(1))[3] . '()' }
367 sub debug { $DEBUG = @_ ? shift : 1 }
369 For our example, we want to able to emit debugging info to help in tracing
370 during development. We keep also one convenience function around
371 internally to help print out warnings; whowasi() returns the function name
374 Here are the methods for the DotFiles tied hash.
378 =item TIEHASH classname, LIST
380 This is the constructor for the class. That means it is expected to
381 return a blessed reference through which the new object (probably but not
382 necessarily an anonymous hash) will be accessed.
384 Here's the constructor:
388 my $user = shift || $>;
389 my $dotdir = shift || '';
390 croak "usage: @{[&whowasi]} [USER [DOTDIR]]" if @_;
391 $user = getpwuid($user) if $user =~ /^\d+$/;
392 my $dir = (getpwnam($user))[7]
393 || croak "@{[&whowasi]}: no user $user";
394 $dir .= "/$dotdir" if $dotdir;
404 || croak "@{[&whowasi]}: can't opendir $dir: $!";
405 foreach $dot ( grep /^\./ && -f "$dir/$_", readdir(DIR)) {
407 $node->{LIST}{$dot} = undef;
410 return bless $node, $self;
413 It's probably worth mentioning that if you're going to filetest the
414 return values out of a readdir, you'd better prepend the directory
415 in question. Otherwise, since we didn't chdir() there, it would
416 have been testing the wrong file.
418 =item FETCH this, key
420 This method will be triggered every time an element in the tied hash is
421 accessed (read). It takes one argument beyond its self reference: the key
422 whose value we're trying to fetch.
424 Here's the fetch for our DotFiles example.
427 carp &whowasi if $DEBUG;
430 my $dir = $self->{HOME};
431 my $file = "$dir/.$dot";
433 unless (exists $self->{LIST}->{$dot} || -f $file) {
434 carp "@{[&whowasi]}: no $dot file" if $DEBUG;
438 if (defined $self->{LIST}->{$dot}) {
439 return $self->{LIST}->{$dot};
441 return $self->{LIST}->{$dot} = `cat $dir/.$dot`;
445 It was easy to write by having it call the Unix cat(1) command, but it
446 would probably be more portable to open the file manually (and somewhat
447 more efficient). Of course, since dot files are a Unixy concept, we're
450 =item STORE this, key, value
452 This method will be triggered every time an element in the tied hash is set
453 (written). It takes two arguments beyond its self reference: the index at
454 which we're trying to store something, and the value we're trying to put
457 Here in our DotFiles example, we'll be careful not to let
458 them try to overwrite the file unless they've called the clobber()
459 method on the original object reference returned by tie().
462 carp &whowasi if $DEBUG;
466 my $file = $self->{HOME} . "/.$dot";
467 my $user = $self->{USER};
469 croak "@{[&whowasi]}: $file not clobberable"
470 unless $self->{CLOBBER};
472 open(F, "> $file") || croak "can't open $file: $!";
477 If they wanted to clobber something, they might say:
479 $ob = tie %daemon_dots, 'daemon';
481 $daemon_dots{signature} = "A true daemon\n";
483 Another way to lay hands on a reference to the underlying object is to
484 use the tied() function, so they might alternately have set clobber
487 tie %daemon_dots, 'daemon';
488 tied(%daemon_dots)->clobber(1);
490 The clobber method is simply:
494 $self->{CLOBBER} = @_ ? shift : 1;
497 =item DELETE this, key
499 This method is triggered when we remove an element from the hash,
500 typically by using the delete() function. Again, we'll
501 be careful to check whether they really want to clobber files.
504 carp &whowasi if $DEBUG;
508 my $file = $self->{HOME} . "/.$dot";
509 croak "@{[&whowasi]}: won't remove file $file"
510 unless $self->{CLOBBER};
511 delete $self->{LIST}->{$dot};
512 unlink($file) || carp "@{[&whowasi]}: can't unlink $file: $!";
517 This method is triggered when the whole hash is to be cleared, usually by
518 assigning the empty list to it.
520 In our example, that would remove all the user's dotfiles! It's such a
521 dangerous thing that they'll have to set CLOBBER to something higher than
525 carp &whowasi if $DEBUG;
527 croak "@{[&whowasi]}: won't remove all dotfiles for $self->{USER}"
528 unless $self->{CLOBBER} > 1;
530 foreach $dot ( keys %{$self->{LIST}}) {
535 =item EXISTS this, key
537 This method is triggered when the user uses the exists() function
538 on a particular hash. In our example, we'll look at the C<{LIST}>
539 hash element for this:
542 carp &whowasi if $DEBUG;
545 return exists $self->{LIST}->{$dot};
550 This method will be triggered when the user is going
551 to iterate through the hash, such as via a keys() or each()
555 carp &whowasi if $DEBUG;
557 my $a = keys %{$self->{LIST}}; # reset each() iterator
558 each %{$self->{LIST}}
561 =item NEXTKEY this, lastkey
563 This method gets triggered during a keys() or each() iteration. It has a
564 second argument which is the last key that had been accessed. This is
565 useful if you're carrying about ordering or calling the iterator from more
566 than one sequence, or not really storing things in a hash anywhere.
568 For our example, we our using a real hash so we'll just do the simple
569 thing, but we'll have to indirect through the LIST field.
572 carp &whowasi if $DEBUG;
574 return each %{ $self->{LIST} }
579 This method is triggered when a tied hash is about to go out of
580 scope. You don't really need it unless you're trying to add debugging
581 or have auxiliary state to clean up. Here's a very simple function:
584 carp &whowasi if $DEBUG;
589 Note that functions such as keys() and values() may return huge array
590 values when used on large objects, like DBM files. You may prefer to
591 use the each() function to iterate over such. Example:
593 # print out history file offsets
595 tie(%HIST, NDBM_File, '/usr/lib/news/history', 1, 0);
596 while (($key,$val) = each %HIST) {
597 print $key, ' = ', unpack('L',$val), "\n";
601 =head2 Tying FileHandles
603 This isn't implemented yet. Sorry; maybe someday.
607 See L<DB_File> or L<Config> for some interesting tie() implementations.
611 Tied arrays are I<incomplete>. They are also distinctly lacking something
612 for the C<$#ARRAY> access (which is hard, as it's an lvalue), as well as
613 the other obvious array functions, like push(), pop(), shift(), unshift(),
616 You cannot easily tie a multilevel data structure (such as a hash of
617 hashes) to a dbm file. The first problem is that all but GDBM and
618 Berkeley DB have size limitations, but beyond that, you also have problems
619 with how references are to be represented on disk. One experimental
620 module that does attempt to partially address this need is the MLDBM
621 module. Check your nearest CPAN site as described in L<perlmod> for
622 source code to MLDBM.