3 perltie - how to hide an object class in a simple variable
7 tie VARIABLE, CLASSNAME, LIST
13 Prior to release 5.0 of Perl, a programmer could use dbmopen()
14 to magically connect an on-disk database in the standard Unix dbm(3x)
15 format to a %HASH in their program. However, their Perl was either
16 built with one particular dbm library or another, but not both, and
17 you couldn't extend this mechanism to other packages or types of variables.
21 The tie() function binds a variable to a class (package) that will provide
22 the implementation for access methods for that variable. Once this magic
23 has been performed, accessing a tied variable automatically triggers
24 method calls in the proper class. All of the complexity of the class is
25 hidden behind magic methods calls. The method names are in ALL CAPS,
26 which is a convention that Perl uses to indicate that they're called
27 implicitly rather than explicitly--just like the BEGIN() and END()
30 In the tie() call, C<VARIABLE> is the name of the variable to be
31 enchanted. C<CLASSNAME> is the name of a class implementing objects of
32 the correct type. Any additional arguments in the C<LIST> are passed to
33 the appropriate constructor method for that class--meaning TIESCALAR(),
34 TIEARRAY(), or TIEHASH(). (Typically these are arguments such as might be
35 passed to the dbminit() function of C.) The object returned by the "new"
36 method is also returned by the tie() function, which would be useful if
37 you wanted to access other methods in C<CLASSNAME>. (You don't actually
38 have to return a reference to a right "type" (e.g. HASH or C<CLASSNAME>)
39 so long as it's a properly blessed object.)
42 Unlike dbmopen(), the tie() function will not C<use> or C<require> a module
43 for you--you need to do that explicitly yourself.
47 A class implementing a tied scalar should define the following methods:
48 TIESCALAR, FETCH, STORE, and possibly DESTROY.
50 Let's look at each in turn, using as an example a tie class for
51 scalars that allows the user to do something like:
53 tie $his_speed, 'Nice', getppid();
54 tie $my_speed, 'Nice', $$;
56 And now whenever either of those variables is accessed, its current
57 system priority is retrieved and returned. If those variables are set,
58 then the process's priority is changed!
60 We'll use Jarkko Hietaniemi F<E<lt>Jarkko.Hietaniemi@hut.fiE<gt>>'s
61 BSD::Resource class (not included) to access the PRIO_PROCESS, PRIO_MIN,
62 and PRIO_MAX constants from your system, as well as the getpriority() and
63 setpriority() system calls. Here's the preamble of the class.
69 $Nice::DEBUG = 0 unless defined $Nice::DEBUG;
73 =item TIESCALAR classname, LIST
75 This is the constructor for the class. That means it is
76 expected to return a blessed reference to a new scalar
77 (probably anonymous) that it's creating. For example:
81 my $pid = shift || $$; # 0 means me
83 if ($pid !~ /^\d+$/) {
84 carp "Nice::TieScalar got non-numeric pid $pid" if $^W;
88 unless (kill 0, $pid) { # EPERM or ERSCH, no doubt
89 carp "Nice::TieScalar got bad pid $pid: $!" if $^W;
93 return bless \$pid, $class;
96 This tie class has chosen to return an error rather than raising an
97 exception if its constructor should fail. While this is how dbmopen() works,
98 other classes may well not wish to be so forgiving. It checks the global
99 variable C<$^W> to see whether to emit a bit of noise anyway.
103 This method will be triggered every time the tied variable is accessed
104 (read). It takes no arguments beyond its self reference, which is the
105 object representing the scalar we're dealing with. Since in this case
106 we're just using a SCALAR ref for the tied scalar object, a simple $$self
107 allows the method to get at the real value stored there. In our example
108 below, that real value is the process ID to which we've tied our variable.
112 confess "wrong type" unless ref $self;
113 croak "usage error" if @_;
116 $nicety = getpriority(PRIO_PROCESS, $$self);
117 if ($!) { croak "getpriority failed: $!" }
121 This time we've decided to blow up (raise an exception) if the renice
122 fails--there's no place for us to return an error otherwise, and it's
123 probably the right thing to do.
125 =item STORE this, value
127 This method will be triggered every time the tied variable is set
128 (assigned). Beyond its self reference, it also expects one (and only one)
129 argument--the new value the user is trying to assign.
133 confess "wrong type" unless ref $self;
134 my $new_nicety = shift;
135 croak "usage error" if @_;
137 if ($new_nicety < PRIO_MIN) {
139 "WARNING: priority %d less than minimum system priority %d",
140 $new_nicety, PRIO_MIN if $^W;
141 $new_nicety = PRIO_MIN;
144 if ($new_nicety > PRIO_MAX) {
146 "WARNING: priority %d greater than maximum system priority %d",
147 $new_nicety, PRIO_MAX if $^W;
148 $new_nicety = PRIO_MAX;
151 unless (defined setpriority(PRIO_PROCESS, $$self, $new_nicety)) {
152 confess "setpriority failed: $!";
159 This method will be triggered when the tied variable needs to be destructed.
160 As with other object classes, such a method is seldom ncessary, since Perl
161 deallocates its moribund object's memory for you automatically--this isn't
162 C++, you know. We'll use a DESTROY method here for debugging purposes only.
166 confess "wrong type" unless ref $self;
167 carp "[ Nice::DESTROY pid $$self ]" if $Nice::DEBUG;
172 That's about all there is to it. Actually, it's more than all there
173 is to it, since we've done a few nice things here for the sake
174 of completeness, robustness, and general aesthetics. Simpler
175 TIESCALAR classes are certainly possible.
179 A class implementing a tied ordinary array should define the following
180 methods: TIEARRAY, FETCH, STORE, and perhaps DESTROY.
182 B<WARNING>: Tied arrays are I<incomplete>. They are also distinctly lacking
183 something for the C<$#ARRAY> access (which is hard, as it's an lvalue), as
184 well as the other obvious array functions, like push(), pop(), shift(),
185 unshift(), and splice().
187 For this discussion, we'll implement an array whose indices are fixed at
188 its creation. If you try to access anything beyond those bounds, you'll
189 take an exception. (Well, if you access an individual element; an
190 aggregate assignment would be missed.) For example:
192 require Bounded_Array;
193 tie @ary, Bounded_Array, 2;
196 print "setting index $i: ";
199 print "value of elt $i now $ary[$i]\n";
202 The preamble code for the class is as follows:
204 package Bounded_Array;
210 =item TIEARRAY classname, LIST
212 This is the constructor for the class. That means it is expected to
213 return a blessed reference through which the new array (probably an
214 anonymous ARRAY ref) will be accessed.
216 In our example, just to show you that you don't I<really> have to return an
217 ARRAY reference, we'll choose a HASH reference to represent our object.
218 A HASH works out well as a generic record type: the C<{BOUND}> field will
219 store the maximum bound allowed, and the C<{ARRAY} field will hold the
220 true ARRAY ref. If someone outside the class tries to dereference the
221 object returned (doubtless thinking it an ARRAY ref), they'll blow up.
222 This just goes to show you that you should respect an object's privacy.
227 confess "usage: tie(\@ary, 'Bounded_Array', max_subscript)"
228 if @_ || $bound =~ /\D/;
235 =item FETCH this, index
237 This method will be triggered every time an individual element the tied array
238 is accessed (read). It takes one argument beyond its self reference: the
239 index whose value we're trying to fetch.
243 if ($idx > $self->{BOUND}) {
244 confess "Array OOB: $idx > $self->{BOUND}";
246 return $self->{ARRAY}[$idx];
249 As you may have noticed, the name of the FETCH method (et al.) is the same
250 for all accesses, even though the constructors differ in names (TIESCALAR
251 vs TIEARRAY). While in theory you could have the same class servicing
252 several tied types, in practice this becomes cumbersome, and it's easiest
253 to simply keep them at one tie type per class.
255 =item STORE this, index, value
257 This method will be triggered every time an element in the tied array is set
258 (written). It takes two arguments beyond its self reference: the index at
259 which we're trying to store something and the value we're trying to put
263 my($self, $idx, $value) = @_;
264 print "[STORE $value at $idx]\n" if _debug;
265 if ($idx > $self->{BOUND} ) {
266 confess "Array OOB: $idx > $self->{BOUND}";
268 return $self->{ARRAY}[$idx] = $value;
273 This method will be triggered when the tied variable needs to be destructed.
274 As with the sclar tie class, this is almost never needed in a
275 language that does its own garbage collection, so this time we'll
280 The code we presented at the top of the tied array class accesses many
281 elements of the array, far more than we've set the bounds to. Therefore,
282 it will blow up once they try to access beyond the 2nd element of @ary, as
283 the following output demonstrates:
285 setting index 0: value of elt 0 now 0
286 setting index 1: value of elt 1 now 10
287 setting index 2: value of elt 2 now 20
288 setting index 3: Array OOB: 3 > 2 at Bounded_Array.pm line 39
289 Bounded_Array::FETCH called at testba line 12
293 As the first Perl data type to be tied (see dbmopen()), associative arrays
294 have the most complete and useful tie() implementation. A class
295 implementing a tied associative array should define the following
296 methods: TIEHASH is the constructor. FETCH and STORE access the key and
297 value pairs. EXISTS reports whether a key is present in the hash, and
298 DELETE deletes one. CLEAR empties the hash by deleting all the key and
299 value pairs. FIRSTKEY and NEXTKEY implement the keys() and each()
300 functions to iterate over all the keys. And DESTROY is called when the
301 tied variable is garbage collected.
303 If this seems like a lot, then feel free to merely inherit
304 from the standard TieHash module for most of your methods, redefining only
305 the interesting ones. See L<TieHash> for details.
307 Remember that Perl distinguishes between a key not existing in the hash,
308 and the key existing in the hash but having a corresponding value of
309 C<undef>. The two possibilities can be tested with the C<exists()> and
310 C<defined()> functions.
312 Here's an example of a somewhat interesting tied hash class: it gives you
313 a hash representing a particular user's dotfiles. You index into the hash
314 with the name of the file (minus the dot) and you get back that dotfile's
315 contents. For example:
319 if ( $dot{profile} =~ /MANPATH/ ||
320 $dot{login} =~ /MANPATH/ ||
321 $dot{cshrc} =~ /MANPATH/ )
323 print "you seem to set your manpath\n";
326 Or here's another sample of using our tied class:
328 tie %him, DotFiles, 'daemon';
329 foreach $f ( keys %him ) {
330 printf "daemon dot file %s is size %d\n",
334 In our tied hash DotFiles example, we use a regular
335 hash for the object containing several important
336 fields, of which only the C<{LIST}> field will be what the
337 user thinks of as the real hash.
343 whose dot files this object represents
347 where those dotfiles live
351 whether we should try to change or remove those dot files
355 the hash of dotfile names and content mappings
359 Here's the start of F<Dotfiles.pm>:
363 sub whowasi { (caller(1))[3] . '()' }
365 sub debug { $DEBUG = @_ ? shift : 1 }
367 For our example, we want to able to emit debugging info to help in tracing
368 during development. We keep also one convenience function around
369 internally to help print out warnings; whowasi() returns the function name
372 Here are the methods for the DotFiles tied hash.
376 =item TIEHASH classname, LIST
378 This is the constructor for the class. That means it is expected to
379 return a blessed reference through which the new object (probably but not
380 necessarily an anonymous hash) will be accessed.
382 Here's the constructor:
386 my $user = shift || $>;
387 my $dotdir = shift || '';
388 croak "usage: @{[&whowasi]} [USER [DOTDIR]]" if @_;
389 $user = getpwuid($user) if $user =~ /^\d+$/;
390 my $dir = (getpwnam($user))[7]
391 || croak "@{[&whowasi]}: no user $user";
392 $dir .= "/$dotdir" if $dotdir;
402 || croak "@{[&whowasi]}: can't opendir $dir: $!";
403 foreach $dot ( grep /^\./ && -f "$dir/$_", readdir(DIR)) {
405 $node->{LIST}{$dot} = undef;
408 return bless $node, $self;
411 It's probably worth mentioning that if you're going to filetest the
412 return values out of a readdir, you'd better prepend the directory
413 in question. Otherwise, since we didn't chdir() there, it would
414 have been testing the wrong file.
416 =item FETCH this, key
418 This method will be triggered every time an element in the tied hash is
419 accessed (read). It takes one argument beyond its self reference: the key
420 whose value we're trying to fetch.
422 Here's the fetch for our DotFiles example.
425 carp &whowasi if $DEBUG;
428 my $dir = $self->{HOME};
429 my $file = "$dir/.$dot";
431 unless (exists $self->{LIST}->{$dot} || -f $file) {
432 carp "@{[&whowasi]}: no $dot file" if $DEBUG;
436 if (defined $self->{LIST}->{$dot}) {
437 return $self->{LIST}->{$dot};
439 return $self->{LIST}->{$dot} = `cat $dir/.$dot`;
443 It was easy to write by having it call the Unix cat(1) command, but it
444 would probably be more portable to open the file manually (and somewhat
445 more efficient). Of course, since dot files are a Unixy concept, we're
448 =item STORE this, key, value
450 This method will be triggered every time an element in the tied hash is set
451 (written). It takes two arguments beyond its self reference: the index at
452 which we're trying to store something, and the value we're trying to put
455 Here in our DotFiles example, we'll be careful not to let
456 them try to overwrite the file unless they've called the clobber()
457 method on the original object reference returned by tie().
460 carp &whowasi if $DEBUG;
464 my $file = $self->{HOME} . "/.$dot";
465 my $user = $self->{USER};
467 croak "@{[&whowasi]}: $file not clobberable"
468 unless $self->{CLOBBER};
470 open(F, "> $file") || croak "can't open $file: $!";
475 If they wanted to clobber something, they might say:
477 $ob = tie %daemon_dots, 'daemon';
479 $daemon_dots{signature} = "A true daemon\n";
481 Where the clobber method is simply:
485 $self->{CLOBBER} = @_ ? shift : 1;
488 =item DELETE this, key
490 This method is triggered when we remove an element from the hash,
491 typically by using the delete() function. Again, we'll
492 be careful to check whether they really want to clobber files.
495 carp &whowasi if $DEBUG;
499 my $file = $self->{HOME} . "/.$dot";
500 croak "@{[&whowasi]}: won't remove file $file"
501 unless $self->{CLOBBER};
502 delete $self->{LIST}->{$dot};
503 unlink($file) || carp "@{[&whowasi]}: can't unlink $file: $!";
508 This method is triggered when the whole hash is to be cleared, usually by
509 assigning the empty list to it.
511 In our example, that would remove all the user's dotfiles! It's such a
512 dangerous thing that they'll have to set CLOBBER to something higher than
516 carp &whowasi if $DEBUG;
518 croak "@{[&whowasi]}: won't remove all dotfiles for $self->{USER}"
519 unless $self->{CLOBBER} > 1;
521 foreach $dot ( keys %{$self->{LIST}}) {
526 =item EXISTS this, key
528 This method is triggered when the user uses the exists() function
529 on a particular hash. In our example, we'll look at the C<{LIST}>
530 hash element for this:
533 carp &whowasi if $DEBUG;
536 return exists $self->{LIST}->{$dot};
541 This method will be triggered when the user is going
542 to iterate through the hash, such as via a keys() or each()
546 carp &whowasi if $DEBUG;
548 my $a = keys %{$self->{LIST}};
549 each %{$self->{LIST}}
552 =item NEXTKEY this, lastkey
554 This method gets triggered during a keys() or each() iteration. It has a
555 second argument which is the last key that had been accessed. This is
556 useful if you're carrying about ordering or calling the iterator from more
557 than one sequence, or not really storing things in a hash anywhere.
559 For our example, we our using a real hash so we'll just do the simple
560 thing, but we'll have to indirect through the LIST field.
563 carp &whowasi if $DEBUG;
565 return each %{ $self->{LIST} }
570 This method is triggered when a tied hash is about to go out of
571 scope. You don't really need it unless you're trying to add debugging
572 or have auxiliary state to clean up. Here's a very simple function:
575 carp &whowasi if $DEBUG;
580 Note that functions such as keys() and values() may return huge array
581 values when used on large objects, like DBM files. You may prefer to
582 use the each() function to iterate over such. Example:
584 # print out history file offsets
586 tie(%HIST, NDBM_File, '/usr/lib/news/history', 1, 0);
587 while (($key,$val) = each %HIST) {
588 print $key, ' = ', unpack('L',$val), "\n";
592 =head2 Tying FileHandles
594 This isn't implemented yet. Sorry; maybe someday.
598 See L<DB_File> or L<Config> for some interesting tie() implementations.
602 Tied arrays are I<incomplete>. They are also distinctly lacking something
603 for the C<$#ARRAY> access (which is hard, as it's an lvalue), as well as
604 the other obvious array functions, like push(), pop(), shift(), unshift(),