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 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
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(), TIEHASH(), or TIEHANDLE(). (Typically these are arguments
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
41 "type" (e.g., HASH or C<CLASSNAME>) so long as it's a properly blessed
42 object.) You can also retrieve a reference to the underlying object
43 using the tied() function.
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.
50 A class implementing a tied scalar should define the following methods:
51 TIESCALAR, FETCH, STORE, and possibly DESTROY.
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:
56 tie $his_speed, 'Nice', getppid();
57 tie $my_speed, 'Nice', $$;
59 And now whenever either of those variables is accessed, its current
60 system priority is retrieved and returned. If those variables are set,
61 then the process's priority is changed!
63 We'll use Jarkko Hietaniemi F<E<lt>Jarkko.Hietaniemi@hut.fiE<gt>>'s
64 BSD::Resource class (not included) to access the PRIO_PROCESS, PRIO_MIN,
65 and PRIO_MAX constants from your system, as well as the getpriority() and
66 setpriority() system calls. Here's the preamble of the class.
72 $Nice::DEBUG = 0 unless defined $Nice::DEBUG;
76 =item TIESCALAR classname, LIST
78 This is the constructor for the class. That means it is
79 expected to return a blessed reference to a new scalar
80 (probably anonymous) that it's creating. For example:
84 my $pid = shift || $$; # 0 means me
86 if ($pid !~ /^\d+$/) {
87 carp "Nice::Tie::Scalar got non-numeric pid $pid" if $^W;
91 unless (kill 0, $pid) { # EPERM or ERSCH, no doubt
92 carp "Nice::Tie::Scalar got bad pid $pid: $!" if $^W;
96 return bless \$pid, $class;
99 This tie class has chosen to return an error rather than raising an
100 exception if its constructor should fail. While this is how dbmopen() works,
101 other classes may well not wish to be so forgiving. It checks the global
102 variable C<$^W> to see whether to emit a bit of noise anyway.
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
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 $$self
110 allows the method to get at the real value stored there. In our example
111 below, that real value is the process ID to which we've tied our variable.
115 confess "wrong type" unless ref $self;
116 croak "usage error" if @_;
119 $nicety = getpriority(PRIO_PROCESS, $$self);
120 if ($!) { croak "getpriority failed: $!" }
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.
128 =item STORE this, value
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.
136 confess "wrong type" unless ref $self;
137 my $new_nicety = shift;
138 croak "usage error" if @_;
140 if ($new_nicety < PRIO_MIN) {
142 "WARNING: priority %d less than minimum system priority %d",
143 $new_nicety, PRIO_MIN if $^W;
144 $new_nicety = PRIO_MIN;
147 if ($new_nicety > PRIO_MAX) {
149 "WARNING: priority %d greater than maximum system priority %d",
150 $new_nicety, PRIO_MAX if $^W;
151 $new_nicety = PRIO_MAX;
154 unless (defined setpriority(PRIO_PROCESS, $$self, $new_nicety)) {
155 confess "setpriority failed: $!";
162 This method will be triggered when the tied variable needs to be destructed.
163 As with other object classes, such a method is seldom necessary, because Perl
164 deallocates its moribund object's memory for you automatically--this isn't
165 C++, you know. We'll use a DESTROY method here for debugging purposes only.
169 confess "wrong type" unless ref $self;
170 carp "[ Nice::DESTROY pid $$self ]" if $Nice::DEBUG;
175 That's about all there is to it. Actually, it's more than all there
176 is to it, because we've done a few nice things here for the sake
177 of completeness, robustness, and general aesthetics. Simpler
178 TIESCALAR classes are certainly possible.
182 A class implementing a tied ordinary array should define the following
183 methods: TIEARRAY, FETCH, STORE, and perhaps DESTROY.
185 B<WARNING>: Tied arrays are I<incomplete>. They are also distinctly lacking
186 something for the C<$#ARRAY> access (which is hard, as it's an lvalue), as
187 well as the other obvious array functions, like push(), pop(), shift(),
188 unshift(), and splice().
190 For this discussion, we'll implement an array whose indices are fixed at
191 its creation. If you try to access anything beyond those bounds, you'll
192 take an exception. (Well, if you access an individual element; an
193 aggregate assignment would be missed.) For example:
195 require Bounded_Array;
196 tie @ary, 'Bounded_Array', 2;
199 print "setting index $i: ";
202 print "value of elt $i now $ary[$i]\n";
205 The preamble code for the class is as follows:
207 package Bounded_Array;
213 =item TIEARRAY classname, LIST
215 This is the constructor for the class. That means it is expected to
216 return a blessed reference through which the new array (probably an
217 anonymous ARRAY ref) will be accessed.
219 In our example, just to show you that you don't I<really> have to return an
220 ARRAY reference, we'll choose a HASH reference to represent our object.
221 A HASH works out well as a generic record type: the C<{BOUND}> field will
222 store the maximum bound allowed, and the C<{ARRAY}> field will hold the
223 true ARRAY ref. If someone outside the class tries to dereference the
224 object returned (doubtless thinking it an ARRAY ref), they'll blow up.
225 This just goes to show you that you should respect an object's privacy.
230 confess "usage: tie(\@ary, 'Bounded_Array', max_subscript)"
231 if @_ || $bound =~ /\D/;
238 =item FETCH this, index
240 This method will be triggered every time an individual element the tied array
241 is accessed (read). It takes one argument beyond its self reference: the
242 index whose value we're trying to fetch.
246 if ($idx > $self->{BOUND}) {
247 confess "Array OOB: $idx > $self->{BOUND}";
249 return $self->{ARRAY}[$idx];
252 As you may have noticed, the name of the FETCH method (et al.) is the same
253 for all accesses, even though the constructors differ in names (TIESCALAR
254 vs TIEARRAY). While in theory you could have the same class servicing
255 several tied types, in practice this becomes cumbersome, and it's easiest
256 to keep them at simply one tie type per class.
258 =item STORE this, index, value
260 This method will be triggered every time an element in the tied array is set
261 (written). It takes two arguments beyond its self reference: the index at
262 which we're trying to store something and the value we're trying to put
266 my($self, $idx, $value) = @_;
267 print "[STORE $value at $idx]\n" if _debug;
268 if ($idx > $self->{BOUND} ) {
269 confess "Array OOB: $idx > $self->{BOUND}";
271 return $self->{ARRAY}[$idx] = $value;
276 This method will be triggered when the tied variable needs to be destructed.
277 As with the scalar tie class, this is almost never needed in a
278 language that does its own garbage collection, so this time we'll
283 The code we presented at the top of the tied array class accesses many
284 elements of the array, far more than we've set the bounds to. Therefore,
285 it will blow up once they try to access beyond the 2nd element of @ary, as
286 the following output demonstrates:
288 setting index 0: value of elt 0 now 0
289 setting index 1: value of elt 1 now 10
290 setting index 2: value of elt 2 now 20
291 setting index 3: Array OOB: 3 > 2 at Bounded_Array.pm line 39
292 Bounded_Array::FETCH called at testba line 12
296 As the first Perl data type to be tied (see dbmopen()), associative arrays
297 have the most complete and useful tie() implementation. A class
298 implementing a tied associative array should define the following
299 methods: TIEHASH is the constructor. FETCH and STORE access the key and
300 value pairs. EXISTS reports whether a key is present in the hash, and
301 DELETE deletes one. CLEAR empties the hash by deleting all the key and
302 value pairs. FIRSTKEY and NEXTKEY implement the keys() and each()
303 functions to iterate over all the keys. And DESTROY is called when the
304 tied variable is garbage collected.
306 If this seems like a lot, then feel free to inherit from
307 merely the standard Tie::Hash module for most of your methods, redefining only
308 the interesting ones. See L<Tie::Hash> for details.
310 Remember that Perl distinguishes between a key not existing in the hash,
311 and the key existing in the hash but having a corresponding value of
312 C<undef>. The two possibilities can be tested with the C<exists()> and
313 C<defined()> functions.
315 Here's an example of a somewhat interesting tied hash class: it gives you
316 a hash representing a particular user's dot files. You index into the hash
317 with the name of the file (minus the dot) and you get back that dot file's
318 contents. For example:
321 tie %dot, 'DotFiles';
322 if ( $dot{profile} =~ /MANPATH/ ||
323 $dot{login} =~ /MANPATH/ ||
324 $dot{cshrc} =~ /MANPATH/ )
326 print "you seem to set your MANPATH\n";
329 Or here's another sample of using our tied class:
331 tie %him, 'DotFiles', 'daemon';
332 foreach $f ( keys %him ) {
333 printf "daemon dot file %s is size %d\n",
337 In our tied hash DotFiles example, we use a regular
338 hash for the object containing several important
339 fields, of which only the C<{LIST}> field will be what the
340 user thinks of as the real hash.
346 whose dot files this object represents
350 where those dot files live
354 whether we should try to change or remove those dot files
358 the hash of dot file names and content mappings
362 Here's the start of F<Dotfiles.pm>:
366 sub whowasi { (caller(1))[3] . '()' }
368 sub debug { $DEBUG = @_ ? shift : 1 }
370 For our example, we want to be able to emit debugging info to help in tracing
371 during development. We keep also one convenience function around
372 internally to help print out warnings; whowasi() returns the function name
375 Here are the methods for the DotFiles tied hash.
379 =item TIEHASH classname, LIST
381 This is the constructor for the class. That means it is expected to
382 return a blessed reference through which the new object (probably but not
383 necessarily an anonymous hash) will be accessed.
385 Here's the constructor:
389 my $user = shift || $>;
390 my $dotdir = shift || '';
391 croak "usage: @{[&whowasi]} [USER [DOTDIR]]" if @_;
392 $user = getpwuid($user) if $user =~ /^\d+$/;
393 my $dir = (getpwnam($user))[7]
394 || croak "@{[&whowasi]}: no user $user";
395 $dir .= "/$dotdir" if $dotdir;
405 || croak "@{[&whowasi]}: can't opendir $dir: $!";
406 foreach $dot ( grep /^\./ && -f "$dir/$_", readdir(DIR)) {
408 $node->{LIST}{$dot} = undef;
411 return bless $node, $self;
414 It's probably worth mentioning that if you're going to filetest the
415 return values out of a readdir, you'd better prepend the directory
416 in question. Otherwise, because we didn't chdir() there, it would
417 have been testing the wrong file.
419 =item FETCH this, key
421 This method will be triggered every time an element in the tied hash is
422 accessed (read). It takes one argument beyond its self reference: the key
423 whose value we're trying to fetch.
425 Here's the fetch for our DotFiles example.
428 carp &whowasi if $DEBUG;
431 my $dir = $self->{HOME};
432 my $file = "$dir/.$dot";
434 unless (exists $self->{LIST}->{$dot} || -f $file) {
435 carp "@{[&whowasi]}: no $dot file" if $DEBUG;
439 if (defined $self->{LIST}->{$dot}) {
440 return $self->{LIST}->{$dot};
442 return $self->{LIST}->{$dot} = `cat $dir/.$dot`;
446 It was easy to write by having it call the Unix cat(1) command, but it
447 would probably be more portable to open the file manually (and somewhat
448 more efficient). Of course, because dot files are a Unixy concept, we're
451 =item STORE this, key, value
453 This method will be triggered every time an element in the tied hash is set
454 (written). It takes two arguments beyond its self reference: the index at
455 which we're trying to store something, and the value we're trying to put
458 Here in our DotFiles example, we'll be careful not to let
459 them try to overwrite the file unless they've called the clobber()
460 method on the original object reference returned by tie().
463 carp &whowasi if $DEBUG;
467 my $file = $self->{HOME} . "/.$dot";
468 my $user = $self->{USER};
470 croak "@{[&whowasi]}: $file not clobberable"
471 unless $self->{CLOBBER};
473 open(F, "> $file") || croak "can't open $file: $!";
478 If they wanted to clobber something, they might say:
480 $ob = tie %daemon_dots, 'daemon';
482 $daemon_dots{signature} = "A true daemon\n";
484 Another way to lay hands on a reference to the underlying object is to
485 use the tied() function, so they might alternately have set clobber
488 tie %daemon_dots, 'daemon';
489 tied(%daemon_dots)->clobber(1);
491 The clobber method is simply:
495 $self->{CLOBBER} = @_ ? shift : 1;
498 =item DELETE this, key
500 This method is triggered when we remove an element from the hash,
501 typically by using the delete() function. Again, we'll
502 be careful to check whether they really want to clobber files.
505 carp &whowasi if $DEBUG;
509 my $file = $self->{HOME} . "/.$dot";
510 croak "@{[&whowasi]}: won't remove file $file"
511 unless $self->{CLOBBER};
512 delete $self->{LIST}->{$dot};
513 my $success = unlink($file);
514 carp "@{[&whowasi]}: can't unlink $file: $!" unless $success;
518 The value returned by DELETE becomes the return value of the call
519 to delete(). If you want to emulate the normal behavior of delete(),
520 you should return whatever FETCH would have returned for this key.
521 In this example, we have chosen instead to return a value which tells
522 the caller whether the file was successfully deleted.
526 This method is triggered when the whole hash is to be cleared, usually by
527 assigning the empty list to it.
529 In our example, that would remove all the user's dot files! It's such a
530 dangerous thing that they'll have to set CLOBBER to something higher than
534 carp &whowasi if $DEBUG;
536 croak "@{[&whowasi]}: won't remove all dot files for $self->{USER}"
537 unless $self->{CLOBBER} > 1;
539 foreach $dot ( keys %{$self->{LIST}}) {
544 =item EXISTS this, key
546 This method is triggered when the user uses the exists() function
547 on a particular hash. In our example, we'll look at the C<{LIST}>
548 hash element for this:
551 carp &whowasi if $DEBUG;
554 return exists $self->{LIST}->{$dot};
559 This method will be triggered when the user is going
560 to iterate through the hash, such as via a keys() or each()
564 carp &whowasi if $DEBUG;
566 my $a = keys %{$self->{LIST}}; # reset each() iterator
567 each %{$self->{LIST}}
570 =item NEXTKEY this, lastkey
572 This method gets triggered during a keys() or each() iteration. It has a
573 second argument which is the last key that had been accessed. This is
574 useful if you're carrying about ordering or calling the iterator from more
575 than one sequence, or not really storing things in a hash anywhere.
577 For our example, we're using a real hash so we'll do just the simple
578 thing, but we'll have to go through the LIST field indirectly.
581 carp &whowasi if $DEBUG;
583 return each %{ $self->{LIST} }
588 This method is triggered when a tied hash is about to go out of
589 scope. You don't really need it unless you're trying to add debugging
590 or have auxiliary state to clean up. Here's a very simple function:
593 carp &whowasi if $DEBUG;
598 Note that functions such as keys() and values() may return huge array
599 values when used on large objects, like DBM files. You may prefer to
600 use the each() function to iterate over such. Example:
602 # print out history file offsets
604 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
605 while (($key,$val) = each %HIST) {
606 print $key, ' = ', unpack('L',$val), "\n";
610 =head2 Tying FileHandles
612 This is partially implemented now.
614 A class implementing a tied filehandle should define the following methods:
615 TIEHANDLE, PRINT and/or READLINE, and possibly DESTROY.
617 It is especially useful when perl is embedded in some other program,
618 where output to STDOUT and STDERR may have to be redirected in some
619 special way. See nvi and the Apache module for examples.
621 In our example we're going to create a shouting handle.
627 =item TIEHANDLE classname, LIST
629 This is the constructor for the class. That means it is expected to
630 return a blessed reference of some sort. The reference can be used to
631 hold some internal information.
633 sub TIEHANDLE { print "<shout>\n"; my $i; bless \$i, shift }
635 =item PRINT this, LIST
637 This method will be triggered every time the tied handle is printed to.
638 Beyond its self reference it also expects the list that was passed to
641 sub PRINT { $r = shift; $$r++; print join($,,map(uc($_),@_)),$\ }
645 This method will be called when the handle is read from. The method
646 should return undef when there is no more data.
648 sub READLINE { $r = shift; "PRINT called $$r times\n"; }
652 As with the other types of ties, this method will be called when the
653 tied handle is about to be destroyed. This is useful for debugging and
654 possibly cleaning up.
656 sub DESTROY { print "</shout>\n" }
660 Here's how to use our little example:
665 print FOO $a, " plus ", $b, " equals ", $a + $b, "\n";
668 =head2 The C<untie> Gotcha
670 If you intend making use of the object returned from either tie() or
671 tied(), and if the tie's target class defines a destructor, there is a
672 subtle gotcha you I<must> guard against.
674 As setup, consider this (admittedly rather contrived) example of a
675 tie; all it does is use a file to keep a log of the values assigned to
685 my $filename = shift;
686 my $handle = new IO::File "> $filename"
687 or die "Cannot open $filename: $!\n";
689 print $handle "The Start\n";
690 bless {FH => $handle, Value => 0}, $class;
695 return $self->{Value};
701 my $handle = $self->{FH};
702 print $handle "$value\n";
703 $self->{Value} = $value;
708 my $handle = $self->{FH};
709 print $handle "The End\n";
715 Here is an example that makes use of this tie:
721 tie $fred, 'Remember', 'myfile.txt';
726 system "cat myfile.txt";
728 This is the output when it is executed:
736 So far so good. Those of you who have been paying attention will have
737 spotted that the tied object hasn't been used so far. So lets add an
738 extra method to the Remember class to allow comments to be included in
739 the file -- say, something like this:
744 my $handle = $self->{FH};
745 print $handle $text, "\n";
748 And here is the previous example modified to use the C<comment> method
749 (which requires the tied object):
755 $x = tie $fred, 'Remember', 'myfile.txt';
758 comment $x "changing...";
761 system "cat myfile.txt";
763 When this code is executed there is no output. Here's why:
765 When a variable is tied, it is associated with the object which is the
766 return value of the TIESCALAR, TIEARRAY, or TIEHASH function. This
767 object normally has only one reference, namely, the implicit reference
768 from the tied variable. When untie() is called, that reference is
769 destroyed. Then, as in the first example above, the object's
770 destructor (DESTROY) is called, which is normal for objects that have
771 no more valid references; and thus the file is closed.
773 In the second example, however, we have stored another reference to
774 the tied object in C<$x>. That means that when untie() gets called
775 there will still be a valid reference to the object in existence, so
776 the destructor is not called at that time, and thus the file is not
777 closed. The reason there is no output is because the file buffers
778 have not been flushed to disk.
780 Now that you know what the problem is, what can you do to avoid it?
781 Well, the good old C<-w> flag will spot any instances where you call
782 untie() and there are still valid references to the tied object. If
783 the second script above is run with the C<-w> flag, Perl prints this
786 untie attempted while 1 inner references still exist
788 To get the script to work properly and silence the warning make sure
789 there are no valid references to the tied object I<before> untie() is
797 See L<DB_File> or L<Config> for some interesting tie() implementations.
801 Tied arrays are I<incomplete>. They are also distinctly lacking something
802 for the C<$#ARRAY> access (which is hard, as it's an lvalue), as well as
803 the other obvious array functions, like push(), pop(), shift(), unshift(),
806 You cannot easily tie a multilevel data structure (such as a hash of
807 hashes) to a dbm file. The first problem is that all but GDBM and
808 Berkeley DB have size limitations, but beyond that, you also have problems
809 with how references are to be represented on disk. One experimental
810 module that does attempt to address this need partially is the MLDBM
811 module. Check your nearest CPAN site as described in L<perlmod> for
812 source code to MLDBM.
818 TIEHANDLE by Sven Verdoolaege E<lt>F<skimo@dns.ufsia.ac.be>E<gt>