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. 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', $$;
59 And now whenever either of those variables is accessed, its current
60 system priority is retrieved and returned. If those variables are set,
61 then the process's priority is changed!
63 We'll use Jarkko Hietaniemi <F<jhi@iki.fi>>'s BSD::Resource class (not
64 included) to access the PRIO_PROCESS, PRIO_MIN, and PRIO_MAX constants
65 from your system, as well as the getpriority() and setpriority() system
66 calls. Here's the preamble of the class.
72 $Nice::DEBUG = 0 unless defined $Nice::DEBUG;
76 =item TIESCALAR classname, LIST
78 This is the constructor for the class. That means it is
79 expected to return a blessed reference to a new scalar
80 (probably anonymous) that it's creating. For example:
84 my $pid = shift || $$; # 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 $Nice::DEBUG;
175 That's about all there is to it. Actually, it's more than all there
176 is to it, because we've done a few nice things here for the sake
177 of completeness, robustness, and general aesthetics. Simpler
178 TIESCALAR classes are certainly possible.
182 A class implementing a tied ordinary array should define the following
183 methods: TIEARRAY, FETCH, STORE, FETCHSIZE, STORESIZE and perhaps DESTROY.
185 FETCHSIZE and STORESIZE are used to provide C<$#array> and
186 equivalent C<scalar(@array)> access.
188 The methods POP, PUSH, SHIFT, UNSHIFT, SPLICE, DELETE, and EXISTS are
189 required if the perl operator with the corresponding (but lowercase) name
190 is to operate on the tied array. The B<Tie::Array> class can be used as a
191 base class to implement the first five of these in terms of the basic
192 methods above. The default implementations of DELETE and EXISTS in
193 B<Tie::Array> simply C<croak>.
195 In addition EXTEND will be called when perl would have pre-extended
196 allocation in a real array.
198 This means that tied arrays are now I<complete>. The example below needs
199 upgrading to illustrate this. (The documentation in B<Tie::Array> is more
202 For this discussion, we'll implement an array whose indices are fixed at
203 its creation. If you try to access anything beyond those bounds, you'll
204 take an exception. For example:
206 require Bounded_Array;
207 tie @ary, 'Bounded_Array', 2;
210 print "setting index $i: ";
213 print "value of elt $i now $ary[$i]\n";
216 The preamble code for the class is as follows:
218 package Bounded_Array;
224 =item TIEARRAY classname, LIST
226 This is the constructor for the class. That means it is expected to
227 return a blessed reference through which the new array (probably an
228 anonymous ARRAY ref) will be accessed.
230 In our example, just to show you that you don't I<really> have to return an
231 ARRAY reference, we'll choose a HASH reference to represent our object.
232 A HASH works out well as a generic record type: the C<{BOUND}> field will
233 store the maximum bound allowed, and the C<{ARRAY}> field will hold the
234 true ARRAY ref. If someone outside the class tries to dereference the
235 object returned (doubtless thinking it an ARRAY ref), they'll blow up.
236 This just goes to show you that you should respect an object's privacy.
241 confess "usage: tie(\@ary, 'Bounded_Array', max_subscript)"
242 if @_ || $bound =~ /\D/;
249 =item FETCH this, index
251 This method will be triggered every time an individual element the tied array
252 is accessed (read). It takes one argument beyond its self reference: the
253 index whose value we're trying to fetch.
257 if ($idx > $self->{BOUND}) {
258 confess "Array OOB: $idx > $self->{BOUND}";
260 return $self->{ARRAY}[$idx];
263 If a negative array index is used to read from an array, the index
264 will be translated to a positive one internally by calling FETCHSIZE
265 before being passed to FETCH.
267 As you may have noticed, the name of the FETCH method (et al.) is the same
268 for all accesses, even though the constructors differ in names (TIESCALAR
269 vs TIEARRAY). While in theory you could have the same class servicing
270 several tied types, in practice this becomes cumbersome, and it's easiest
271 to keep them at simply one tie type per class.
273 =item STORE this, index, value
275 This method will be triggered every time an element in the tied array is set
276 (written). It takes two arguments beyond its self reference: the index at
277 which we're trying to store something and the value we're trying to put
281 my($self, $idx, $value) = @_;
282 print "[STORE $value at $idx]\n" if _debug;
283 if ($idx > $self->{BOUND} ) {
284 confess "Array OOB: $idx > $self->{BOUND}";
286 return $self->{ARRAY}[$idx] = $value;
289 Negative indexes are treated the same as with FETCH.
293 This method will be triggered when the tied variable needs to be destructed.
294 As with the scalar tie class, this is almost never needed in a
295 language that does its own garbage collection, so this time we'll
300 The code we presented at the top of the tied array class accesses many
301 elements of the array, far more than we've set the bounds to. Therefore,
302 it will blow up once they try to access beyond the 2nd element of @ary, as
303 the following output demonstrates:
305 setting index 0: value of elt 0 now 0
306 setting index 1: value of elt 1 now 10
307 setting index 2: value of elt 2 now 20
308 setting index 3: Array OOB: 3 > 2 at Bounded_Array.pm line 39
309 Bounded_Array::FETCH called at testba line 12
313 Hashes were the first Perl data type to be tied (see dbmopen()). A class
314 implementing a tied hash should define the following methods: TIEHASH is
315 the constructor. FETCH and STORE access the key and value pairs. EXISTS
316 reports whether a key is present in the hash, and DELETE deletes one.
317 CLEAR empties the hash by deleting all the key and value pairs. FIRSTKEY
318 and NEXTKEY implement the keys() and each() functions to iterate over all
319 the keys. And DESTROY is called when the tied variable is garbage
322 If this seems like a lot, then feel free to inherit from merely the
323 standard Tie::Hash module for most of your methods, redefining only the
324 interesting ones. See L<Tie::Hash> for details.
326 Remember that Perl distinguishes between a key not existing in the hash,
327 and the key existing in the hash but having a corresponding value of
328 C<undef>. The two possibilities can be tested with the C<exists()> and
329 C<defined()> functions.
331 Here's an example of a somewhat interesting tied hash class: it gives you
332 a hash representing a particular user's dot files. You index into the hash
333 with the name of the file (minus the dot) and you get back that dot file's
334 contents. For example:
337 tie %dot, 'DotFiles';
338 if ( $dot{profile} =~ /MANPATH/ ||
339 $dot{login} =~ /MANPATH/ ||
340 $dot{cshrc} =~ /MANPATH/ )
342 print "you seem to set your MANPATH\n";
345 Or here's another sample of using our tied class:
347 tie %him, 'DotFiles', 'daemon';
348 foreach $f ( keys %him ) {
349 printf "daemon dot file %s is size %d\n",
353 In our tied hash DotFiles example, we use a regular
354 hash for the object containing several important
355 fields, of which only the C<{LIST}> field will be what the
356 user thinks of as the real hash.
362 whose dot files this object represents
366 where those dot files live
370 whether we should try to change or remove those dot files
374 the hash of dot file names and content mappings
378 Here's the start of F<Dotfiles.pm>:
382 sub whowasi { (caller(1))[3] . '()' }
384 sub debug { $DEBUG = @_ ? shift : 1 }
386 For our example, we want to be able to emit debugging info to help in tracing
387 during development. We keep also one convenience function around
388 internally to help print out warnings; whowasi() returns the function name
391 Here are the methods for the DotFiles tied hash.
395 =item TIEHASH classname, LIST
397 This is the constructor for the class. That means it is expected to
398 return a blessed reference through which the new object (probably but not
399 necessarily an anonymous hash) will be accessed.
401 Here's the constructor:
405 my $user = shift || $>;
406 my $dotdir = shift || '';
407 croak "usage: @{[&whowasi]} [USER [DOTDIR]]" if @_;
408 $user = getpwuid($user) if $user =~ /^\d+$/;
409 my $dir = (getpwnam($user))[7]
410 || croak "@{[&whowasi]}: no user $user";
411 $dir .= "/$dotdir" if $dotdir;
421 || croak "@{[&whowasi]}: can't opendir $dir: $!";
422 foreach $dot ( grep /^\./ && -f "$dir/$_", readdir(DIR)) {
424 $node->{LIST}{$dot} = undef;
427 return bless $node, $self;
430 It's probably worth mentioning that if you're going to filetest the
431 return values out of a readdir, you'd better prepend the directory
432 in question. Otherwise, because we didn't chdir() there, it would
433 have been testing the wrong file.
435 =item FETCH this, key
437 This method will be triggered every time an element in the tied hash is
438 accessed (read). It takes one argument beyond its self reference: the key
439 whose value we're trying to fetch.
441 Here's the fetch for our DotFiles example.
444 carp &whowasi if $DEBUG;
447 my $dir = $self->{HOME};
448 my $file = "$dir/.$dot";
450 unless (exists $self->{LIST}->{$dot} || -f $file) {
451 carp "@{[&whowasi]}: no $dot file" if $DEBUG;
455 if (defined $self->{LIST}->{$dot}) {
456 return $self->{LIST}->{$dot};
458 return $self->{LIST}->{$dot} = `cat $dir/.$dot`;
462 It was easy to write by having it call the Unix cat(1) command, but it
463 would probably be more portable to open the file manually (and somewhat
464 more efficient). Of course, because dot files are a Unixy concept, we're
467 =item STORE this, key, value
469 This method will be triggered every time an element in the tied hash is set
470 (written). It takes two arguments beyond its self reference: the index at
471 which we're trying to store something, and the value we're trying to put
474 Here in our DotFiles example, we'll be careful not to let
475 them try to overwrite the file unless they've called the clobber()
476 method on the original object reference returned by tie().
479 carp &whowasi if $DEBUG;
483 my $file = $self->{HOME} . "/.$dot";
484 my $user = $self->{USER};
486 croak "@{[&whowasi]}: $file not clobberable"
487 unless $self->{CLOBBER};
489 open(F, "> $file") || croak "can't open $file: $!";
494 If they wanted to clobber something, they might say:
496 $ob = tie %daemon_dots, 'daemon';
498 $daemon_dots{signature} = "A true daemon\n";
500 Another way to lay hands on a reference to the underlying object is to
501 use the tied() function, so they might alternately have set clobber
504 tie %daemon_dots, 'daemon';
505 tied(%daemon_dots)->clobber(1);
507 The clobber method is simply:
511 $self->{CLOBBER} = @_ ? shift : 1;
514 =item DELETE this, key
516 This method is triggered when we remove an element from the hash,
517 typically by using the delete() function. Again, we'll
518 be careful to check whether they really want to clobber files.
521 carp &whowasi if $DEBUG;
525 my $file = $self->{HOME} . "/.$dot";
526 croak "@{[&whowasi]}: won't remove file $file"
527 unless $self->{CLOBBER};
528 delete $self->{LIST}->{$dot};
529 my $success = unlink($file);
530 carp "@{[&whowasi]}: can't unlink $file: $!" unless $success;
534 The value returned by DELETE becomes the return value of the call
535 to delete(). If you want to emulate the normal behavior of delete(),
536 you should return whatever FETCH would have returned for this key.
537 In this example, we have chosen instead to return a value which tells
538 the caller whether the file was successfully deleted.
542 This method is triggered when the whole hash is to be cleared, usually by
543 assigning the empty list to it.
545 In our example, that would remove all the user's dot files! It's such a
546 dangerous thing that they'll have to set CLOBBER to something higher than
550 carp &whowasi if $DEBUG;
552 croak "@{[&whowasi]}: won't remove all dot files for $self->{USER}"
553 unless $self->{CLOBBER} > 1;
555 foreach $dot ( keys %{$self->{LIST}}) {
560 =item EXISTS this, key
562 This method is triggered when the user uses the exists() function
563 on a particular hash. In our example, we'll look at the C<{LIST}>
564 hash element for this:
567 carp &whowasi if $DEBUG;
570 return exists $self->{LIST}->{$dot};
575 This method will be triggered when the user is going
576 to iterate through the hash, such as via a keys() or each()
580 carp &whowasi if $DEBUG;
582 my $a = keys %{$self->{LIST}}; # reset each() iterator
583 each %{$self->{LIST}}
586 =item NEXTKEY this, lastkey
588 This method gets triggered during a keys() or each() iteration. It has a
589 second argument which is the last key that had been accessed. This is
590 useful if you're carrying about ordering or calling the iterator from more
591 than one sequence, or not really storing things in a hash anywhere.
593 For our example, we're using a real hash so we'll do just the simple
594 thing, but we'll have to go through the LIST field indirectly.
597 carp &whowasi if $DEBUG;
599 return each %{ $self->{LIST} }
604 This method is triggered when a tied hash is about to go out of
605 scope. You don't really need it unless you're trying to add debugging
606 or have auxiliary state to clean up. Here's a very simple function:
609 carp &whowasi if $DEBUG;
614 Note that functions such as keys() and values() may return huge lists
615 when used on large objects, like DBM files. You may prefer to use the
616 each() function to iterate over such. Example:
618 # print out history file offsets
620 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
621 while (($key,$val) = each %HIST) {
622 print $key, ' = ', unpack('L',$val), "\n";
626 =head2 Tying FileHandles
628 This is partially implemented now.
630 A class implementing a tied filehandle should define the following
631 methods: TIEHANDLE, at least one of PRINT, PRINTF, WRITE, READLINE, GETC,
632 READ, and possibly CLOSE and DESTROY. The class can also provide: BINMODE,
633 OPEN, EOF, FILENO, SEEK, TELL - if the corresponding perl operators are
636 It is especially useful when perl is embedded in some other program,
637 where output to STDOUT and STDERR may have to be redirected in some
638 special way. See nvi and the Apache module for examples.
640 In our example we're going to create a shouting handle.
646 =item TIEHANDLE classname, LIST
648 This is the constructor for the class. That means it is expected to
649 return a blessed reference of some sort. The reference can be used to
650 hold some internal information.
652 sub TIEHANDLE { print "<shout>\n"; my $i; bless \$i, shift }
654 =item WRITE this, LIST
656 This method will be called when the handle is written to via the
657 C<syswrite> function.
661 my($buf,$len,$offset) = @_;
662 print "WRITE called, \$buf=$buf, \$len=$len, \$offset=$offset";
665 =item PRINT this, LIST
667 This method will be triggered every time the tied handle is printed to
668 with the C<print()> function.
669 Beyond its self reference it also expects the list that was passed to
672 sub PRINT { $r = shift; $$r++; print join($,,map(uc($_),@_)),$\ }
674 =item PRINTF this, LIST
676 This method will be triggered every time the tied handle is printed to
677 with the C<printf()> function.
678 Beyond its self reference it also expects the format and list that was
679 passed to the printf function.
684 print sprintf($fmt, @_)."\n";
687 =item READ this, LIST
689 This method will be called when the handle is read from via the C<read>
690 or C<sysread> functions.
694 my $$bufref = \$_[0];
695 my(undef,$len,$offset) = @_;
696 print "READ called, \$buf=$bufref, \$len=$len, \$offset=$offset";
697 # add to $$bufref, set $len to number of characters read
703 This method will be called when the handle is read from via <HANDLE>.
704 The method should return undef when there is no more data.
706 sub READLINE { $r = shift; "READLINE called $$r times\n"; }
710 This method will be called when the C<getc> function is called.
712 sub GETC { print "Don't GETC, Get Perl"; return "a"; }
716 This method will be called when the handle is closed via the C<close>
719 sub CLOSE { print "CLOSE called.\n" }
723 As with the other types of ties, this method will be called when the
724 tied handle is about to be destroyed. This is useful for debugging and
725 possibly cleaning up.
727 sub DESTROY { print "</shout>\n" }
731 Here's how to use our little example:
736 print FOO $a, " plus ", $b, " equals ", $a + $b, "\n";
739 =head2 The C<untie> Gotcha
741 If you intend making use of the object returned from either tie() or
742 tied(), and if the tie's target class defines a destructor, there is a
743 subtle gotcha you I<must> guard against.
745 As setup, consider this (admittedly rather contrived) example of a
746 tie; all it does is use a file to keep a log of the values assigned to
757 my $filename = shift;
758 my $handle = new IO::File "> $filename"
759 or die "Cannot open $filename: $!\n";
761 print $handle "The Start\n";
762 bless {FH => $handle, Value => 0}, $class;
767 return $self->{Value};
773 my $handle = $self->{FH};
774 print $handle "$value\n";
775 $self->{Value} = $value;
780 my $handle = $self->{FH};
781 print $handle "The End\n";
787 Here is an example that makes use of this tie:
793 tie $fred, 'Remember', 'myfile.txt';
798 system "cat myfile.txt";
800 This is the output when it is executed:
808 So far so good. Those of you who have been paying attention will have
809 spotted that the tied object hasn't been used so far. So lets add an
810 extra method to the Remember class to allow comments to be included in
811 the file -- say, something like this:
816 my $handle = $self->{FH};
817 print $handle $text, "\n";
820 And here is the previous example modified to use the C<comment> method
821 (which requires the tied object):
827 $x = tie $fred, 'Remember', 'myfile.txt';
830 comment $x "changing...";
833 system "cat myfile.txt";
835 When this code is executed there is no output. Here's why:
837 When a variable is tied, it is associated with the object which is the
838 return value of the TIESCALAR, TIEARRAY, or TIEHASH function. This
839 object normally has only one reference, namely, the implicit reference
840 from the tied variable. When untie() is called, that reference is
841 destroyed. Then, as in the first example above, the object's
842 destructor (DESTROY) is called, which is normal for objects that have
843 no more valid references; and thus the file is closed.
845 In the second example, however, we have stored another reference to
846 the tied object in $x. That means that when untie() gets called
847 there will still be a valid reference to the object in existence, so
848 the destructor is not called at that time, and thus the file is not
849 closed. The reason there is no output is because the file buffers
850 have not been flushed to disk.
852 Now that you know what the problem is, what can you do to avoid it?
853 Well, the good old C<-w> flag will spot any instances where you call
854 untie() and there are still valid references to the tied object. If
855 the second script above this near the top C<use warnings 'untie'>
856 or was run with the C<-w> flag, Perl prints this
859 untie attempted while 1 inner references still exist
861 To get the script to work properly and silence the warning make sure
862 there are no valid references to the tied object I<before> untie() is
870 See L<DB_File> or L<Config> for some interesting tie() implementations.
871 A good starting point for many tie() implementations is with one of the
872 modules L<Tie::Scalar>, L<Tie::Array>, L<Tie::Hash>, or L<Tie::Handle>.
876 You cannot easily tie a multilevel data structure (such as a hash of
877 hashes) to a dbm file. The first problem is that all but GDBM and
878 Berkeley DB have size limitations, but beyond that, you also have problems
879 with how references are to be represented on disk. One experimental
880 module that does attempt to address this need partially is the MLDBM
881 module. Check your nearest CPAN site as described in L<perlmodlib> for
882 source code to MLDBM.
888 TIEHANDLE by Sven Verdoolaege <F<skimo@dns.ufsia.ac.be>> and Doug MacEachern <F<dougm@osf.org>>