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 UNTIE and/or 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, $$self, $new_nicety)) {
155 confess "setpriority failed: $!";
162 This method will be triggered when the C<untie> occurs. This can be useful
163 if the class needs to know when no further calls will be made. (Except DESTROY
164 of course.) See below for more details.
168 This method will be triggered when the tied variable needs to be destructed.
169 As with other object classes, such a method is seldom necessary, because Perl
170 deallocates its moribund object's memory for you automatically--this isn't
171 C++, you know. We'll use a DESTROY method here for debugging purposes only.
175 confess "wrong type" unless ref $self;
176 carp "[ Nice::DESTROY pid $$self ]" if $Nice::DEBUG;
181 That's about all there is to it. Actually, it's more than all there
182 is to it, because we've done a few nice things here for the sake
183 of completeness, robustness, and general aesthetics. Simpler
184 TIESCALAR classes are certainly possible.
188 A class implementing a tied ordinary array should define the following
189 methods: TIEARRAY, FETCH, STORE, FETCHSIZE, STORESIZE and perhaps UNTIE and/or DESTROY.
191 FETCHSIZE and STORESIZE are used to provide C<$#array> and
192 equivalent C<scalar(@array)> access.
194 The methods POP, PUSH, SHIFT, UNSHIFT, SPLICE, DELETE, and EXISTS are
195 required if the perl operator with the corresponding (but lowercase) name
196 is to operate on the tied array. The B<Tie::Array> class can be used as a
197 base class to implement the first five of these in terms of the basic
198 methods above. The default implementations of DELETE and EXISTS in
199 B<Tie::Array> simply C<croak>.
201 In addition EXTEND will be called when perl would have pre-extended
202 allocation in a real array.
204 For this discussion, we'll implement an array whose elements are a fixed
205 size at creation. If you try to create an element larger than the fixed
206 size, you'll take an exception. For example:
209 tie @array, 'FixedElem_Array', 3;
210 $array[0] = 'cat'; # ok.
211 $array[1] = 'dogs'; # exception, length('dogs') > 3.
213 The preamble code for the class is as follows:
215 package FixedElem_Array;
221 =item TIEARRAY classname, LIST
223 This is the constructor for the class. That means it is expected to
224 return a blessed reference through which the new array (probably an
225 anonymous ARRAY ref) will be accessed.
227 In our example, just to show you that you don't I<really> have to return an
228 ARRAY reference, we'll choose a HASH reference to represent our object.
229 A HASH works out well as a generic record type: the C<{ELEMSIZE}> field will
230 store the maximum element size allowed, and the C<{ARRAY}> field will hold the
231 true ARRAY ref. If someone outside the class tries to dereference the
232 object returned (doubtless thinking it an ARRAY ref), they'll blow up.
233 This just goes to show you that you should respect an object's privacy.
237 my $elemsize = shift;
238 if ( @_ || $elemsize =~ /\D/ ) {
239 croak "usage: tie ARRAY, '" . __PACKAGE__ . "', elem_size";
242 ELEMSIZE => $elemsize,
247 =item FETCH this, index
249 This method will be triggered every time an individual element the tied array
250 is accessed (read). It takes one argument beyond its self reference: the
251 index whose value we're trying to fetch.
256 return $self->{ARRAY}->[$index];
259 If a negative array index is used to read from an array, the index
260 will be translated to a positive one internally by calling FETCHSIZE
261 before being passed to FETCH.
263 As you may have noticed, the name of the FETCH method (et al.) is the same
264 for all accesses, even though the constructors differ in names (TIESCALAR
265 vs TIEARRAY). While in theory you could have the same class servicing
266 several tied types, in practice this becomes cumbersome, and it's easiest
267 to keep them at simply one tie type per class.
269 =item STORE this, index, value
271 This method will be triggered every time an element in the tied array is set
272 (written). It takes two arguments beyond its self reference: the index at
273 which we're trying to store something and the value we're trying to put
276 In our example, C<undef> is really C<$self-E<gt>{ELEMSIZE}> number of
277 spaces so we have a little more work to do here:
281 my( $index, $value ) = @_;
282 if ( length $value > $self->{ELEMSIZE} ) {
283 croak "length of $value is greater than $self->{ELEMSIZE}";
286 $self->EXTEND( $index ) if $index > $self->FETCHSIZE();
287 # right justify to keep element size for smaller elements
288 $self->{ARRAY}->[$index] = sprintf "%$self->{ELEMSIZE}s", $value;
291 Negative indexes are treated the same as with FETCH.
295 Returns the total number of items in the tied array associated with
296 object I<this>. (Equivalent to C<scalar(@array)>). For example:
300 return scalar @{$self->{ARRAY}};
303 =item STORESIZE this, count
305 Sets the total number of items in the tied array associated with
306 object I<this> to be I<count>. If this makes the array larger then
307 class's mapping of C<undef> should be returned for new positions.
308 If the array becomes smaller then entries beyond count should be
311 In our example, 'undef' is really an element containing
312 C<$self-E<gt>{ELEMSIZE}> number of spaces. Observe:
317 if ( $count > $self->FETCHSIZE() ) {
318 $self->STORE( $_, '' ) foreach $count - $self->FETCHSIZE() + 1 .. $count;
319 } elsif ( $count < $self->FETCHSIZE() ) {
320 $self->POP() foreach 0 .. $self->FETCHSIZE() - $count + 1;
324 =item EXTEND this, count
326 Informative call that array is likely to grow to have I<count> entries.
327 Can be used to optimize allocation. This method need do nothing.
329 In our example, we want to make sure there are no blank (C<undef>)
330 entries, so C<EXTEND> will make use of C<STORESIZE> to fill elements
336 $self->STORESIZE( $count );
339 =item EXISTS this, key
341 Verify that the element at index I<key> exists in the tied array I<this>.
343 In our example, we will determine that if an element consists of
344 C<$self-E<gt>{ELEMSIZE}> spaces only, it does not exist:
349 return $self->{ARRAY}->[$index] eq ' ' x $self->{ELEMSIZE} ? 0 : 1;
352 =item DELETE this, key
354 Delete the element at index I<key> from the tied array I<this>.
356 In our example, a deleted item is C<$self->{ELEMSIZE}> spaces:
361 return $self->STORE( $index, '' );
366 Clear (remove, delete, ...) all values from the tied array associated with
367 object I<this>. For example:
371 return $self->{ARRAY} = [];
374 =item PUSH this, LIST
376 Append elements of I<LIST> to the array. For example:
381 my $last = $self->FETCHSIZE();
382 $self->STORE( $last + $_, $list[$_] ) foreach 0 .. $#list;
383 return $self->FETCHSIZE();
388 Remove last element of the array and return it. For example:
392 return pop @{$self->{ARRAY}};
397 Remove the first element of the array (shifting other elements down)
398 and return it. For example:
402 return shift @{$self->{ARRAY}};
405 =item UNSHIFT this, LIST
407 Insert LIST elements at the beginning of the array, moving existing elements
408 up to make room. For example:
413 my $size = scalar( @list );
414 # make room for our list
415 @{$self->{ARRAY}}[ $size .. $#{$self->{ARRAY}} + $size ]
417 $self->STORE( $_, $list[$_] ) foreach 0 .. $#list;
420 =item SPLICE this, offset, length, LIST
422 Perform the equivalent of C<splice> on the array.
424 I<offset> is optional and defaults to zero, negative values count back
425 from the end of the array.
427 I<length> is optional and defaults to rest of the array.
429 I<LIST> may be empty.
431 Returns a list of the original I<length> elements at I<offset>.
433 In our example, we'll use a little shortcut if there is a I<LIST>:
437 my $offset = shift || 0;
438 my $length = shift || $self->FETCHSIZE() - $offset;
441 tie @list, __PACKAGE__, $self->{ELEMSIZE};
444 return splice @{$self->{ARRAY}}, $offset, $length, @list;
449 Will be called when C<untie> happens. (See below.)
453 This method will be triggered when the tied variable needs to be destructed.
454 As with the scalar tie class, this is almost never needed in a
455 language that does its own garbage collection, so this time we'll
460 The code we presented at the top of the tied array class accesses many
461 elements of the array, far more than we've set the bounds to. Therefore,
462 it will blow up once they try to access beyond the 2nd element of @ary, as
463 the following output demonstrates:
465 setting index 0: value of elt 0 now 0
466 setting index 1: value of elt 1 now 10
467 setting index 2: value of elt 2 now 20
468 setting index 3: Array OOB: 3 > 2 at Bounded_Array.pm line 39
469 Bounded_Array::FETCH called at testba line 12
473 Hashes were the first Perl data type to be tied (see dbmopen()). A class
474 implementing a tied hash should define the following methods: TIEHASH is
475 the constructor. FETCH and STORE access the key and value pairs. EXISTS
476 reports whether a key is present in the hash, and DELETE deletes one.
477 CLEAR empties the hash by deleting all the key and value pairs. FIRSTKEY
478 and NEXTKEY implement the keys() and each() functions to iterate over all
479 the keys. UNTIE is called when C<untie> happens, and DESTROY is called when
480 the tied variable is garbage collected.
482 If this seems like a lot, then feel free to inherit from merely the
483 standard Tie::Hash module for most of your methods, redefining only the
484 interesting ones. See L<Tie::Hash> for details.
486 Remember that Perl distinguishes between a key not existing in the hash,
487 and the key existing in the hash but having a corresponding value of
488 C<undef>. The two possibilities can be tested with the C<exists()> and
489 C<defined()> functions.
491 Here's an example of a somewhat interesting tied hash class: it gives you
492 a hash representing a particular user's dot files. You index into the hash
493 with the name of the file (minus the dot) and you get back that dot file's
494 contents. For example:
497 tie %dot, 'DotFiles';
498 if ( $dot{profile} =~ /MANPATH/ ||
499 $dot{login} =~ /MANPATH/ ||
500 $dot{cshrc} =~ /MANPATH/ )
502 print "you seem to set your MANPATH\n";
505 Or here's another sample of using our tied class:
507 tie %him, 'DotFiles', 'daemon';
508 foreach $f ( keys %him ) {
509 printf "daemon dot file %s is size %d\n",
513 In our tied hash DotFiles example, we use a regular
514 hash for the object containing several important
515 fields, of which only the C<{LIST}> field will be what the
516 user thinks of as the real hash.
522 whose dot files this object represents
526 where those dot files live
530 whether we should try to change or remove those dot files
534 the hash of dot file names and content mappings
538 Here's the start of F<Dotfiles.pm>:
542 sub whowasi { (caller(1))[3] . '()' }
544 sub debug { $DEBUG = @_ ? shift : 1 }
546 For our example, we want to be able to emit debugging info to help in tracing
547 during development. We keep also one convenience function around
548 internally to help print out warnings; whowasi() returns the function name
551 Here are the methods for the DotFiles tied hash.
555 =item TIEHASH classname, LIST
557 This is the constructor for the class. That means it is expected to
558 return a blessed reference through which the new object (probably but not
559 necessarily an anonymous hash) will be accessed.
561 Here's the constructor:
565 my $user = shift || $>;
566 my $dotdir = shift || '';
567 croak "usage: @{[&whowasi]} [USER [DOTDIR]]" if @_;
568 $user = getpwuid($user) if $user =~ /^\d+$/;
569 my $dir = (getpwnam($user))[7]
570 || croak "@{[&whowasi]}: no user $user";
571 $dir .= "/$dotdir" if $dotdir;
581 || croak "@{[&whowasi]}: can't opendir $dir: $!";
582 foreach $dot ( grep /^\./ && -f "$dir/$_", readdir(DIR)) {
584 $node->{LIST}{$dot} = undef;
587 return bless $node, $self;
590 It's probably worth mentioning that if you're going to filetest the
591 return values out of a readdir, you'd better prepend the directory
592 in question. Otherwise, because we didn't chdir() there, it would
593 have been testing the wrong file.
595 =item FETCH this, key
597 This method will be triggered every time an element in the tied hash is
598 accessed (read). It takes one argument beyond its self reference: the key
599 whose value we're trying to fetch.
601 Here's the fetch for our DotFiles example.
604 carp &whowasi if $DEBUG;
607 my $dir = $self->{HOME};
608 my $file = "$dir/.$dot";
610 unless (exists $self->{LIST}->{$dot} || -f $file) {
611 carp "@{[&whowasi]}: no $dot file" if $DEBUG;
615 if (defined $self->{LIST}->{$dot}) {
616 return $self->{LIST}->{$dot};
618 return $self->{LIST}->{$dot} = `cat $dir/.$dot`;
622 It was easy to write by having it call the Unix cat(1) command, but it
623 would probably be more portable to open the file manually (and somewhat
624 more efficient). Of course, because dot files are a Unixy concept, we're
627 =item STORE this, key, value
629 This method will be triggered every time an element in the tied hash is set
630 (written). It takes two arguments beyond its self reference: the index at
631 which we're trying to store something, and the value we're trying to put
634 Here in our DotFiles example, we'll be careful not to let
635 them try to overwrite the file unless they've called the clobber()
636 method on the original object reference returned by tie().
639 carp &whowasi if $DEBUG;
643 my $file = $self->{HOME} . "/.$dot";
644 my $user = $self->{USER};
646 croak "@{[&whowasi]}: $file not clobberable"
647 unless $self->{CLOBBER};
649 open(F, "> $file") || croak "can't open $file: $!";
654 If they wanted to clobber something, they might say:
656 $ob = tie %daemon_dots, 'daemon';
658 $daemon_dots{signature} = "A true daemon\n";
660 Another way to lay hands on a reference to the underlying object is to
661 use the tied() function, so they might alternately have set clobber
664 tie %daemon_dots, 'daemon';
665 tied(%daemon_dots)->clobber(1);
667 The clobber method is simply:
671 $self->{CLOBBER} = @_ ? shift : 1;
674 =item DELETE this, key
676 This method is triggered when we remove an element from the hash,
677 typically by using the delete() function. Again, we'll
678 be careful to check whether they really want to clobber files.
681 carp &whowasi if $DEBUG;
685 my $file = $self->{HOME} . "/.$dot";
686 croak "@{[&whowasi]}: won't remove file $file"
687 unless $self->{CLOBBER};
688 delete $self->{LIST}->{$dot};
689 my $success = unlink($file);
690 carp "@{[&whowasi]}: can't unlink $file: $!" unless $success;
694 The value returned by DELETE becomes the return value of the call
695 to delete(). If you want to emulate the normal behavior of delete(),
696 you should return whatever FETCH would have returned for this key.
697 In this example, we have chosen instead to return a value which tells
698 the caller whether the file was successfully deleted.
702 This method is triggered when the whole hash is to be cleared, usually by
703 assigning the empty list to it.
705 In our example, that would remove all the user's dot files! It's such a
706 dangerous thing that they'll have to set CLOBBER to something higher than
710 carp &whowasi if $DEBUG;
712 croak "@{[&whowasi]}: won't remove all dot files for $self->{USER}"
713 unless $self->{CLOBBER} > 1;
715 foreach $dot ( keys %{$self->{LIST}}) {
720 =item EXISTS this, key
722 This method is triggered when the user uses the exists() function
723 on a particular hash. In our example, we'll look at the C<{LIST}>
724 hash element for this:
727 carp &whowasi if $DEBUG;
730 return exists $self->{LIST}->{$dot};
735 This method will be triggered when the user is going
736 to iterate through the hash, such as via a keys() or each()
740 carp &whowasi if $DEBUG;
742 my $a = keys %{$self->{LIST}}; # reset each() iterator
743 each %{$self->{LIST}}
746 =item NEXTKEY this, lastkey
748 This method gets triggered during a keys() or each() iteration. It has a
749 second argument which is the last key that had been accessed. This is
750 useful if you're carrying about ordering or calling the iterator from more
751 than one sequence, or not really storing things in a hash anywhere.
753 For our example, we're using a real hash so we'll do just the simple
754 thing, but we'll have to go through the LIST field indirectly.
757 carp &whowasi if $DEBUG;
759 return each %{ $self->{LIST} }
764 This is called when C<untie> occurs.
768 This method is triggered when a tied hash is about to go out of
769 scope. You don't really need it unless you're trying to add debugging
770 or have auxiliary state to clean up. Here's a very simple function:
773 carp &whowasi if $DEBUG;
778 Note that functions such as keys() and values() may return huge lists
779 when used on large objects, like DBM files. You may prefer to use the
780 each() function to iterate over such. Example:
782 # print out history file offsets
784 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
785 while (($key,$val) = each %HIST) {
786 print $key, ' = ', unpack('L',$val), "\n";
790 =head2 Tying FileHandles
792 This is partially implemented now.
794 A class implementing a tied filehandle should define the following
795 methods: TIEHANDLE, at least one of PRINT, PRINTF, WRITE, READLINE, GETC,
796 READ, and possibly CLOSE, UNTIE and DESTROY. The class can also provide: BINMODE,
797 OPEN, EOF, FILENO, SEEK, TELL - if the corresponding perl operators are
800 It is especially useful when perl is embedded in some other program,
801 where output to STDOUT and STDERR may have to be redirected in some
802 special way. See nvi and the Apache module for examples.
804 In our example we're going to create a shouting handle.
810 =item TIEHANDLE classname, LIST
812 This is the constructor for the class. That means it is expected to
813 return a blessed reference of some sort. The reference can be used to
814 hold some internal information.
816 sub TIEHANDLE { print "<shout>\n"; my $i; bless \$i, shift }
818 =item WRITE this, LIST
820 This method will be called when the handle is written to via the
821 C<syswrite> function.
825 my($buf,$len,$offset) = @_;
826 print "WRITE called, \$buf=$buf, \$len=$len, \$offset=$offset";
829 =item PRINT this, LIST
831 This method will be triggered every time the tied handle is printed to
832 with the C<print()> function.
833 Beyond its self reference it also expects the list that was passed to
836 sub PRINT { $r = shift; $$r++; print join($,,map(uc($_),@_)),$\ }
838 =item PRINTF this, LIST
840 This method will be triggered every time the tied handle is printed to
841 with the C<printf()> function.
842 Beyond its self reference it also expects the format and list that was
843 passed to the printf function.
848 print sprintf($fmt, @_)."\n";
851 =item READ this, LIST
853 This method will be called when the handle is read from via the C<read>
854 or C<sysread> functions.
858 my $$bufref = \$_[0];
859 my(undef,$len,$offset) = @_;
860 print "READ called, \$buf=$bufref, \$len=$len, \$offset=$offset";
861 # add to $$bufref, set $len to number of characters read
867 This method will be called when the handle is read from via <HANDLE>.
868 The method should return undef when there is no more data.
870 sub READLINE { $r = shift; "READLINE called $$r times\n"; }
874 This method will be called when the C<getc> function is called.
876 sub GETC { print "Don't GETC, Get Perl"; return "a"; }
880 This method will be called when the handle is closed via the C<close>
883 sub CLOSE { print "CLOSE called.\n" }
887 As with the other types of ties, this method will be called when C<untie> happens.
888 It may be appropriate to "auto CLOSE" when this occurs.
892 As with the other types of ties, this method will be called when the
893 tied handle is about to be destroyed. This is useful for debugging and
894 possibly cleaning up.
896 sub DESTROY { print "</shout>\n" }
900 Here's how to use our little example:
905 print FOO $a, " plus ", $b, " equals ", $a + $b, "\n";
910 You can define for all tie types an UNTIE method that will be called
913 =head2 The C<untie> Gotcha
915 If you intend making use of the object returned from either tie() or
916 tied(), and if the tie's target class defines a destructor, there is a
917 subtle gotcha you I<must> guard against.
919 As setup, consider this (admittedly rather contrived) example of a
920 tie; all it does is use a file to keep a log of the values assigned to
931 my $filename = shift;
932 my $handle = new IO::File "> $filename"
933 or die "Cannot open $filename: $!\n";
935 print $handle "The Start\n";
936 bless {FH => $handle, Value => 0}, $class;
941 return $self->{Value};
947 my $handle = $self->{FH};
948 print $handle "$value\n";
949 $self->{Value} = $value;
954 my $handle = $self->{FH};
955 print $handle "The End\n";
961 Here is an example that makes use of this tie:
967 tie $fred, 'Remember', 'myfile.txt';
972 system "cat myfile.txt";
974 This is the output when it is executed:
982 So far so good. Those of you who have been paying attention will have
983 spotted that the tied object hasn't been used so far. So lets add an
984 extra method to the Remember class to allow comments to be included in
985 the file -- say, something like this:
990 my $handle = $self->{FH};
991 print $handle $text, "\n";
994 And here is the previous example modified to use the C<comment> method
995 (which requires the tied object):
1001 $x = tie $fred, 'Remember', 'myfile.txt';
1004 comment $x "changing...";
1007 system "cat myfile.txt";
1009 When this code is executed there is no output. Here's why:
1011 When a variable is tied, it is associated with the object which is the
1012 return value of the TIESCALAR, TIEARRAY, or TIEHASH function. This
1013 object normally has only one reference, namely, the implicit reference
1014 from the tied variable. When untie() is called, that reference is
1015 destroyed. Then, as in the first example above, the object's
1016 destructor (DESTROY) is called, which is normal for objects that have
1017 no more valid references; and thus the file is closed.
1019 In the second example, however, we have stored another reference to
1020 the tied object in $x. That means that when untie() gets called
1021 there will still be a valid reference to the object in existence, so
1022 the destructor is not called at that time, and thus the file is not
1023 closed. The reason there is no output is because the file buffers
1024 have not been flushed to disk.
1026 Now that you know what the problem is, what can you do to avoid it?
1027 Prior to the introduction of the optional UNTIE method the only way
1028 was the good old C<-w> flag. Which will spot any instances where you call
1029 untie() and there are still valid references to the tied object. If
1030 the second script above this near the top C<use warnings 'untie'>
1031 or was run with the C<-w> flag, Perl prints this
1034 untie attempted while 1 inner references still exist
1036 To get the script to work properly and silence the warning make sure
1037 there are no valid references to the tied object I<before> untie() is
1043 Now that UNTIE exists the class designer can decide which parts of the
1044 class functionality are really associated with C<untie> and which with
1045 the object being destroyed. What makes sense for a given class depends
1046 on whether the inner references are being kept so that non-tie-related
1047 methods can be called on the object. But in most cases it probably makes
1048 sense to move the functionality that would have been in DESTROY to the UNTIE
1051 If the UNTIE method exists then the warning above does not occur. Instead the
1052 UNTIE method is passed the count of "extra" references and can issue its own
1053 warning if appropriate. e.g. to replicate the no UNTIE case this method can
1058 my ($obj,$count) = @_;
1059 carp "untie attempted while $count inner references still exist" if $count;
1064 See L<DB_File> or L<Config> for some interesting tie() implementations.
1065 A good starting point for many tie() implementations is with one of the
1066 modules L<Tie::Scalar>, L<Tie::Array>, L<Tie::Hash>, or L<Tie::Handle>.
1070 You cannot easily tie a multilevel data structure (such as a hash of
1071 hashes) to a dbm file. The first problem is that all but GDBM and
1072 Berkeley DB have size limitations, but beyond that, you also have problems
1073 with how references are to be represented on disk. One experimental
1074 module that does attempt to address this need partially is the MLDBM
1075 module. Check your nearest CPAN site as described in L<perlmodlib> for
1076 source code to MLDBM.
1078 Tied filehandles are still incomplete. sysopen(), truncate(),
1079 flock(), fcntl(), stat() and -X can't currently be trapped.
1085 TIEHANDLE by Sven Verdoolaege <F<skimo@dns.ufsia.ac.be>> and Doug MacEachern <F<dougm@osf.org>>
1087 UNTIE by Nick Ing-Simmons <F<nick@ing-simmons.net>>
1089 Tying Arrays by Casey Tweten <F<crt@kiski.net>>