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. Don't worry about
133 returning a value from STORE -- the semantic of assignment returning the
134 assigned value is implemented with FETCH.
138 confess "wrong type" unless ref $self;
139 my $new_nicety = shift;
140 croak "usage error" if @_;
142 if ($new_nicety < PRIO_MIN) {
144 "WARNING: priority %d less than minimum system priority %d",
145 $new_nicety, PRIO_MIN if $^W;
146 $new_nicety = PRIO_MIN;
149 if ($new_nicety > PRIO_MAX) {
151 "WARNING: priority %d greater than maximum system priority %d",
152 $new_nicety, PRIO_MAX if $^W;
153 $new_nicety = PRIO_MAX;
156 unless (defined setpriority(PRIO_PROCESS, $$self, $new_nicety)) {
157 confess "setpriority failed: $!";
163 This method will be triggered when the C<untie> occurs. This can be useful
164 if the class needs to know when no further calls will be made. (Except DESTROY
165 of course.) See L<The C<untie> Gotcha> below for more details.
169 This method will be triggered when the tied variable needs to be destructed.
170 As with other object classes, such a method is seldom necessary, because Perl
171 deallocates its moribund object's memory for you automatically--this isn't
172 C++, you know. We'll use a DESTROY method here for debugging purposes only.
176 confess "wrong type" unless ref $self;
177 carp "[ Nice::DESTROY pid $$self ]" if $Nice::DEBUG;
182 That's about all there is to it. Actually, it's more than all there
183 is to it, because we've done a few nice things here for the sake
184 of completeness, robustness, and general aesthetics. Simpler
185 TIESCALAR classes are certainly possible.
189 A class implementing a tied ordinary array should define the following
190 methods: TIEARRAY, FETCH, STORE, FETCHSIZE, STORESIZE and perhaps UNTIE and/or DESTROY.
192 FETCHSIZE and STORESIZE are used to provide C<$#array> and
193 equivalent C<scalar(@array)> access.
195 The methods POP, PUSH, SHIFT, UNSHIFT, SPLICE, DELETE, and EXISTS are
196 required if the perl operator with the corresponding (but lowercase) name
197 is to operate on the tied array. The B<Tie::Array> class can be used as a
198 base class to implement the first five of these in terms of the basic
199 methods above. The default implementations of DELETE and EXISTS in
200 B<Tie::Array> simply C<croak>.
202 In addition EXTEND will be called when perl would have pre-extended
203 allocation in a real array.
205 For this discussion, we'll implement an array whose elements are a fixed
206 size at creation. If you try to create an element larger than the fixed
207 size, you'll take an exception. For example:
210 tie @array, 'FixedElem_Array', 3;
211 $array[0] = 'cat'; # ok.
212 $array[1] = 'dogs'; # exception, length('dogs') > 3.
214 The preamble code for the class is as follows:
216 package FixedElem_Array;
222 =item TIEARRAY classname, LIST
224 This is the constructor for the class. That means it is expected to
225 return a blessed reference through which the new array (probably an
226 anonymous ARRAY ref) will be accessed.
228 In our example, just to show you that you don't I<really> have to return an
229 ARRAY reference, we'll choose a HASH reference to represent our object.
230 A HASH works out well as a generic record type: the C<{ELEMSIZE}> field will
231 store the maximum element size allowed, and the C<{ARRAY}> field will hold the
232 true ARRAY ref. If someone outside the class tries to dereference the
233 object returned (doubtless thinking it an ARRAY ref), they'll blow up.
234 This just goes to show you that you should respect an object's privacy.
238 my $elemsize = shift;
239 if ( @_ || $elemsize =~ /\D/ ) {
240 croak "usage: tie ARRAY, '" . __PACKAGE__ . "', elem_size";
243 ELEMSIZE => $elemsize,
248 =item FETCH this, index
250 This method will be triggered every time an individual element the tied array
251 is accessed (read). It takes one argument beyond its self reference: the
252 index whose value we're trying to fetch.
257 return $self->{ARRAY}->[$index];
260 If a negative array index is used to read from an array, the index
261 will be translated to a positive one internally by calling FETCHSIZE
262 before being passed to FETCH. You may disable this feature by
263 assigning a true value to the variable C<$NEGATIVE_INDICES> in the
266 As you may have noticed, the name of the FETCH method (et al.) is the same
267 for all accesses, even though the constructors differ in names (TIESCALAR
268 vs TIEARRAY). While in theory you could have the same class servicing
269 several tied types, in practice this becomes cumbersome, and it's easiest
270 to keep them at simply one tie type per class.
272 =item STORE this, index, value
274 This method will be triggered every time an element in the tied array is set
275 (written). It takes two arguments beyond its self reference: the index at
276 which we're trying to store something and the value we're trying to put
279 In our example, C<undef> is really C<$self-E<gt>{ELEMSIZE}> number of
280 spaces so we have a little more work to do here:
284 my( $index, $value ) = @_;
285 if ( length $value > $self->{ELEMSIZE} ) {
286 croak "length of $value is greater than $self->{ELEMSIZE}";
289 $self->EXTEND( $index ) if $index > $self->FETCHSIZE();
290 # right justify to keep element size for smaller elements
291 $self->{ARRAY}->[$index] = sprintf "%$self->{ELEMSIZE}s", $value;
294 Negative indexes are treated the same as with FETCH.
298 Returns the total number of items in the tied array associated with
299 object I<this>. (Equivalent to C<scalar(@array)>). For example:
303 return scalar @{$self->{ARRAY}};
306 =item STORESIZE this, count
308 Sets the total number of items in the tied array associated with
309 object I<this> to be I<count>. If this makes the array larger then
310 class's mapping of C<undef> should be returned for new positions.
311 If the array becomes smaller then entries beyond count should be
314 In our example, 'undef' is really an element containing
315 C<$self-E<gt>{ELEMSIZE}> number of spaces. Observe:
320 if ( $count > $self->FETCHSIZE() ) {
321 foreach ( $count - $self->FETCHSIZE() .. $count ) {
322 $self->STORE( $_, '' );
324 } elsif ( $count < $self->FETCHSIZE() ) {
325 foreach ( 0 .. $self->FETCHSIZE() - $count - 2 ) {
331 =item EXTEND this, count
333 Informative call that array is likely to grow to have I<count> entries.
334 Can be used to optimize allocation. This method need do nothing.
336 In our example, we want to make sure there are no blank (C<undef>)
337 entries, so C<EXTEND> will make use of C<STORESIZE> to fill elements
343 $self->STORESIZE( $count );
346 =item EXISTS this, key
348 Verify that the element at index I<key> exists in the tied array I<this>.
350 In our example, we will determine that if an element consists of
351 C<$self-E<gt>{ELEMSIZE}> spaces only, it does not exist:
356 return 0 if ! defined $self->{ARRAY}->[$index] ||
357 $self->{ARRAY}->[$index] eq ' ' x $self->{ELEMSIZE};
361 =item DELETE this, key
363 Delete the element at index I<key> from the tied array I<this>.
365 In our example, a deleted item is C<$self-E<gt>{ELEMSIZE}> spaces:
370 return $self->STORE( $index, '' );
375 Clear (remove, delete, ...) all values from the tied array associated with
376 object I<this>. For example:
380 return $self->{ARRAY} = [];
383 =item PUSH this, LIST
385 Append elements of I<LIST> to the array. For example:
390 my $last = $self->FETCHSIZE();
391 $self->STORE( $last + $_, $list[$_] ) foreach 0 .. $#list;
392 return $self->FETCHSIZE();
397 Remove last element of the array and return it. For example:
401 return pop @{$self->{ARRAY}};
406 Remove the first element of the array (shifting other elements down)
407 and return it. For example:
411 return shift @{$self->{ARRAY}};
414 =item UNSHIFT this, LIST
416 Insert LIST elements at the beginning of the array, moving existing elements
417 up to make room. For example:
422 my $size = scalar( @list );
423 # make room for our list
424 @{$self->{ARRAY}}[ $size .. $#{$self->{ARRAY}} + $size ]
426 $self->STORE( $_, $list[$_] ) foreach 0 .. $#list;
429 =item SPLICE this, offset, length, LIST
431 Perform the equivalent of C<splice> on the array.
433 I<offset> is optional and defaults to zero, negative values count back
434 from the end of the array.
436 I<length> is optional and defaults to rest of the array.
438 I<LIST> may be empty.
440 Returns a list of the original I<length> elements at I<offset>.
442 In our example, we'll use a little shortcut if there is a I<LIST>:
446 my $offset = shift || 0;
447 my $length = shift || $self->FETCHSIZE() - $offset;
450 tie @list, __PACKAGE__, $self->{ELEMSIZE};
453 return splice @{$self->{ARRAY}}, $offset, $length, @list;
458 Will be called when C<untie> happens. (See L<The C<untie> Gotcha> below.)
462 This method will be triggered when the tied variable needs to be destructed.
463 As with the scalar tie class, this is almost never needed in a
464 language that does its own garbage collection, so this time we'll
471 Hashes were the first Perl data type to be tied (see dbmopen()). A class
472 implementing a tied hash should define the following methods: TIEHASH is
473 the constructor. FETCH and STORE access the key and value pairs. EXISTS
474 reports whether a key is present in the hash, and DELETE deletes one.
475 CLEAR empties the hash by deleting all the key and value pairs. FIRSTKEY
476 and NEXTKEY implement the keys() and each() functions to iterate over all
477 the keys. SCALAR is triggered when the tied hash is evaluated in scalar
478 context. UNTIE is called when C<untie> happens, and DESTROY is called when
479 the tied variable is garbage collected.
481 If this seems like a lot, then feel free to inherit from merely the
482 standard Tie::StdHash module for most of your methods, redefining only the
483 interesting ones. See L<Tie::Hash> for details.
485 Remember that Perl distinguishes between a key not existing in the hash,
486 and the key existing in the hash but having a corresponding value of
487 C<undef>. The two possibilities can be tested with the C<exists()> and
488 C<defined()> functions.
490 Here's an example of a somewhat interesting tied hash class: it gives you
491 a hash representing a particular user's dot files. You index into the hash
492 with the name of the file (minus the dot) and you get back that dot file's
493 contents. For example:
496 tie %dot, 'DotFiles';
497 if ( $dot{profile} =~ /MANPATH/ ||
498 $dot{login} =~ /MANPATH/ ||
499 $dot{cshrc} =~ /MANPATH/ )
501 print "you seem to set your MANPATH\n";
504 Or here's another sample of using our tied class:
506 tie %him, 'DotFiles', 'daemon';
507 foreach $f ( keys %him ) {
508 printf "daemon dot file %s is size %d\n",
512 In our tied hash DotFiles example, we use a regular
513 hash for the object containing several important
514 fields, of which only the C<{LIST}> field will be what the
515 user thinks of as the real hash.
521 whose dot files this object represents
525 where those dot files live
529 whether we should try to change or remove those dot files
533 the hash of dot file names and content mappings
537 Here's the start of F<Dotfiles.pm>:
541 sub whowasi { (caller(1))[3] . '()' }
543 sub debug { $DEBUG = @_ ? shift : 1 }
545 For our example, we want to be able to emit debugging info to help in tracing
546 during development. We keep also one convenience function around
547 internally to help print out warnings; whowasi() returns the function name
550 Here are the methods for the DotFiles tied hash.
554 =item TIEHASH classname, LIST
556 This is the constructor for the class. That means it is expected to
557 return a blessed reference through which the new object (probably but not
558 necessarily an anonymous hash) will be accessed.
560 Here's the constructor:
564 my $user = shift || $>;
565 my $dotdir = shift || '';
566 croak "usage: @{[&whowasi]} [USER [DOTDIR]]" if @_;
567 $user = getpwuid($user) if $user =~ /^\d+$/;
568 my $dir = (getpwnam($user))[7]
569 || croak "@{[&whowasi]}: no user $user";
570 $dir .= "/$dotdir" if $dotdir;
580 || croak "@{[&whowasi]}: can't opendir $dir: $!";
581 foreach $dot ( grep /^\./ && -f "$dir/$_", readdir(DIR)) {
583 $node->{LIST}{$dot} = undef;
586 return bless $node, $self;
589 It's probably worth mentioning that if you're going to filetest the
590 return values out of a readdir, you'd better prepend the directory
591 in question. Otherwise, because we didn't chdir() there, it would
592 have been testing the wrong file.
594 =item FETCH this, key
596 This method will be triggered every time an element in the tied hash is
597 accessed (read). It takes one argument beyond its self reference: the key
598 whose value we're trying to fetch.
600 Here's the fetch for our DotFiles example.
603 carp &whowasi if $DEBUG;
606 my $dir = $self->{HOME};
607 my $file = "$dir/.$dot";
609 unless (exists $self->{LIST}->{$dot} || -f $file) {
610 carp "@{[&whowasi]}: no $dot file" if $DEBUG;
614 if (defined $self->{LIST}->{$dot}) {
615 return $self->{LIST}->{$dot};
617 return $self->{LIST}->{$dot} = `cat $dir/.$dot`;
621 It was easy to write by having it call the Unix cat(1) command, but it
622 would probably be more portable to open the file manually (and somewhat
623 more efficient). Of course, because dot files are a Unixy concept, we're
626 =item STORE this, key, value
628 This method will be triggered every time an element in the tied hash is set
629 (written). It takes two arguments beyond its self reference: the index at
630 which we're trying to store something, and the value we're trying to put
633 Here in our DotFiles example, we'll be careful not to let
634 them try to overwrite the file unless they've called the clobber()
635 method on the original object reference returned by tie().
638 carp &whowasi if $DEBUG;
642 my $file = $self->{HOME} . "/.$dot";
643 my $user = $self->{USER};
645 croak "@{[&whowasi]}: $file not clobberable"
646 unless $self->{CLOBBER};
648 open(F, "> $file") || croak "can't open $file: $!";
653 If they wanted to clobber something, they might say:
655 $ob = tie %daemon_dots, 'daemon';
657 $daemon_dots{signature} = "A true daemon\n";
659 Another way to lay hands on a reference to the underlying object is to
660 use the tied() function, so they might alternately have set clobber
663 tie %daemon_dots, 'daemon';
664 tied(%daemon_dots)->clobber(1);
666 The clobber method is simply:
670 $self->{CLOBBER} = @_ ? shift : 1;
673 =item DELETE this, key
675 This method is triggered when we remove an element from the hash,
676 typically by using the delete() function. Again, we'll
677 be careful to check whether they really want to clobber files.
680 carp &whowasi if $DEBUG;
684 my $file = $self->{HOME} . "/.$dot";
685 croak "@{[&whowasi]}: won't remove file $file"
686 unless $self->{CLOBBER};
687 delete $self->{LIST}->{$dot};
688 my $success = unlink($file);
689 carp "@{[&whowasi]}: can't unlink $file: $!" unless $success;
693 The value returned by DELETE becomes the return value of the call
694 to delete(). If you want to emulate the normal behavior of delete(),
695 you should return whatever FETCH would have returned for this key.
696 In this example, we have chosen instead to return a value which tells
697 the caller whether the file was successfully deleted.
701 This method is triggered when the whole hash is to be cleared, usually by
702 assigning the empty list to it.
704 In our example, that would remove all the user's dot files! It's such a
705 dangerous thing that they'll have to set CLOBBER to something higher than
709 carp &whowasi if $DEBUG;
711 croak "@{[&whowasi]}: won't remove all dot files for $self->{USER}"
712 unless $self->{CLOBBER} > 1;
714 foreach $dot ( keys %{$self->{LIST}}) {
719 =item EXISTS this, key
721 This method is triggered when the user uses the exists() function
722 on a particular hash. In our example, we'll look at the C<{LIST}>
723 hash element for this:
726 carp &whowasi if $DEBUG;
729 return exists $self->{LIST}->{$dot};
734 This method will be triggered when the user is going
735 to iterate through the hash, such as via a keys() or each()
739 carp &whowasi if $DEBUG;
741 my $a = keys %{$self->{LIST}}; # reset each() iterator
742 each %{$self->{LIST}}
745 =item NEXTKEY this, lastkey
747 This method gets triggered during a keys() or each() iteration. It has a
748 second argument which is the last key that had been accessed. This is
749 useful if you're carrying about ordering or calling the iterator from more
750 than one sequence, or not really storing things in a hash anywhere.
752 For our example, we're using a real hash so we'll do just the simple
753 thing, but we'll have to go through the LIST field indirectly.
756 carp &whowasi if $DEBUG;
758 return each %{ $self->{LIST} }
763 This is called when the hash is evaluated in scalar context. In order
764 to mimic the behaviour of untied hashes, this method should return a
765 false value when the tied hash is considered empty. If this method does
766 not exist, perl will make some educated guesses and return true when
767 the hash is inside an iteration. If this isn't the case, FIRSTKEY is
768 called, and the result will be a false value if FIRSTKEY returns the empty
769 list, true otherwise.
771 However, you should B<not> blindly rely on perl always doing the right
772 thing. Particularly, perl will mistakenly return true when you clear the
773 hash by repeatedly calling DELETE until it is empty. You are therefore
774 advised to supply your own SCALAR method when you want to be absolutely
775 sure that your hash behaves nicely in scalar context.
777 In our example we can just call C<scalar> on the underlying hash
778 referenced by C<$self-E<gt>{LIST}>:
781 carp &whowasi if $DEBUG;
783 return scalar %{ $self->{LIST} }
788 This is called when C<untie> occurs. See L<The C<untie> Gotcha> below.
792 This method is triggered when a tied hash is about to go out of
793 scope. You don't really need it unless you're trying to add debugging
794 or have auxiliary state to clean up. Here's a very simple function:
797 carp &whowasi if $DEBUG;
802 Note that functions such as keys() and values() may return huge lists
803 when used on large objects, like DBM files. You may prefer to use the
804 each() function to iterate over such. Example:
806 # print out history file offsets
808 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
809 while (($key,$val) = each %HIST) {
810 print $key, ' = ', unpack('L',$val), "\n";
814 =head2 Tying FileHandles
816 This is partially implemented now.
818 A class implementing a tied filehandle should define the following
819 methods: TIEHANDLE, at least one of PRINT, PRINTF, WRITE, READLINE, GETC,
820 READ, and possibly CLOSE, UNTIE and DESTROY. The class can also provide: BINMODE,
821 OPEN, EOF, FILENO, SEEK, TELL - if the corresponding perl operators are
824 When STDERR is tied, its PRINT method will be called to issue warnings
825 and error messages. This feature is temporarily disabled during the call,
826 which means you can use C<warn()> inside PRINT without starting a recursive
827 loop. And just like C<__WARN__> and C<__DIE__> handlers, STDERR's PRINT
828 method may be called to report parser errors, so the caveats mentioned under
829 L<perlvar/%SIG> apply.
831 All of this is especially useful when perl is embedded in some other
832 program, where output to STDOUT and STDERR may have to be redirected
833 in some special way. See nvi and the Apache module for examples.
835 In our example we're going to create a shouting handle.
841 =item TIEHANDLE classname, LIST
843 This is the constructor for the class. That means it is expected to
844 return a blessed reference of some sort. The reference can be used to
845 hold some internal information.
847 sub TIEHANDLE { print "<shout>\n"; my $i; bless \$i, shift }
849 =item WRITE this, LIST
851 This method will be called when the handle is written to via the
852 C<syswrite> function.
856 my($buf,$len,$offset) = @_;
857 print "WRITE called, \$buf=$buf, \$len=$len, \$offset=$offset";
860 =item PRINT this, LIST
862 This method will be triggered every time the tied handle is printed to
863 with the C<print()> function.
864 Beyond its self reference it also expects the list that was passed to
867 sub PRINT { $r = shift; $$r++; print join($,,map(uc($_),@_)),$\ }
869 =item PRINTF this, LIST
871 This method will be triggered every time the tied handle is printed to
872 with the C<printf()> function.
873 Beyond its self reference it also expects the format and list that was
874 passed to the printf function.
879 print sprintf($fmt, @_);
882 =item READ this, LIST
884 This method will be called when the handle is read from via the C<read>
885 or C<sysread> functions.
890 my(undef,$len,$offset) = @_;
891 print "READ called, \$buf=$bufref, \$len=$len, \$offset=$offset";
892 # add to $$bufref, set $len to number of characters read
898 This method will be called when the handle is read from via <HANDLE>.
899 The method should return undef when there is no more data.
901 sub READLINE { $r = shift; "READLINE called $$r times\n"; }
905 This method will be called when the C<getc> function is called.
907 sub GETC { print "Don't GETC, Get Perl"; return "a"; }
911 This method will be called when the handle is closed via the C<close>
914 sub CLOSE { print "CLOSE called.\n" }
918 As with the other types of ties, this method will be called when C<untie> happens.
919 It may be appropriate to "auto CLOSE" when this occurs. See
920 L<The C<untie> Gotcha> below.
924 As with the other types of ties, this method will be called when the
925 tied handle is about to be destroyed. This is useful for debugging and
926 possibly cleaning up.
928 sub DESTROY { print "</shout>\n" }
932 Here's how to use our little example:
937 print FOO $a, " plus ", $b, " equals ", $a + $b, "\n";
942 You can define for all tie types an UNTIE method that will be called
943 at untie(). See L<The C<untie> Gotcha> below.
945 =head2 The C<untie> Gotcha
947 If you intend making use of the object returned from either tie() or
948 tied(), and if the tie's target class defines a destructor, there is a
949 subtle gotcha you I<must> guard against.
951 As setup, consider this (admittedly rather contrived) example of a
952 tie; all it does is use a file to keep a log of the values assigned to
963 my $filename = shift;
964 my $handle = new IO::File "> $filename"
965 or die "Cannot open $filename: $!\n";
967 print $handle "The Start\n";
968 bless {FH => $handle, Value => 0}, $class;
973 return $self->{Value};
979 my $handle = $self->{FH};
980 print $handle "$value\n";
981 $self->{Value} = $value;
986 my $handle = $self->{FH};
987 print $handle "The End\n";
993 Here is an example that makes use of this tie:
999 tie $fred, 'Remember', 'myfile.txt';
1004 system "cat myfile.txt";
1006 This is the output when it is executed:
1014 So far so good. Those of you who have been paying attention will have
1015 spotted that the tied object hasn't been used so far. So lets add an
1016 extra method to the Remember class to allow comments to be included in
1017 the file -- say, something like this:
1022 my $handle = $self->{FH};
1023 print $handle $text, "\n";
1026 And here is the previous example modified to use the C<comment> method
1027 (which requires the tied object):
1033 $x = tie $fred, 'Remember', 'myfile.txt';
1036 comment $x "changing...";
1039 system "cat myfile.txt";
1041 When this code is executed there is no output. Here's why:
1043 When a variable is tied, it is associated with the object which is the
1044 return value of the TIESCALAR, TIEARRAY, or TIEHASH function. This
1045 object normally has only one reference, namely, the implicit reference
1046 from the tied variable. When untie() is called, that reference is
1047 destroyed. Then, as in the first example above, the object's
1048 destructor (DESTROY) is called, which is normal for objects that have
1049 no more valid references; and thus the file is closed.
1051 In the second example, however, we have stored another reference to
1052 the tied object in $x. That means that when untie() gets called
1053 there will still be a valid reference to the object in existence, so
1054 the destructor is not called at that time, and thus the file is not
1055 closed. The reason there is no output is because the file buffers
1056 have not been flushed to disk.
1058 Now that you know what the problem is, what can you do to avoid it?
1059 Prior to the introduction of the optional UNTIE method the only way
1060 was the good old C<-w> flag. Which will spot any instances where you call
1061 untie() and there are still valid references to the tied object. If
1062 the second script above this near the top C<use warnings 'untie'>
1063 or was run with the C<-w> flag, Perl prints this
1066 untie attempted while 1 inner references still exist
1068 To get the script to work properly and silence the warning make sure
1069 there are no valid references to the tied object I<before> untie() is
1075 Now that UNTIE exists the class designer can decide which parts of the
1076 class functionality are really associated with C<untie> and which with
1077 the object being destroyed. What makes sense for a given class depends
1078 on whether the inner references are being kept so that non-tie-related
1079 methods can be called on the object. But in most cases it probably makes
1080 sense to move the functionality that would have been in DESTROY to the UNTIE
1083 If the UNTIE method exists then the warning above does not occur. Instead the
1084 UNTIE method is passed the count of "extra" references and can issue its own
1085 warning if appropriate. e.g. to replicate the no UNTIE case this method can
1090 my ($obj,$count) = @_;
1091 carp "untie attempted while $count inner references still exist" if $count;
1096 See L<DB_File> or L<Config> for some interesting tie() implementations.
1097 A good starting point for many tie() implementations is with one of the
1098 modules L<Tie::Scalar>, L<Tie::Array>, L<Tie::Hash>, or L<Tie::Handle>.
1102 The bucket usage information provided by C<scalar(%hash)> is not
1103 available. What this means is that using %tied_hash in boolean
1104 context doesn't work right (currently this always tests false,
1105 regardless of whether the hash is empty or hash elements).
1107 Localizing tied arrays or hashes does not work. After exiting the
1108 scope the arrays or the hashes are not restored.
1110 Counting the number of entries in a hash via C<scalar(keys(%hash))>
1111 or C<scalar(values(%hash)>) is inefficient since it needs to iterate
1112 through all the entries with FIRSTKEY/NEXTKEY.
1114 Tied hash/array slices cause multiple FETCH/STORE pairs, there are no
1115 tie methods for slice operations.
1117 You cannot easily tie a multilevel data structure (such as a hash of
1118 hashes) to a dbm file. The first problem is that all but GDBM and
1119 Berkeley DB have size limitations, but beyond that, you also have problems
1120 with how references are to be represented on disk. One experimental
1121 module that does attempt to address this need partially is the MLDBM
1122 module. Check your nearest CPAN site as described in L<perlmodlib> for
1123 source code to MLDBM.
1125 Tied filehandles are still incomplete. sysopen(), truncate(),
1126 flock(), fcntl(), stat() and -X can't currently be trapped.
1132 TIEHANDLE by Sven Verdoolaege <F<skimo@dns.ufsia.ac.be>> and Doug MacEachern <F<dougm@osf.org>>
1134 UNTIE by Nick Ing-Simmons <F<nick@ing-simmons.net>>
1136 SCALAR by Tassilo von Parseval <F<tassilo.von.parseval@rwth-aachen.de>>
1138 Tying Arrays by Casey West <F<casey@geeknest.com>>