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, $$self, $new_nicety)) {
155 confess "setpriority failed: $!";
162 This method will be triggered when the C<untie> occurs. This can be useful
163 if the class needs to know when no further calls will be made. (Except DESTROY
164 of course.) See L<The C<untie> Gotcha> below for more details.
168 This method will be triggered when the tied variable needs to be destructed.
169 As with other object classes, such a method is seldom necessary, because Perl
170 deallocates its moribund object's memory for you automatically--this isn't
171 C++, you know. We'll use a DESTROY method here for debugging purposes only.
175 confess "wrong type" unless ref $self;
176 carp "[ Nice::DESTROY pid $$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. You may disable this feature by
262 assigning a true value to the variable C<$NEGATIVE_INDICES> in the
265 As you may have noticed, the name of the FETCH method (et al.) is the same
266 for all accesses, even though the constructors differ in names (TIESCALAR
267 vs TIEARRAY). While in theory you could have the same class servicing
268 several tied types, in practice this becomes cumbersome, and it's easiest
269 to keep them at simply one tie type per class.
271 =item STORE this, index, value
273 This method will be triggered every time an element in the tied array is set
274 (written). It takes two arguments beyond its self reference: the index at
275 which we're trying to store something and the value we're trying to put
278 In our example, C<undef> is really C<$self-E<gt>{ELEMSIZE}> number of
279 spaces so we have a little more work to do here:
283 my( $index, $value ) = @_;
284 if ( length $value > $self->{ELEMSIZE} ) {
285 croak "length of $value is greater than $self->{ELEMSIZE}";
288 $self->EXTEND( $index ) if $index > $self->FETCHSIZE();
289 # right justify to keep element size for smaller elements
290 $self->{ARRAY}->[$index] = sprintf "%$self->{ELEMSIZE}s", $value;
293 Negative indexes are treated the same as with FETCH.
297 Returns the total number of items in the tied array associated with
298 object I<this>. (Equivalent to C<scalar(@array)>). For example:
302 return scalar @{$self->{ARRAY}};
305 =item STORESIZE this, count
307 Sets the total number of items in the tied array associated with
308 object I<this> to be I<count>. If this makes the array larger then
309 class's mapping of C<undef> should be returned for new positions.
310 If the array becomes smaller then entries beyond count should be
313 In our example, 'undef' is really an element containing
314 C<$self-E<gt>{ELEMSIZE}> number of spaces. Observe:
319 if ( $count > $self->FETCHSIZE() ) {
320 foreach ( $count - $self->FETCHSIZE() .. $count ) {
321 $self->STORE( $_, '' );
323 } elsif ( $count < $self->FETCHSIZE() ) {
324 foreach ( 0 .. $self->FETCHSIZE() - $count - 2 ) {
330 =item EXTEND this, count
332 Informative call that array is likely to grow to have I<count> entries.
333 Can be used to optimize allocation. This method need do nothing.
335 In our example, we want to make sure there are no blank (C<undef>)
336 entries, so C<EXTEND> will make use of C<STORESIZE> to fill elements
342 $self->STORESIZE( $count );
345 =item EXISTS this, key
347 Verify that the element at index I<key> exists in the tied array I<this>.
349 In our example, we will determine that if an element consists of
350 C<$self-E<gt>{ELEMSIZE}> spaces only, it does not exist:
355 return 0 if ! defined $self->{ARRAY}->[$index] ||
356 $self->{ARRAY}->[$index] eq ' ' x $self->{ELEMSIZE};
360 =item DELETE this, key
362 Delete the element at index I<key> from the tied array I<this>.
364 In our example, a deleted item is C<$self->{ELEMSIZE}> spaces:
369 return $self->STORE( $index, '' );
374 Clear (remove, delete, ...) all values from the tied array associated with
375 object I<this>. For example:
379 return $self->{ARRAY} = [];
382 =item PUSH this, LIST
384 Append elements of I<LIST> to the array. For example:
389 my $last = $self->FETCHSIZE();
390 $self->STORE( $last + $_, $list[$_] ) foreach 0 .. $#list;
391 return $self->FETCHSIZE();
396 Remove last element of the array and return it. For example:
400 return pop @{$self->{ARRAY}};
405 Remove the first element of the array (shifting other elements down)
406 and return it. For example:
410 return shift @{$self->{ARRAY}};
413 =item UNSHIFT this, LIST
415 Insert LIST elements at the beginning of the array, moving existing elements
416 up to make room. For example:
421 my $size = scalar( @list );
422 # make room for our list
423 @{$self->{ARRAY}}[ $size .. $#{$self->{ARRAY}} + $size ]
425 $self->STORE( $_, $list[$_] ) foreach 0 .. $#list;
428 =item SPLICE this, offset, length, LIST
430 Perform the equivalent of C<splice> on the array.
432 I<offset> is optional and defaults to zero, negative values count back
433 from the end of the array.
435 I<length> is optional and defaults to rest of the array.
437 I<LIST> may be empty.
439 Returns a list of the original I<length> elements at I<offset>.
441 In our example, we'll use a little shortcut if there is a I<LIST>:
445 my $offset = shift || 0;
446 my $length = shift || $self->FETCHSIZE() - $offset;
449 tie @list, __PACKAGE__, $self->{ELEMSIZE};
452 return splice @{$self->{ARRAY}}, $offset, $length, @list;
457 Will be called when C<untie> happens. (See L<The C<untie> Gotcha> below.)
461 This method will be triggered when the tied variable needs to be destructed.
462 As with the scalar tie class, this is almost never needed in a
463 language that does its own garbage collection, so this time we'll
470 Hashes were the first Perl data type to be tied (see dbmopen()). A class
471 implementing a tied hash should define the following methods: TIEHASH is
472 the constructor. FETCH and STORE access the key and value pairs. EXISTS
473 reports whether a key is present in the hash, and DELETE deletes one.
474 CLEAR empties the hash by deleting all the key and value pairs. FIRSTKEY
475 and NEXTKEY implement the keys() and each() functions to iterate over all
476 the keys. UNTIE is called when C<untie> happens, and DESTROY is called when
477 the tied variable is garbage collected.
479 If this seems like a lot, then feel free to inherit from merely the
480 standard Tie::StdHash module for most of your methods, redefining only the
481 interesting ones. See L<Tie::Hash> for details.
483 Remember that Perl distinguishes between a key not existing in the hash,
484 and the key existing in the hash but having a corresponding value of
485 C<undef>. The two possibilities can be tested with the C<exists()> and
486 C<defined()> functions.
488 Here's an example of a somewhat interesting tied hash class: it gives you
489 a hash representing a particular user's dot files. You index into the hash
490 with the name of the file (minus the dot) and you get back that dot file's
491 contents. For example:
494 tie %dot, 'DotFiles';
495 if ( $dot{profile} =~ /MANPATH/ ||
496 $dot{login} =~ /MANPATH/ ||
497 $dot{cshrc} =~ /MANPATH/ )
499 print "you seem to set your MANPATH\n";
502 Or here's another sample of using our tied class:
504 tie %him, 'DotFiles', 'daemon';
505 foreach $f ( keys %him ) {
506 printf "daemon dot file %s is size %d\n",
510 In our tied hash DotFiles example, we use a regular
511 hash for the object containing several important
512 fields, of which only the C<{LIST}> field will be what the
513 user thinks of as the real hash.
519 whose dot files this object represents
523 where those dot files live
527 whether we should try to change or remove those dot files
531 the hash of dot file names and content mappings
535 Here's the start of F<Dotfiles.pm>:
539 sub whowasi { (caller(1))[3] . '()' }
541 sub debug { $DEBUG = @_ ? shift : 1 }
543 For our example, we want to be able to emit debugging info to help in tracing
544 during development. We keep also one convenience function around
545 internally to help print out warnings; whowasi() returns the function name
548 Here are the methods for the DotFiles tied hash.
552 =item TIEHASH classname, LIST
554 This is the constructor for the class. That means it is expected to
555 return a blessed reference through which the new object (probably but not
556 necessarily an anonymous hash) will be accessed.
558 Here's the constructor:
562 my $user = shift || $>;
563 my $dotdir = shift || '';
564 croak "usage: @{[&whowasi]} [USER [DOTDIR]]" if @_;
565 $user = getpwuid($user) if $user =~ /^\d+$/;
566 my $dir = (getpwnam($user))[7]
567 || croak "@{[&whowasi]}: no user $user";
568 $dir .= "/$dotdir" if $dotdir;
578 || croak "@{[&whowasi]}: can't opendir $dir: $!";
579 foreach $dot ( grep /^\./ && -f "$dir/$_", readdir(DIR)) {
581 $node->{LIST}{$dot} = undef;
584 return bless $node, $self;
587 It's probably worth mentioning that if you're going to filetest the
588 return values out of a readdir, you'd better prepend the directory
589 in question. Otherwise, because we didn't chdir() there, it would
590 have been testing the wrong file.
592 =item FETCH this, key
594 This method will be triggered every time an element in the tied hash is
595 accessed (read). It takes one argument beyond its self reference: the key
596 whose value we're trying to fetch.
598 Here's the fetch for our DotFiles example.
601 carp &whowasi if $DEBUG;
604 my $dir = $self->{HOME};
605 my $file = "$dir/.$dot";
607 unless (exists $self->{LIST}->{$dot} || -f $file) {
608 carp "@{[&whowasi]}: no $dot file" if $DEBUG;
612 if (defined $self->{LIST}->{$dot}) {
613 return $self->{LIST}->{$dot};
615 return $self->{LIST}->{$dot} = `cat $dir/.$dot`;
619 It was easy to write by having it call the Unix cat(1) command, but it
620 would probably be more portable to open the file manually (and somewhat
621 more efficient). Of course, because dot files are a Unixy concept, we're
624 =item STORE this, key, value
626 This method will be triggered every time an element in the tied hash is set
627 (written). It takes two arguments beyond its self reference: the index at
628 which we're trying to store something, and the value we're trying to put
631 Here in our DotFiles example, we'll be careful not to let
632 them try to overwrite the file unless they've called the clobber()
633 method on the original object reference returned by tie().
636 carp &whowasi if $DEBUG;
640 my $file = $self->{HOME} . "/.$dot";
641 my $user = $self->{USER};
643 croak "@{[&whowasi]}: $file not clobberable"
644 unless $self->{CLOBBER};
646 open(F, "> $file") || croak "can't open $file: $!";
651 If they wanted to clobber something, they might say:
653 $ob = tie %daemon_dots, 'daemon';
655 $daemon_dots{signature} = "A true daemon\n";
657 Another way to lay hands on a reference to the underlying object is to
658 use the tied() function, so they might alternately have set clobber
661 tie %daemon_dots, 'daemon';
662 tied(%daemon_dots)->clobber(1);
664 The clobber method is simply:
668 $self->{CLOBBER} = @_ ? shift : 1;
671 =item DELETE this, key
673 This method is triggered when we remove an element from the hash,
674 typically by using the delete() function. Again, we'll
675 be careful to check whether they really want to clobber files.
678 carp &whowasi if $DEBUG;
682 my $file = $self->{HOME} . "/.$dot";
683 croak "@{[&whowasi]}: won't remove file $file"
684 unless $self->{CLOBBER};
685 delete $self->{LIST}->{$dot};
686 my $success = unlink($file);
687 carp "@{[&whowasi]}: can't unlink $file: $!" unless $success;
691 The value returned by DELETE becomes the return value of the call
692 to delete(). If you want to emulate the normal behavior of delete(),
693 you should return whatever FETCH would have returned for this key.
694 In this example, we have chosen instead to return a value which tells
695 the caller whether the file was successfully deleted.
699 This method is triggered when the whole hash is to be cleared, usually by
700 assigning the empty list to it.
702 In our example, that would remove all the user's dot files! It's such a
703 dangerous thing that they'll have to set CLOBBER to something higher than
707 carp &whowasi if $DEBUG;
709 croak "@{[&whowasi]}: won't remove all dot files for $self->{USER}"
710 unless $self->{CLOBBER} > 1;
712 foreach $dot ( keys %{$self->{LIST}}) {
717 =item EXISTS this, key
719 This method is triggered when the user uses the exists() function
720 on a particular hash. In our example, we'll look at the C<{LIST}>
721 hash element for this:
724 carp &whowasi if $DEBUG;
727 return exists $self->{LIST}->{$dot};
732 This method will be triggered when the user is going
733 to iterate through the hash, such as via a keys() or each()
737 carp &whowasi if $DEBUG;
739 my $a = keys %{$self->{LIST}}; # reset each() iterator
740 each %{$self->{LIST}}
743 =item NEXTKEY this, lastkey
745 This method gets triggered during a keys() or each() iteration. It has a
746 second argument which is the last key that had been accessed. This is
747 useful if you're carrying about ordering or calling the iterator from more
748 than one sequence, or not really storing things in a hash anywhere.
750 For our example, we're using a real hash so we'll do just the simple
751 thing, but we'll have to go through the LIST field indirectly.
754 carp &whowasi if $DEBUG;
756 return each %{ $self->{LIST} }
761 This is called when C<untie> occurs. See L<The C<untie> Gotcha> below.
765 This method is triggered when a tied hash is about to go out of
766 scope. You don't really need it unless you're trying to add debugging
767 or have auxiliary state to clean up. Here's a very simple function:
770 carp &whowasi if $DEBUG;
775 Note that functions such as keys() and values() may return huge lists
776 when used on large objects, like DBM files. You may prefer to use the
777 each() function to iterate over such. Example:
779 # print out history file offsets
781 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
782 while (($key,$val) = each %HIST) {
783 print $key, ' = ', unpack('L',$val), "\n";
787 =head2 Tying FileHandles
789 This is partially implemented now.
791 A class implementing a tied filehandle should define the following
792 methods: TIEHANDLE, at least one of PRINT, PRINTF, WRITE, READLINE, GETC,
793 READ, and possibly CLOSE, UNTIE and DESTROY. The class can also provide: BINMODE,
794 OPEN, EOF, FILENO, SEEK, TELL - if the corresponding perl operators are
797 It is especially useful when perl is embedded in some other program,
798 where output to STDOUT and STDERR may have to be redirected in some
799 special way. See nvi and the Apache module for examples.
801 In our example we're going to create a shouting handle.
807 =item TIEHANDLE classname, LIST
809 This is the constructor for the class. That means it is expected to
810 return a blessed reference of some sort. The reference can be used to
811 hold some internal information.
813 sub TIEHANDLE { print "<shout>\n"; my $i; bless \$i, shift }
815 =item WRITE this, LIST
817 This method will be called when the handle is written to via the
818 C<syswrite> function.
822 my($buf,$len,$offset) = @_;
823 print "WRITE called, \$buf=$buf, \$len=$len, \$offset=$offset";
826 =item PRINT this, LIST
828 This method will be triggered every time the tied handle is printed to
829 with the C<print()> function.
830 Beyond its self reference it also expects the list that was passed to
833 sub PRINT { $r = shift; $$r++; print join($,,map(uc($_),@_)),$\ }
835 =item PRINTF this, LIST
837 This method will be triggered every time the tied handle is printed to
838 with the C<printf()> function.
839 Beyond its self reference it also expects the format and list that was
840 passed to the printf function.
845 print sprintf($fmt, @_)."\n";
848 =item READ this, LIST
850 This method will be called when the handle is read from via the C<read>
851 or C<sysread> functions.
856 my(undef,$len,$offset) = @_;
857 print "READ called, \$buf=$bufref, \$len=$len, \$offset=$offset";
858 # add to $$bufref, set $len to number of characters read
864 This method will be called when the handle is read from via <HANDLE>.
865 The method should return undef when there is no more data.
867 sub READLINE { $r = shift; "READLINE called $$r times\n"; }
871 This method will be called when the C<getc> function is called.
873 sub GETC { print "Don't GETC, Get Perl"; return "a"; }
877 This method will be called when the handle is closed via the C<close>
880 sub CLOSE { print "CLOSE called.\n" }
884 As with the other types of ties, this method will be called when C<untie> happens.
885 It may be appropriate to "auto CLOSE" when this occurs. See
886 L<The C<untie> Gotcha> below.
890 As with the other types of ties, this method will be called when the
891 tied handle is about to be destroyed. This is useful for debugging and
892 possibly cleaning up.
894 sub DESTROY { print "</shout>\n" }
898 Here's how to use our little example:
903 print FOO $a, " plus ", $b, " equals ", $a + $b, "\n";
908 You can define for all tie types an UNTIE method that will be called
909 at untie(). See L<The C<untie> Gotcha> below.
911 =head2 The C<untie> Gotcha
913 If you intend making use of the object returned from either tie() or
914 tied(), and if the tie's target class defines a destructor, there is a
915 subtle gotcha you I<must> guard against.
917 As setup, consider this (admittedly rather contrived) example of a
918 tie; all it does is use a file to keep a log of the values assigned to
929 my $filename = shift;
930 my $handle = new IO::File "> $filename"
931 or die "Cannot open $filename: $!\n";
933 print $handle "The Start\n";
934 bless {FH => $handle, Value => 0}, $class;
939 return $self->{Value};
945 my $handle = $self->{FH};
946 print $handle "$value\n";
947 $self->{Value} = $value;
952 my $handle = $self->{FH};
953 print $handle "The End\n";
959 Here is an example that makes use of this tie:
965 tie $fred, 'Remember', 'myfile.txt';
970 system "cat myfile.txt";
972 This is the output when it is executed:
980 So far so good. Those of you who have been paying attention will have
981 spotted that the tied object hasn't been used so far. So lets add an
982 extra method to the Remember class to allow comments to be included in
983 the file -- say, something like this:
988 my $handle = $self->{FH};
989 print $handle $text, "\n";
992 And here is the previous example modified to use the C<comment> method
993 (which requires the tied object):
999 $x = tie $fred, 'Remember', 'myfile.txt';
1002 comment $x "changing...";
1005 system "cat myfile.txt";
1007 When this code is executed there is no output. Here's why:
1009 When a variable is tied, it is associated with the object which is the
1010 return value of the TIESCALAR, TIEARRAY, or TIEHASH function. This
1011 object normally has only one reference, namely, the implicit reference
1012 from the tied variable. When untie() is called, that reference is
1013 destroyed. Then, as in the first example above, the object's
1014 destructor (DESTROY) is called, which is normal for objects that have
1015 no more valid references; and thus the file is closed.
1017 In the second example, however, we have stored another reference to
1018 the tied object in $x. That means that when untie() gets called
1019 there will still be a valid reference to the object in existence, so
1020 the destructor is not called at that time, and thus the file is not
1021 closed. The reason there is no output is because the file buffers
1022 have not been flushed to disk.
1024 Now that you know what the problem is, what can you do to avoid it?
1025 Prior to the introduction of the optional UNTIE method the only way
1026 was the good old C<-w> flag. Which will spot any instances where you call
1027 untie() and there are still valid references to the tied object. If
1028 the second script above this near the top C<use warnings 'untie'>
1029 or was run with the C<-w> flag, Perl prints this
1032 untie attempted while 1 inner references still exist
1034 To get the script to work properly and silence the warning make sure
1035 there are no valid references to the tied object I<before> untie() is
1041 Now that UNTIE exists the class designer can decide which parts of the
1042 class functionality are really associated with C<untie> and which with
1043 the object being destroyed. What makes sense for a given class depends
1044 on whether the inner references are being kept so that non-tie-related
1045 methods can be called on the object. But in most cases it probably makes
1046 sense to move the functionality that would have been in DESTROY to the UNTIE
1049 If the UNTIE method exists then the warning above does not occur. Instead the
1050 UNTIE method is passed the count of "extra" references and can issue its own
1051 warning if appropriate. e.g. to replicate the no UNTIE case this method can
1056 my ($obj,$count) = @_;
1057 carp "untie attempted while $count inner references still exist" if $count;
1062 See L<DB_File> or L<Config> for some interesting tie() implementations.
1063 A good starting point for many tie() implementations is with one of the
1064 modules L<Tie::Scalar>, L<Tie::Array>, L<Tie::Hash>, or L<Tie::Handle>.
1068 You cannot easily tie a multilevel data structure (such as a hash of
1069 hashes) to a dbm file. The first problem is that all but GDBM and
1070 Berkeley DB have size limitations, but beyond that, you also have problems
1071 with how references are to be represented on disk. One experimental
1072 module that does attempt to address this need partially is the MLDBM
1073 module. Check your nearest CPAN site as described in L<perlmodlib> for
1074 source code to MLDBM.
1076 Tied filehandles are still incomplete. sysopen(), truncate(),
1077 flock(), fcntl(), stat() and -X can't currently be trapped.
1083 TIEHANDLE by Sven Verdoolaege <F<skimo@dns.ufsia.ac.be>> and Doug MacEachern <F<dougm@osf.org>>
1085 UNTIE by Nick Ing-Simmons <F<nick@ing-simmons.net>>
1087 Tying Arrays by Casey West <F<casey@geeknest.com>>