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. UNTIE is called when C<untie> happens, and DESTROY is called when
478 the tied variable is garbage collected.
480 If this seems like a lot, then feel free to inherit from merely the
481 standard Tie::StdHash module for most of your methods, redefining only the
482 interesting ones. See L<Tie::Hash> for details.
484 Remember that Perl distinguishes between a key not existing in the hash,
485 and the key existing in the hash but having a corresponding value of
486 C<undef>. The two possibilities can be tested with the C<exists()> and
487 C<defined()> functions.
489 Here's an example of a somewhat interesting tied hash class: it gives you
490 a hash representing a particular user's dot files. You index into the hash
491 with the name of the file (minus the dot) and you get back that dot file's
492 contents. For example:
495 tie %dot, 'DotFiles';
496 if ( $dot{profile} =~ /MANPATH/ ||
497 $dot{login} =~ /MANPATH/ ||
498 $dot{cshrc} =~ /MANPATH/ )
500 print "you seem to set your MANPATH\n";
503 Or here's another sample of using our tied class:
505 tie %him, 'DotFiles', 'daemon';
506 foreach $f ( keys %him ) {
507 printf "daemon dot file %s is size %d\n",
511 In our tied hash DotFiles example, we use a regular
512 hash for the object containing several important
513 fields, of which only the C<{LIST}> field will be what the
514 user thinks of as the real hash.
520 whose dot files this object represents
524 where those dot files live
528 whether we should try to change or remove those dot files
532 the hash of dot file names and content mappings
536 Here's the start of F<Dotfiles.pm>:
540 sub whowasi { (caller(1))[3] . '()' }
542 sub debug { $DEBUG = @_ ? shift : 1 }
544 For our example, we want to be able to emit debugging info to help in tracing
545 during development. We keep also one convenience function around
546 internally to help print out warnings; whowasi() returns the function name
549 Here are the methods for the DotFiles tied hash.
553 =item TIEHASH classname, LIST
555 This is the constructor for the class. That means it is expected to
556 return a blessed reference through which the new object (probably but not
557 necessarily an anonymous hash) will be accessed.
559 Here's the constructor:
563 my $user = shift || $>;
564 my $dotdir = shift || '';
565 croak "usage: @{[&whowasi]} [USER [DOTDIR]]" if @_;
566 $user = getpwuid($user) if $user =~ /^\d+$/;
567 my $dir = (getpwnam($user))[7]
568 || croak "@{[&whowasi]}: no user $user";
569 $dir .= "/$dotdir" if $dotdir;
579 || croak "@{[&whowasi]}: can't opendir $dir: $!";
580 foreach $dot ( grep /^\./ && -f "$dir/$_", readdir(DIR)) {
582 $node->{LIST}{$dot} = undef;
585 return bless $node, $self;
588 It's probably worth mentioning that if you're going to filetest the
589 return values out of a readdir, you'd better prepend the directory
590 in question. Otherwise, because we didn't chdir() there, it would
591 have been testing the wrong file.
593 =item FETCH this, key
595 This method will be triggered every time an element in the tied hash is
596 accessed (read). It takes one argument beyond its self reference: the key
597 whose value we're trying to fetch.
599 Here's the fetch for our DotFiles example.
602 carp &whowasi if $DEBUG;
605 my $dir = $self->{HOME};
606 my $file = "$dir/.$dot";
608 unless (exists $self->{LIST}->{$dot} || -f $file) {
609 carp "@{[&whowasi]}: no $dot file" if $DEBUG;
613 if (defined $self->{LIST}->{$dot}) {
614 return $self->{LIST}->{$dot};
616 return $self->{LIST}->{$dot} = `cat $dir/.$dot`;
620 It was easy to write by having it call the Unix cat(1) command, but it
621 would probably be more portable to open the file manually (and somewhat
622 more efficient). Of course, because dot files are a Unixy concept, we're
625 =item STORE this, key, value
627 This method will be triggered every time an element in the tied hash is set
628 (written). It takes two arguments beyond its self reference: the index at
629 which we're trying to store something, and the value we're trying to put
632 Here in our DotFiles example, we'll be careful not to let
633 them try to overwrite the file unless they've called the clobber()
634 method on the original object reference returned by tie().
637 carp &whowasi if $DEBUG;
641 my $file = $self->{HOME} . "/.$dot";
642 my $user = $self->{USER};
644 croak "@{[&whowasi]}: $file not clobberable"
645 unless $self->{CLOBBER};
647 open(F, "> $file") || croak "can't open $file: $!";
652 If they wanted to clobber something, they might say:
654 $ob = tie %daemon_dots, 'daemon';
656 $daemon_dots{signature} = "A true daemon\n";
658 Another way to lay hands on a reference to the underlying object is to
659 use the tied() function, so they might alternately have set clobber
662 tie %daemon_dots, 'daemon';
663 tied(%daemon_dots)->clobber(1);
665 The clobber method is simply:
669 $self->{CLOBBER} = @_ ? shift : 1;
672 =item DELETE this, key
674 This method is triggered when we remove an element from the hash,
675 typically by using the delete() function. Again, we'll
676 be careful to check whether they really want to clobber files.
679 carp &whowasi if $DEBUG;
683 my $file = $self->{HOME} . "/.$dot";
684 croak "@{[&whowasi]}: won't remove file $file"
685 unless $self->{CLOBBER};
686 delete $self->{LIST}->{$dot};
687 my $success = unlink($file);
688 carp "@{[&whowasi]}: can't unlink $file: $!" unless $success;
692 The value returned by DELETE becomes the return value of the call
693 to delete(). If you want to emulate the normal behavior of delete(),
694 you should return whatever FETCH would have returned for this key.
695 In this example, we have chosen instead to return a value which tells
696 the caller whether the file was successfully deleted.
700 This method is triggered when the whole hash is to be cleared, usually by
701 assigning the empty list to it.
703 In our example, that would remove all the user's dot files! It's such a
704 dangerous thing that they'll have to set CLOBBER to something higher than
708 carp &whowasi if $DEBUG;
710 croak "@{[&whowasi]}: won't remove all dot files for $self->{USER}"
711 unless $self->{CLOBBER} > 1;
713 foreach $dot ( keys %{$self->{LIST}}) {
718 =item EXISTS this, key
720 This method is triggered when the user uses the exists() function
721 on a particular hash. In our example, we'll look at the C<{LIST}>
722 hash element for this:
725 carp &whowasi if $DEBUG;
728 return exists $self->{LIST}->{$dot};
733 This method will be triggered when the user is going
734 to iterate through the hash, such as via a keys() or each()
738 carp &whowasi if $DEBUG;
740 my $a = keys %{$self->{LIST}}; # reset each() iterator
741 each %{$self->{LIST}}
744 =item NEXTKEY this, lastkey
746 This method gets triggered during a keys() or each() iteration. It has a
747 second argument which is the last key that had been accessed. This is
748 useful if you're carrying about ordering or calling the iterator from more
749 than one sequence, or not really storing things in a hash anywhere.
751 For our example, we're using a real hash so we'll do just the simple
752 thing, but we'll have to go through the LIST field indirectly.
755 carp &whowasi if $DEBUG;
757 return each %{ $self->{LIST} }
762 This is called when C<untie> occurs. See L<The C<untie> Gotcha> below.
766 This method is triggered when a tied hash is about to go out of
767 scope. You don't really need it unless you're trying to add debugging
768 or have auxiliary state to clean up. Here's a very simple function:
771 carp &whowasi if $DEBUG;
776 Note that functions such as keys() and values() may return huge lists
777 when used on large objects, like DBM files. You may prefer to use the
778 each() function to iterate over such. Example:
780 # print out history file offsets
782 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
783 while (($key,$val) = each %HIST) {
784 print $key, ' = ', unpack('L',$val), "\n";
788 =head2 Tying FileHandles
790 This is partially implemented now.
792 A class implementing a tied filehandle should define the following
793 methods: TIEHANDLE, at least one of PRINT, PRINTF, WRITE, READLINE, GETC,
794 READ, and possibly CLOSE, UNTIE and DESTROY. The class can also provide: BINMODE,
795 OPEN, EOF, FILENO, SEEK, TELL - if the corresponding perl operators are
798 When STDERR is tied, its PRINT method will be called to issue warnings
799 and error messages. This feature is temporarily disabled during the call,
800 which means you can use C<warn()> inside PRINT without starting a recursive
801 loop. And just like C<__WARN__> and C<__DIE__> handlers, STDERR's PRINT
802 method may be called to report parser errors, so the caveats mentioned under
803 L<perlvar/%SIG> apply.
805 All of this is especially useful when perl is embedded in some other
806 program, where output to STDOUT and STDERR may have to be redirected
807 in some special way. See nvi and the Apache module for examples.
809 In our example we're going to create a shouting handle.
815 =item TIEHANDLE classname, LIST
817 This is the constructor for the class. That means it is expected to
818 return a blessed reference of some sort. The reference can be used to
819 hold some internal information.
821 sub TIEHANDLE { print "<shout>\n"; my $i; bless \$i, shift }
823 =item WRITE this, LIST
825 This method will be called when the handle is written to via the
826 C<syswrite> function.
830 my($buf,$len,$offset) = @_;
831 print "WRITE called, \$buf=$buf, \$len=$len, \$offset=$offset";
834 =item PRINT this, LIST
836 This method will be triggered every time the tied handle is printed to
837 with the C<print()> function.
838 Beyond its self reference it also expects the list that was passed to
841 sub PRINT { $r = shift; $$r++; print join($,,map(uc($_),@_)),$\ }
843 =item PRINTF this, LIST
845 This method will be triggered every time the tied handle is printed to
846 with the C<printf()> function.
847 Beyond its self reference it also expects the format and list that was
848 passed to the printf function.
853 print sprintf($fmt, @_)."\n";
856 =item READ this, LIST
858 This method will be called when the handle is read from via the C<read>
859 or C<sysread> functions.
864 my(undef,$len,$offset) = @_;
865 print "READ called, \$buf=$bufref, \$len=$len, \$offset=$offset";
866 # add to $$bufref, set $len to number of characters read
872 This method will be called when the handle is read from via <HANDLE>.
873 The method should return undef when there is no more data.
875 sub READLINE { $r = shift; "READLINE called $$r times\n"; }
879 This method will be called when the C<getc> function is called.
881 sub GETC { print "Don't GETC, Get Perl"; return "a"; }
885 This method will be called when the handle is closed via the C<close>
888 sub CLOSE { print "CLOSE called.\n" }
892 As with the other types of ties, this method will be called when C<untie> happens.
893 It may be appropriate to "auto CLOSE" when this occurs. See
894 L<The C<untie> Gotcha> below.
898 As with the other types of ties, this method will be called when the
899 tied handle is about to be destroyed. This is useful for debugging and
900 possibly cleaning up.
902 sub DESTROY { print "</shout>\n" }
906 Here's how to use our little example:
911 print FOO $a, " plus ", $b, " equals ", $a + $b, "\n";
916 You can define for all tie types an UNTIE method that will be called
917 at untie(). See L<The C<untie> Gotcha> below.
919 =head2 The C<untie> Gotcha
921 If you intend making use of the object returned from either tie() or
922 tied(), and if the tie's target class defines a destructor, there is a
923 subtle gotcha you I<must> guard against.
925 As setup, consider this (admittedly rather contrived) example of a
926 tie; all it does is use a file to keep a log of the values assigned to
937 my $filename = shift;
938 my $handle = new IO::File "> $filename"
939 or die "Cannot open $filename: $!\n";
941 print $handle "The Start\n";
942 bless {FH => $handle, Value => 0}, $class;
947 return $self->{Value};
953 my $handle = $self->{FH};
954 print $handle "$value\n";
955 $self->{Value} = $value;
960 my $handle = $self->{FH};
961 print $handle "The End\n";
967 Here is an example that makes use of this tie:
973 tie $fred, 'Remember', 'myfile.txt';
978 system "cat myfile.txt";
980 This is the output when it is executed:
988 So far so good. Those of you who have been paying attention will have
989 spotted that the tied object hasn't been used so far. So lets add an
990 extra method to the Remember class to allow comments to be included in
991 the file -- say, something like this:
996 my $handle = $self->{FH};
997 print $handle $text, "\n";
1000 And here is the previous example modified to use the C<comment> method
1001 (which requires the tied object):
1007 $x = tie $fred, 'Remember', 'myfile.txt';
1010 comment $x "changing...";
1013 system "cat myfile.txt";
1015 When this code is executed there is no output. Here's why:
1017 When a variable is tied, it is associated with the object which is the
1018 return value of the TIESCALAR, TIEARRAY, or TIEHASH function. This
1019 object normally has only one reference, namely, the implicit reference
1020 from the tied variable. When untie() is called, that reference is
1021 destroyed. Then, as in the first example above, the object's
1022 destructor (DESTROY) is called, which is normal for objects that have
1023 no more valid references; and thus the file is closed.
1025 In the second example, however, we have stored another reference to
1026 the tied object in $x. That means that when untie() gets called
1027 there will still be a valid reference to the object in existence, so
1028 the destructor is not called at that time, and thus the file is not
1029 closed. The reason there is no output is because the file buffers
1030 have not been flushed to disk.
1032 Now that you know what the problem is, what can you do to avoid it?
1033 Prior to the introduction of the optional UNTIE method the only way
1034 was the good old C<-w> flag. Which will spot any instances where you call
1035 untie() and there are still valid references to the tied object. If
1036 the second script above this near the top C<use warnings 'untie'>
1037 or was run with the C<-w> flag, Perl prints this
1040 untie attempted while 1 inner references still exist
1042 To get the script to work properly and silence the warning make sure
1043 there are no valid references to the tied object I<before> untie() is
1049 Now that UNTIE exists the class designer can decide which parts of the
1050 class functionality are really associated with C<untie> and which with
1051 the object being destroyed. What makes sense for a given class depends
1052 on whether the inner references are being kept so that non-tie-related
1053 methods can be called on the object. But in most cases it probably makes
1054 sense to move the functionality that would have been in DESTROY to the UNTIE
1057 If the UNTIE method exists then the warning above does not occur. Instead the
1058 UNTIE method is passed the count of "extra" references and can issue its own
1059 warning if appropriate. e.g. to replicate the no UNTIE case this method can
1064 my ($obj,$count) = @_;
1065 carp "untie attempted while $count inner references still exist" if $count;
1070 See L<DB_File> or L<Config> for some interesting tie() implementations.
1071 A good starting point for many tie() implementations is with one of the
1072 modules L<Tie::Scalar>, L<Tie::Array>, L<Tie::Hash>, or L<Tie::Handle>.
1076 The bucket usage information provided by C<scalar(%hash)> is not
1077 available. What this means is that using %tied_hash in boolean
1078 context doesn't work right (currently this always tests false,
1079 regardless of whether the hash is empty or hash elements).
1081 Localizing tied arrays or hashes does not work. After exiting the
1082 scope the arrays or the hashes are not restored.
1084 Counting the number of entries in a hash via C<scalar(keys(%hash))>
1085 or C<scalar(values(%hash)>) is inefficient since it needs to iterate
1086 through all the entries with FIRSTKEY/NEXTKEY.
1088 Tied hash/array slices cause multiple FETCH/STORE pairs, there are no
1089 tie methods for slice operations.
1091 You cannot easily tie a multilevel data structure (such as a hash of
1092 hashes) to a dbm file. The first problem is that all but GDBM and
1093 Berkeley DB have size limitations, but beyond that, you also have problems
1094 with how references are to be represented on disk. One experimental
1095 module that does attempt to address this need partially is the MLDBM
1096 module. Check your nearest CPAN site as described in L<perlmodlib> for
1097 source code to MLDBM.
1099 Tied filehandles are still incomplete. sysopen(), truncate(),
1100 flock(), fcntl(), stat() and -X can't currently be trapped.
1106 TIEHANDLE by Sven Verdoolaege <F<skimo@dns.ufsia.ac.be>> and Doug MacEachern <F<dougm@osf.org>>
1108 UNTIE by Nick Ing-Simmons <F<nick@ing-simmons.net>>
1110 Tying Arrays by Casey West <F<casey@geeknest.com>>