3 perltie - how to hide an object class in a simple variable
7 tie VARIABLE, CLASSNAME, LIST
9 $object = tied VARIABLE
15 Prior to release 5.0 of Perl, a programmer could use dbmopen()
16 to connect an on-disk database in the standard Unix dbm(3x)
17 format magically to a %HASH in their program. However, their Perl was either
18 built with one particular dbm library or another, but not both, and
19 you couldn't extend this mechanism to other packages or types of variables.
23 The tie() function binds a variable to a class (package) that will provide
24 the implementation for access methods for that variable. Once this magic
25 has been performed, accessing a tied variable automatically triggers
26 method calls in the proper class. The complexity of the class is
27 hidden behind magic methods calls. The method names are in ALL CAPS,
28 which is a convention that Perl uses to indicate that they're called
29 implicitly rather than explicitly--just like the BEGIN() and END()
32 In the tie() call, C<VARIABLE> is the name of the variable to be
33 enchanted. C<CLASSNAME> is the name of a class implementing objects of
34 the correct type. Any additional arguments in the C<LIST> are passed to
35 the appropriate constructor method for that class--meaning TIESCALAR(),
36 TIEARRAY(), TIEHASH(), or TIEHANDLE(). (Typically these are arguments
37 such as might be passed to the dbminit() function of C.) The object
38 returned by the "new" method is also returned by the tie() function,
39 which would be useful if you wanted to access other methods in
40 C<CLASSNAME>. (You don't actually have to return a reference to a right
41 "type" (e.g., HASH or C<CLASSNAME>) so long as it's a properly blessed
42 object.) You can also retrieve a reference to the underlying object
43 using the tied() function.
45 Unlike dbmopen(), the tie() function will not C<use> or C<require> a module
46 for you--you need to do that explicitly yourself.
50 A class implementing a tied scalar should define the following methods:
51 TIESCALAR, FETCH, STORE, and possibly UNTIE and/or DESTROY.
53 Let's look at each in turn, using as an example a tie class for
54 scalars that allows the user to do something like:
56 tie $his_speed, 'Nice', getppid();
57 tie $my_speed, 'Nice', $$;
59 And now whenever either of those variables is accessed, its current
60 system priority is retrieved and returned. If those variables are set,
61 then the process's priority is changed!
63 We'll use Jarkko Hietaniemi <F<jhi@iki.fi>>'s BSD::Resource class (not
64 included) to access the PRIO_PROCESS, PRIO_MIN, and PRIO_MAX constants
65 from your system, as well as the getpriority() and setpriority() system
66 calls. Here's the preamble of the class.
72 $Nice::DEBUG = 0 unless defined $Nice::DEBUG;
76 =item TIESCALAR classname, LIST
78 This is the constructor for the class. That means it is
79 expected to return a blessed reference to a new scalar
80 (probably anonymous) that it's creating. For example:
84 my $pid = shift || $$; # 0 means me
86 if ($pid !~ /^\d+$/) {
87 carp "Nice::Tie::Scalar got non-numeric pid $pid" if $^W;
91 unless (kill 0, $pid) { # EPERM or ERSCH, no doubt
92 carp "Nice::Tie::Scalar got bad pid $pid: $!" if $^W;
96 return bless \$pid, $class;
99 This tie class has chosen to return an error rather than raising an
100 exception if its constructor should fail. While this is how dbmopen() works,
101 other classes may well not wish to be so forgiving. It checks the global
102 variable C<$^W> to see whether to emit a bit of noise anyway.
106 This method will be triggered every time the tied variable is accessed
107 (read). It takes no arguments beyond its self reference, which is the
108 object representing the scalar we're dealing with. Because in this case
109 we're using just a SCALAR ref for the tied scalar object, a simple $$self
110 allows the method to get at the real value stored there. In our example
111 below, that real value is the process ID to which we've tied our variable.
115 confess "wrong type" unless ref $self;
116 croak "usage error" if @_;
119 $nicety = getpriority(PRIO_PROCESS, $$self);
120 if ($!) { croak "getpriority failed: $!" }
124 This time we've decided to blow up (raise an exception) if the renice
125 fails--there's no place for us to return an error otherwise, and it's
126 probably the right thing to do.
128 =item STORE this, value
130 This method will be triggered every time the tied variable is set
131 (assigned). Beyond its self reference, it also expects one (and only one)
132 argument--the new value the user is trying to assign.
136 confess "wrong type" unless ref $self;
137 my $new_nicety = shift;
138 croak "usage error" if @_;
140 if ($new_nicety < PRIO_MIN) {
142 "WARNING: priority %d less than minimum system priority %d",
143 $new_nicety, PRIO_MIN if $^W;
144 $new_nicety = PRIO_MIN;
147 if ($new_nicety > PRIO_MAX) {
149 "WARNING: priority %d greater than maximum system priority %d",
150 $new_nicety, PRIO_MAX if $^W;
151 $new_nicety = PRIO_MAX;
154 unless (defined setpriority(PRIO_PROCESS, $$self, $new_nicety)) {
155 confess "setpriority failed: $!";
162 This method will be triggered when the C<untie> occurs. This can be useful
163 if the class needs to know when no further calls will be made. (Except DESTROY
164 of course.) See below for more details.
168 This method will be triggered when the tied variable needs to be destructed.
169 As with other object classes, such a method is seldom necessary, because Perl
170 deallocates its moribund object's memory for you automatically--this isn't
171 C++, you know. We'll use a DESTROY method here for debugging purposes only.
175 confess "wrong type" unless ref $self;
176 carp "[ Nice::DESTROY pid $$self ]" if $Nice::DEBUG;
181 That's about all there is to it. Actually, it's more than all there
182 is to it, because we've done a few nice things here for the sake
183 of completeness, robustness, and general aesthetics. Simpler
184 TIESCALAR classes are certainly possible.
188 A class implementing a tied ordinary array should define the following
189 methods: TIEARRAY, FETCH, STORE, FETCHSIZE, STORESIZE and perhaps UNTIE and/or DESTROY.
191 FETCHSIZE and STORESIZE are used to provide C<$#array> and
192 equivalent C<scalar(@array)> access.
194 The methods POP, PUSH, SHIFT, UNSHIFT, SPLICE, DELETE, and EXISTS are
195 required if the perl operator with the corresponding (but lowercase) name
196 is to operate on the tied array. The B<Tie::Array> class can be used as a
197 base class to implement the first five of these in terms of the basic
198 methods above. The default implementations of DELETE and EXISTS in
199 B<Tie::Array> simply C<croak>.
201 In addition EXTEND will be called when perl would have pre-extended
202 allocation in a real array.
204 For this discussion, we'll implement an array whose elements are a fixed
205 size at creation. If you try to create an element larger than the fixed
206 size, you'll take an exception. For example:
209 tie @array, 'FixedElem_Array', 3;
210 $array[0] = 'cat'; # ok.
211 $array[1] = 'dogs'; # exception, length('dogs') > 3.
213 The preamble code for the class is as follows:
215 package FixedElem_Array;
221 =item TIEARRAY classname, LIST
223 This is the constructor for the class. That means it is expected to
224 return a blessed reference through which the new array (probably an
225 anonymous ARRAY ref) will be accessed.
227 In our example, just to show you that you don't I<really> have to return an
228 ARRAY reference, we'll choose a HASH reference to represent our object.
229 A HASH works out well as a generic record type: the C<{ELEMSIZE}> field will
230 store the maximum element size allowed, and the C<{ARRAY}> field will hold the
231 true ARRAY ref. If someone outside the class tries to dereference the
232 object returned (doubtless thinking it an ARRAY ref), they'll blow up.
233 This just goes to show you that you should respect an object's privacy.
237 my $elemsize = shift;
238 if ( @_ || $elemsize =~ /\D/ ) {
239 croak "usage: tie ARRAY, '" . __PACKAGE__ . "', elem_size";
242 ELEMSIZE => $elemsize,
247 =item FETCH this, index
249 This method will be triggered every time an individual element the tied array
250 is accessed (read). It takes one argument beyond its self reference: the
251 index whose value we're trying to fetch.
256 return $self->{ARRAY}->[$index];
259 If a negative array index is used to read from an array, the index
260 will be translated to a positive one internally by calling FETCHSIZE
261 before being passed to FETCH.
263 As you may have noticed, the name of the FETCH method (et al.) is the same
264 for all accesses, even though the constructors differ in names (TIESCALAR
265 vs TIEARRAY). While in theory you could have the same class servicing
266 several tied types, in practice this becomes cumbersome, and it's easiest
267 to keep them at simply one tie type per class.
269 =item STORE this, index, value
271 This method will be triggered every time an element in the tied array is set
272 (written). It takes two arguments beyond its self reference: the index at
273 which we're trying to store something and the value we're trying to put
276 In our example, C<undef> is really C<$self-E<gt>{ELEMSIZE}> number of
277 spaces so we have a little more work to do here:
281 my( $index, $value ) = @_;
282 if ( length $value > $self->{ELEMSIZE} ) {
283 croak "length of $value is greater than $self->{ELEMSIZE}";
286 $self->EXTEND( $index ) if $index > $self->FETCHSIZE();
287 # right justify to keep element size for smaller elements
288 $self->{ARRAY}->[$index] = sprintf "%$self->{ELEMSIZE}s", $value;
291 Negative indexes are treated the same as with FETCH.
295 Returns the total number of items in the tied array associated with
296 object I<this>. (Equivalent to C<scalar(@array)>). For example:
300 return scalar @{$self->{ARRAY}};
303 =item STORESIZE this, count
305 Sets the total number of items in the tied array associated with
306 object I<this> to be I<count>. If this makes the array larger then
307 class's mapping of C<undef> should be returned for new positions.
308 If the array becomes smaller then entries beyond count should be
311 In our example, 'undef' is really an element containing
312 C<$self-E<gt>{ELEMSIZE}> number of spaces. Observe:
317 if ( $count > $self->FETCHSIZE() ) {
318 foreach ( $count - $self->FETCHSIZE() .. $count ) {
319 $self->STORE( $_, '' );
321 } elsif ( $count < $self->FETCHSIZE() ) {
322 foreach ( 0 .. $self->FETCHSIZE() - $count - 2 ) {
328 =item EXTEND this, count
330 Informative call that array is likely to grow to have I<count> entries.
331 Can be used to optimize allocation. This method need do nothing.
333 In our example, we want to make sure there are no blank (C<undef>)
334 entries, so C<EXTEND> will make use of C<STORESIZE> to fill elements
340 $self->STORESIZE( $count );
343 =item EXISTS this, key
345 Verify that the element at index I<key> exists in the tied array I<this>.
347 In our example, we will determine that if an element consists of
348 C<$self-E<gt>{ELEMSIZE}> spaces only, it does not exist:
353 return 0 if ! defined $self->{ARRAY}->[$index] ||
354 $self->{ARRAY}->[$index] eq ' ' x $self->{ELEMSIZE};
358 =item DELETE this, key
360 Delete the element at index I<key> from the tied array I<this>.
362 In our example, a deleted item is C<$self->{ELEMSIZE}> spaces:
367 return $self->STORE( $index, '' );
372 Clear (remove, delete, ...) all values from the tied array associated with
373 object I<this>. For example:
377 return $self->{ARRAY} = [];
380 =item PUSH this, LIST
382 Append elements of I<LIST> to the array. For example:
387 my $last = $self->FETCHSIZE();
388 $self->STORE( $last + $_, $list[$_] ) foreach 0 .. $#list;
389 return $self->FETCHSIZE();
394 Remove last element of the array and return it. For example:
398 return pop @{$self->{ARRAY}};
403 Remove the first element of the array (shifting other elements down)
404 and return it. For example:
408 return shift @{$self->{ARRAY}};
411 =item UNSHIFT this, LIST
413 Insert LIST elements at the beginning of the array, moving existing elements
414 up to make room. For example:
419 my $size = scalar( @list );
420 # make room for our list
421 @{$self->{ARRAY}}[ $size .. $#{$self->{ARRAY}} + $size ]
423 $self->STORE( $_, $list[$_] ) foreach 0 .. $#list;
426 =item SPLICE this, offset, length, LIST
428 Perform the equivalent of C<splice> on the array.
430 I<offset> is optional and defaults to zero, negative values count back
431 from the end of the array.
433 I<length> is optional and defaults to rest of the array.
435 I<LIST> may be empty.
437 Returns a list of the original I<length> elements at I<offset>.
439 In our example, we'll use a little shortcut if there is a I<LIST>:
443 my $offset = shift || 0;
444 my $length = shift || $self->FETCHSIZE() - $offset;
447 tie @list, __PACKAGE__, $self->{ELEMSIZE};
450 return splice @{$self->{ARRAY}}, $offset, $length, @list;
455 Will be called when C<untie> happens. (See below.)
459 This method will be triggered when the tied variable needs to be destructed.
460 As with the scalar tie class, this is almost never needed in a
461 language that does its own garbage collection, so this time we'll
466 The code we presented at the top of the tied array class accesses many
467 elements of the array, far more than we've set the bounds to. Therefore,
468 it will blow up once they try to access beyond the 2nd element of @ary, as
469 the following output demonstrates:
471 setting index 0: value of elt 0 now 0
472 setting index 1: value of elt 1 now 10
473 setting index 2: value of elt 2 now 20
474 setting index 3: Array OOB: 3 > 2 at Bounded_Array.pm line 39
475 Bounded_Array::FETCH called at testba line 12
479 Hashes were the first Perl data type to be tied (see dbmopen()). A class
480 implementing a tied hash should define the following methods: TIEHASH is
481 the constructor. FETCH and STORE access the key and value pairs. EXISTS
482 reports whether a key is present in the hash, and DELETE deletes one.
483 CLEAR empties the hash by deleting all the key and value pairs. FIRSTKEY
484 and NEXTKEY implement the keys() and each() functions to iterate over all
485 the keys. UNTIE is called when C<untie> happens, and DESTROY is called when
486 the tied variable is garbage collected.
488 If this seems like a lot, then feel free to inherit from merely the
489 standard Tie::Hash module for most of your methods, redefining only the
490 interesting ones. See L<Tie::Hash> for details.
492 Remember that Perl distinguishes between a key not existing in the hash,
493 and the key existing in the hash but having a corresponding value of
494 C<undef>. The two possibilities can be tested with the C<exists()> and
495 C<defined()> functions.
497 Here's an example of a somewhat interesting tied hash class: it gives you
498 a hash representing a particular user's dot files. You index into the hash
499 with the name of the file (minus the dot) and you get back that dot file's
500 contents. For example:
503 tie %dot, 'DotFiles';
504 if ( $dot{profile} =~ /MANPATH/ ||
505 $dot{login} =~ /MANPATH/ ||
506 $dot{cshrc} =~ /MANPATH/ )
508 print "you seem to set your MANPATH\n";
511 Or here's another sample of using our tied class:
513 tie %him, 'DotFiles', 'daemon';
514 foreach $f ( keys %him ) {
515 printf "daemon dot file %s is size %d\n",
519 In our tied hash DotFiles example, we use a regular
520 hash for the object containing several important
521 fields, of which only the C<{LIST}> field will be what the
522 user thinks of as the real hash.
528 whose dot files this object represents
532 where those dot files live
536 whether we should try to change or remove those dot files
540 the hash of dot file names and content mappings
544 Here's the start of F<Dotfiles.pm>:
548 sub whowasi { (caller(1))[3] . '()' }
550 sub debug { $DEBUG = @_ ? shift : 1 }
552 For our example, we want to be able to emit debugging info to help in tracing
553 during development. We keep also one convenience function around
554 internally to help print out warnings; whowasi() returns the function name
557 Here are the methods for the DotFiles tied hash.
561 =item TIEHASH classname, LIST
563 This is the constructor for the class. That means it is expected to
564 return a blessed reference through which the new object (probably but not
565 necessarily an anonymous hash) will be accessed.
567 Here's the constructor:
571 my $user = shift || $>;
572 my $dotdir = shift || '';
573 croak "usage: @{[&whowasi]} [USER [DOTDIR]]" if @_;
574 $user = getpwuid($user) if $user =~ /^\d+$/;
575 my $dir = (getpwnam($user))[7]
576 || croak "@{[&whowasi]}: no user $user";
577 $dir .= "/$dotdir" if $dotdir;
587 || croak "@{[&whowasi]}: can't opendir $dir: $!";
588 foreach $dot ( grep /^\./ && -f "$dir/$_", readdir(DIR)) {
590 $node->{LIST}{$dot} = undef;
593 return bless $node, $self;
596 It's probably worth mentioning that if you're going to filetest the
597 return values out of a readdir, you'd better prepend the directory
598 in question. Otherwise, because we didn't chdir() there, it would
599 have been testing the wrong file.
601 =item FETCH this, key
603 This method will be triggered every time an element in the tied hash is
604 accessed (read). It takes one argument beyond its self reference: the key
605 whose value we're trying to fetch.
607 Here's the fetch for our DotFiles example.
610 carp &whowasi if $DEBUG;
613 my $dir = $self->{HOME};
614 my $file = "$dir/.$dot";
616 unless (exists $self->{LIST}->{$dot} || -f $file) {
617 carp "@{[&whowasi]}: no $dot file" if $DEBUG;
621 if (defined $self->{LIST}->{$dot}) {
622 return $self->{LIST}->{$dot};
624 return $self->{LIST}->{$dot} = `cat $dir/.$dot`;
628 It was easy to write by having it call the Unix cat(1) command, but it
629 would probably be more portable to open the file manually (and somewhat
630 more efficient). Of course, because dot files are a Unixy concept, we're
633 =item STORE this, key, value
635 This method will be triggered every time an element in the tied hash is set
636 (written). It takes two arguments beyond its self reference: the index at
637 which we're trying to store something, and the value we're trying to put
640 Here in our DotFiles example, we'll be careful not to let
641 them try to overwrite the file unless they've called the clobber()
642 method on the original object reference returned by tie().
645 carp &whowasi if $DEBUG;
649 my $file = $self->{HOME} . "/.$dot";
650 my $user = $self->{USER};
652 croak "@{[&whowasi]}: $file not clobberable"
653 unless $self->{CLOBBER};
655 open(F, "> $file") || croak "can't open $file: $!";
660 If they wanted to clobber something, they might say:
662 $ob = tie %daemon_dots, 'daemon';
664 $daemon_dots{signature} = "A true daemon\n";
666 Another way to lay hands on a reference to the underlying object is to
667 use the tied() function, so they might alternately have set clobber
670 tie %daemon_dots, 'daemon';
671 tied(%daemon_dots)->clobber(1);
673 The clobber method is simply:
677 $self->{CLOBBER} = @_ ? shift : 1;
680 =item DELETE this, key
682 This method is triggered when we remove an element from the hash,
683 typically by using the delete() function. Again, we'll
684 be careful to check whether they really want to clobber files.
687 carp &whowasi if $DEBUG;
691 my $file = $self->{HOME} . "/.$dot";
692 croak "@{[&whowasi]}: won't remove file $file"
693 unless $self->{CLOBBER};
694 delete $self->{LIST}->{$dot};
695 my $success = unlink($file);
696 carp "@{[&whowasi]}: can't unlink $file: $!" unless $success;
700 The value returned by DELETE becomes the return value of the call
701 to delete(). If you want to emulate the normal behavior of delete(),
702 you should return whatever FETCH would have returned for this key.
703 In this example, we have chosen instead to return a value which tells
704 the caller whether the file was successfully deleted.
708 This method is triggered when the whole hash is to be cleared, usually by
709 assigning the empty list to it.
711 In our example, that would remove all the user's dot files! It's such a
712 dangerous thing that they'll have to set CLOBBER to something higher than
716 carp &whowasi if $DEBUG;
718 croak "@{[&whowasi]}: won't remove all dot files for $self->{USER}"
719 unless $self->{CLOBBER} > 1;
721 foreach $dot ( keys %{$self->{LIST}}) {
726 =item EXISTS this, key
728 This method is triggered when the user uses the exists() function
729 on a particular hash. In our example, we'll look at the C<{LIST}>
730 hash element for this:
733 carp &whowasi if $DEBUG;
736 return exists $self->{LIST}->{$dot};
741 This method will be triggered when the user is going
742 to iterate through the hash, such as via a keys() or each()
746 carp &whowasi if $DEBUG;
748 my $a = keys %{$self->{LIST}}; # reset each() iterator
749 each %{$self->{LIST}}
752 =item NEXTKEY this, lastkey
754 This method gets triggered during a keys() or each() iteration. It has a
755 second argument which is the last key that had been accessed. This is
756 useful if you're carrying about ordering or calling the iterator from more
757 than one sequence, or not really storing things in a hash anywhere.
759 For our example, we're using a real hash so we'll do just the simple
760 thing, but we'll have to go through the LIST field indirectly.
763 carp &whowasi if $DEBUG;
765 return each %{ $self->{LIST} }
770 This is called when C<untie> occurs.
774 This method is triggered when a tied hash is about to go out of
775 scope. You don't really need it unless you're trying to add debugging
776 or have auxiliary state to clean up. Here's a very simple function:
779 carp &whowasi if $DEBUG;
784 Note that functions such as keys() and values() may return huge lists
785 when used on large objects, like DBM files. You may prefer to use the
786 each() function to iterate over such. Example:
788 # print out history file offsets
790 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
791 while (($key,$val) = each %HIST) {
792 print $key, ' = ', unpack('L',$val), "\n";
796 =head2 Tying FileHandles
798 This is partially implemented now.
800 A class implementing a tied filehandle should define the following
801 methods: TIEHANDLE, at least one of PRINT, PRINTF, WRITE, READLINE, GETC,
802 READ, and possibly CLOSE, UNTIE and DESTROY. The class can also provide: BINMODE,
803 OPEN, EOF, FILENO, SEEK, TELL - if the corresponding perl operators are
806 It is especially useful when perl is embedded in some other program,
807 where output to STDOUT and STDERR may have to be redirected in some
808 special way. See nvi and the Apache module for examples.
810 In our example we're going to create a shouting handle.
816 =item TIEHANDLE classname, LIST
818 This is the constructor for the class. That means it is expected to
819 return a blessed reference of some sort. The reference can be used to
820 hold some internal information.
822 sub TIEHANDLE { print "<shout>\n"; my $i; bless \$i, shift }
824 =item WRITE this, LIST
826 This method will be called when the handle is written to via the
827 C<syswrite> function.
831 my($buf,$len,$offset) = @_;
832 print "WRITE called, \$buf=$buf, \$len=$len, \$offset=$offset";
835 =item PRINT this, LIST
837 This method will be triggered every time the tied handle is printed to
838 with the C<print()> function.
839 Beyond its self reference it also expects the list that was passed to
842 sub PRINT { $r = shift; $$r++; print join($,,map(uc($_),@_)),$\ }
844 =item PRINTF this, LIST
846 This method will be triggered every time the tied handle is printed to
847 with the C<printf()> function.
848 Beyond its self reference it also expects the format and list that was
849 passed to the printf function.
854 print sprintf($fmt, @_)."\n";
857 =item READ this, LIST
859 This method will be called when the handle is read from via the C<read>
860 or C<sysread> functions.
864 my $$bufref = \$_[0];
865 my(undef,$len,$offset) = @_;
866 print "READ called, \$buf=$bufref, \$len=$len, \$offset=$offset";
867 # add to $$bufref, set $len to number of characters read
873 This method will be called when the handle is read from via <HANDLE>.
874 The method should return undef when there is no more data.
876 sub READLINE { $r = shift; "READLINE called $$r times\n"; }
880 This method will be called when the C<getc> function is called.
882 sub GETC { print "Don't GETC, Get Perl"; return "a"; }
886 This method will be called when the handle is closed via the C<close>
889 sub CLOSE { print "CLOSE called.\n" }
893 As with the other types of ties, this method will be called when C<untie> happens.
894 It may be appropriate to "auto CLOSE" when this occurs.
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
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 You cannot easily tie a multilevel data structure (such as a hash of
1077 hashes) to a dbm file. The first problem is that all but GDBM and
1078 Berkeley DB have size limitations, but beyond that, you also have problems
1079 with how references are to be represented on disk. One experimental
1080 module that does attempt to address this need partially is the MLDBM
1081 module. Check your nearest CPAN site as described in L<perlmodlib> for
1082 source code to MLDBM.
1084 Tied filehandles are still incomplete. sysopen(), truncate(),
1085 flock(), fcntl(), stat() and -X can't currently be trapped.
1091 TIEHANDLE by Sven Verdoolaege <F<skimo@dns.ufsia.ac.be>> and Doug MacEachern <F<dougm@osf.org>>
1093 UNTIE by Nick Ing-Simmons <F<nick@ing-simmons.net>>
1095 Tying Arrays by Casey Tweten <F<crt@kiski.net>>