4 perltie - how to hide an object class in a simple variable
8 tie VARIABLE, CLASSNAME, LIST
10 $object = tied VARIABLE
16 Prior to release 5.0 of Perl, a programmer could use dbmopen()
17 to connect an on-disk database in the standard Unix dbm(3x)
18 format magically to a %HASH in their program. However, their Perl was either
19 built with one particular dbm library or another, but not both, and
20 you couldn't extend this mechanism to other packages or types of variables.
24 The tie() function binds a variable to a class (package) that will provide
25 the implementation for access methods for that variable. Once this magic
26 has been performed, accessing a tied variable automatically triggers
27 method calls in the proper class. The complexity of the class is
28 hidden behind magic methods calls. The method names are in ALL CAPS,
29 which is a convention that Perl uses to indicate that they're called
30 implicitly rather than explicitly--just like the BEGIN() and END()
33 In the tie() call, C<VARIABLE> is the name of the variable to be
34 enchanted. C<CLASSNAME> is the name of a class implementing objects of
35 the correct type. Any additional arguments in the C<LIST> are passed to
36 the appropriate constructor method for that class--meaning TIESCALAR(),
37 TIEARRAY(), TIEHASH(), or TIEHANDLE(). (Typically these are arguments
38 such as might be passed to the dbminit() function of C.) The object
39 returned by the "new" method is also returned by the tie() function,
40 which would be useful if you wanted to access other methods in
41 C<CLASSNAME>. (You don't actually have to return a reference to a right
42 "type" (e.g., HASH or C<CLASSNAME>) so long as it's a properly blessed
43 object.) You can also retrieve a reference to the underlying object
44 using the tied() function.
46 Unlike dbmopen(), the tie() function will not C<use> or C<require> a module
47 for you--you need to do that explicitly yourself.
52 A class implementing a tied scalar should define the following methods:
53 TIESCALAR, FETCH, STORE, and possibly UNTIE and/or DESTROY.
55 Let's look at each in turn, using as an example a tie class for
56 scalars that allows the user to do something like:
58 tie $his_speed, 'Nice', getppid();
59 tie $my_speed, 'Nice', $$;
61 And now whenever either of those variables is accessed, its current
62 system priority is retrieved and returned. If those variables are set,
63 then the process's priority is changed!
65 We'll use Jarkko Hietaniemi <F<jhi@iki.fi>>'s BSD::Resource class (not
66 included) to access the PRIO_PROCESS, PRIO_MIN, and PRIO_MAX constants
67 from your system, as well as the getpriority() and setpriority() system
68 calls. Here's the preamble of the class.
74 $Nice::DEBUG = 0 unless defined $Nice::DEBUG;
78 =item TIESCALAR classname, LIST
81 This is the constructor for the class. That means it is
82 expected to return a blessed reference to a new scalar
83 (probably anonymous) that it's creating. For example:
87 my $pid = shift || $$; # 0 means me
89 if ($pid !~ /^\d+$/) {
90 carp "Nice::Tie::Scalar got non-numeric pid $pid" if $^W;
94 unless (kill 0, $pid) { # EPERM or ERSCH, no doubt
95 carp "Nice::Tie::Scalar got bad pid $pid: $!" if $^W;
99 return bless \$pid, $class;
102 This tie class has chosen to return an error rather than raising an
103 exception if its constructor should fail. While this is how dbmopen() works,
104 other classes may well not wish to be so forgiving. It checks the global
105 variable C<$^W> to see whether to emit a bit of noise anyway.
110 This method will be triggered every time the tied variable is accessed
111 (read). It takes no arguments beyond its self reference, which is the
112 object representing the scalar we're dealing with. Because in this case
113 we're using just a SCALAR ref for the tied scalar object, a simple $$self
114 allows the method to get at the real value stored there. In our example
115 below, that real value is the process ID to which we've tied our variable.
119 confess "wrong type" unless ref $self;
120 croak "usage error" if @_;
123 $nicety = getpriority(PRIO_PROCESS, $$self);
124 if ($!) { croak "getpriority failed: $!" }
128 This time we've decided to blow up (raise an exception) if the renice
129 fails--there's no place for us to return an error otherwise, and it's
130 probably the right thing to do.
132 =item STORE this, value
135 This method will be triggered every time the tied variable is set
136 (assigned). Beyond its self reference, it also expects one (and only one)
137 argument: the new value the user is trying to assign. Don't worry about
138 returning a value from STORE; the semantic of assignment returning the
139 assigned value is implemented with FETCH.
143 confess "wrong type" unless ref $self;
144 my $new_nicety = shift;
145 croak "usage error" if @_;
147 if ($new_nicety < PRIO_MIN) {
149 "WARNING: priority %d less than minimum system priority %d",
150 $new_nicety, PRIO_MIN if $^W;
151 $new_nicety = PRIO_MIN;
154 if ($new_nicety > PRIO_MAX) {
156 "WARNING: priority %d greater than maximum system priority %d",
157 $new_nicety, PRIO_MAX if $^W;
158 $new_nicety = PRIO_MAX;
161 unless (defined setpriority(PRIO_PROCESS, $$self, $new_nicety)) {
162 confess "setpriority failed: $!";
169 This method will be triggered when the C<untie> occurs. This can be useful
170 if the class needs to know when no further calls will be made. (Except DESTROY
171 of course.) See L<The C<untie> Gotcha> below for more details.
176 This method will be triggered when the tied variable needs to be destructed.
177 As with other object classes, such a method is seldom necessary, because Perl
178 deallocates its moribund object's memory for you automatically--this isn't
179 C++, you know. We'll use a DESTROY method here for debugging purposes only.
183 confess "wrong type" unless ref $self;
184 carp "[ Nice::DESTROY pid $$self ]" if $Nice::DEBUG;
189 That's about all there is to it. Actually, it's more than all there
190 is to it, because we've done a few nice things here for the sake
191 of completeness, robustness, and general aesthetics. Simpler
192 TIESCALAR classes are certainly possible.
197 A class implementing a tied ordinary array should define the following
198 methods: TIEARRAY, FETCH, STORE, FETCHSIZE, STORESIZE, CLEAR
199 and perhaps UNTIE and/or DESTROY.
201 FETCHSIZE and STORESIZE are used to provide C<$#array> and
202 equivalent C<scalar(@array)> access.
204 The methods POP, PUSH, SHIFT, UNSHIFT, SPLICE, DELETE, and EXISTS are
205 required if the perl operator with the corresponding (but lowercase) name
206 is to operate on the tied array. The B<Tie::Array> class can be used as a
207 base class to implement the first five of these in terms of the basic
208 methods above. The default implementations of DELETE and EXISTS in
209 B<Tie::Array> simply C<croak>.
211 In addition EXTEND will be called when perl would have pre-extended
212 allocation in a real array.
214 For this discussion, we'll implement an array whose elements are a fixed
215 size at creation. If you try to create an element larger than the fixed
216 size, you'll take an exception. For example:
219 tie @array, 'FixedElem_Array', 3;
220 $array[0] = 'cat'; # ok.
221 $array[1] = 'dogs'; # exception, length('dogs') > 3.
223 The preamble code for the class is as follows:
225 package FixedElem_Array;
231 =item TIEARRAY classname, LIST
234 This is the constructor for the class. That means it is expected to
235 return a blessed reference through which the new array (probably an
236 anonymous ARRAY ref) will be accessed.
238 In our example, just to show you that you don't I<really> have to return an
239 ARRAY reference, we'll choose a HASH reference to represent our object.
240 A HASH works out well as a generic record type: the C<{ELEMSIZE}> field will
241 store the maximum element size allowed, and the C<{ARRAY}> field will hold the
242 true ARRAY ref. If someone outside the class tries to dereference the
243 object returned (doubtless thinking it an ARRAY ref), they'll blow up.
244 This just goes to show you that you should respect an object's privacy.
248 my $elemsize = shift;
249 if ( @_ || $elemsize =~ /\D/ ) {
250 croak "usage: tie ARRAY, '" . __PACKAGE__ . "', elem_size";
253 ELEMSIZE => $elemsize,
258 =item FETCH this, index
261 This method will be triggered every time an individual element the tied array
262 is accessed (read). It takes one argument beyond its self reference: the
263 index whose value we're trying to fetch.
268 return $self->{ARRAY}->[$index];
271 If a negative array index is used to read from an array, the index
272 will be translated to a positive one internally by calling FETCHSIZE
273 before being passed to FETCH. You may disable this feature by
274 assigning a true value to the variable C<$NEGATIVE_INDICES> in the
277 As you may have noticed, the name of the FETCH method (et al.) is the same
278 for all accesses, even though the constructors differ in names (TIESCALAR
279 vs TIEARRAY). While in theory you could have the same class servicing
280 several tied types, in practice this becomes cumbersome, and it's easiest
281 to keep them at simply one tie type per class.
283 =item STORE this, index, value
286 This method will be triggered every time an element in the tied array is set
287 (written). It takes two arguments beyond its self reference: the index at
288 which we're trying to store something and the value we're trying to put
291 In our example, C<undef> is really C<$self-E<gt>{ELEMSIZE}> number of
292 spaces so we have a little more work to do here:
296 my( $index, $value ) = @_;
297 if ( length $value > $self->{ELEMSIZE} ) {
298 croak "length of $value is greater than $self->{ELEMSIZE}";
301 $self->EXTEND( $index ) if $index > $self->FETCHSIZE();
302 # right justify to keep element size for smaller elements
303 $self->{ARRAY}->[$index] = sprintf "%$self->{ELEMSIZE}s", $value;
306 Negative indexes are treated the same as with FETCH.
311 Returns the total number of items in the tied array associated with
312 object I<this>. (Equivalent to C<scalar(@array)>). For example:
316 return scalar @{$self->{ARRAY}};
319 =item STORESIZE this, count
322 Sets the total number of items in the tied array associated with
323 object I<this> to be I<count>. If this makes the array larger then
324 class's mapping of C<undef> should be returned for new positions.
325 If the array becomes smaller then entries beyond count should be
328 In our example, 'undef' is really an element containing
329 C<$self-E<gt>{ELEMSIZE}> number of spaces. Observe:
334 if ( $count > $self->FETCHSIZE() ) {
335 foreach ( $count - $self->FETCHSIZE() .. $count ) {
336 $self->STORE( $_, '' );
338 } elsif ( $count < $self->FETCHSIZE() ) {
339 foreach ( 0 .. $self->FETCHSIZE() - $count - 2 ) {
345 =item EXTEND this, count
348 Informative call that array is likely to grow to have I<count> entries.
349 Can be used to optimize allocation. This method need do nothing.
351 In our example, we want to make sure there are no blank (C<undef>)
352 entries, so C<EXTEND> will make use of C<STORESIZE> to fill elements
358 $self->STORESIZE( $count );
361 =item EXISTS this, key
364 Verify that the element at index I<key> exists in the tied array I<this>.
366 In our example, we will determine that if an element consists of
367 C<$self-E<gt>{ELEMSIZE}> spaces only, it does not exist:
372 return 0 if ! defined $self->{ARRAY}->[$index] ||
373 $self->{ARRAY}->[$index] eq ' ' x $self->{ELEMSIZE};
377 =item DELETE this, key
380 Delete the element at index I<key> from the tied array I<this>.
382 In our example, a deleted item is C<$self-E<gt>{ELEMSIZE}> spaces:
387 return $self->STORE( $index, '' );
393 Clear (remove, delete, ...) all values from the tied array associated with
394 object I<this>. For example:
398 return $self->{ARRAY} = [];
401 =item PUSH this, LIST
404 Append elements of I<LIST> to the array. For example:
409 my $last = $self->FETCHSIZE();
410 $self->STORE( $last + $_, $list[$_] ) foreach 0 .. $#list;
411 return $self->FETCHSIZE();
417 Remove last element of the array and return it. For example:
421 return pop @{$self->{ARRAY}};
427 Remove the first element of the array (shifting other elements down)
428 and return it. For example:
432 return shift @{$self->{ARRAY}};
435 =item UNSHIFT this, LIST
438 Insert LIST elements at the beginning of the array, moving existing elements
439 up to make room. For example:
444 my $size = scalar( @list );
445 # make room for our list
446 @{$self->{ARRAY}}[ $size .. $#{$self->{ARRAY}} + $size ]
448 $self->STORE( $_, $list[$_] ) foreach 0 .. $#list;
451 =item SPLICE this, offset, length, LIST
454 Perform the equivalent of C<splice> on the array.
456 I<offset> is optional and defaults to zero, negative values count back
457 from the end of the array.
459 I<length> is optional and defaults to rest of the array.
461 I<LIST> may be empty.
463 Returns a list of the original I<length> elements at I<offset>.
465 In our example, we'll use a little shortcut if there is a I<LIST>:
469 my $offset = shift || 0;
470 my $length = shift || $self->FETCHSIZE() - $offset;
473 tie @list, __PACKAGE__, $self->{ELEMSIZE};
476 return splice @{$self->{ARRAY}}, $offset, $length, @list;
482 Will be called when C<untie> happens. (See L<The C<untie> Gotcha> below.)
487 This method will be triggered when the tied variable needs to be destructed.
488 As with the scalar tie class, this is almost never needed in a
489 language that does its own garbage collection, so this time we'll
497 Hashes were the first Perl data type to be tied (see dbmopen()). A class
498 implementing a tied hash should define the following methods: TIEHASH is
499 the constructor. FETCH and STORE access the key and value pairs. EXISTS
500 reports whether a key is present in the hash, and DELETE deletes one.
501 CLEAR empties the hash by deleting all the key and value pairs. FIRSTKEY
502 and NEXTKEY implement the keys() and each() functions to iterate over all
503 the keys. SCALAR is triggered when the tied hash is evaluated in scalar
504 context. UNTIE is called when C<untie> happens, and DESTROY is called when
505 the tied variable is garbage collected.
507 If this seems like a lot, then feel free to inherit from merely the
508 standard Tie::StdHash module for most of your methods, redefining only the
509 interesting ones. See L<Tie::Hash> for details.
511 Remember that Perl distinguishes between a key not existing in the hash,
512 and the key existing in the hash but having a corresponding value of
513 C<undef>. The two possibilities can be tested with the C<exists()> and
514 C<defined()> functions.
516 Here's an example of a somewhat interesting tied hash class: it gives you
517 a hash representing a particular user's dot files. You index into the hash
518 with the name of the file (minus the dot) and you get back that dot file's
519 contents. For example:
522 tie %dot, 'DotFiles';
523 if ( $dot{profile} =~ /MANPATH/ ||
524 $dot{login} =~ /MANPATH/ ||
525 $dot{cshrc} =~ /MANPATH/ )
527 print "you seem to set your MANPATH\n";
530 Or here's another sample of using our tied class:
532 tie %him, 'DotFiles', 'daemon';
533 foreach $f ( keys %him ) {
534 printf "daemon dot file %s is size %d\n",
538 In our tied hash DotFiles example, we use a regular
539 hash for the object containing several important
540 fields, of which only the C<{LIST}> field will be what the
541 user thinks of as the real hash.
547 whose dot files this object represents
551 where those dot files live
555 whether we should try to change or remove those dot files
559 the hash of dot file names and content mappings
563 Here's the start of F<Dotfiles.pm>:
567 sub whowasi { (caller(1))[3] . '()' }
569 sub debug { $DEBUG = @_ ? shift : 1 }
571 For our example, we want to be able to emit debugging info to help in tracing
572 during development. We keep also one convenience function around
573 internally to help print out warnings; whowasi() returns the function name
576 Here are the methods for the DotFiles tied hash.
580 =item TIEHASH classname, LIST
583 This is the constructor for the class. That means it is expected to
584 return a blessed reference through which the new object (probably but not
585 necessarily an anonymous hash) will be accessed.
587 Here's the constructor:
591 my $user = shift || $>;
592 my $dotdir = shift || '';
593 croak "usage: @{[&whowasi]} [USER [DOTDIR]]" if @_;
594 $user = getpwuid($user) if $user =~ /^\d+$/;
595 my $dir = (getpwnam($user))[7]
596 || croak "@{[&whowasi]}: no user $user";
597 $dir .= "/$dotdir" if $dotdir;
607 || croak "@{[&whowasi]}: can't opendir $dir: $!";
608 foreach $dot ( grep /^\./ && -f "$dir/$_", readdir(DIR)) {
610 $node->{LIST}{$dot} = undef;
613 return bless $node, $self;
616 It's probably worth mentioning that if you're going to filetest the
617 return values out of a readdir, you'd better prepend the directory
618 in question. Otherwise, because we didn't chdir() there, it would
619 have been testing the wrong file.
621 =item FETCH this, key
624 This method will be triggered every time an element in the tied hash is
625 accessed (read). It takes one argument beyond its self reference: the key
626 whose value we're trying to fetch.
628 Here's the fetch for our DotFiles example.
631 carp &whowasi if $DEBUG;
634 my $dir = $self->{HOME};
635 my $file = "$dir/.$dot";
637 unless (exists $self->{LIST}->{$dot} || -f $file) {
638 carp "@{[&whowasi]}: no $dot file" if $DEBUG;
642 if (defined $self->{LIST}->{$dot}) {
643 return $self->{LIST}->{$dot};
645 return $self->{LIST}->{$dot} = `cat $dir/.$dot`;
649 It was easy to write by having it call the Unix cat(1) command, but it
650 would probably be more portable to open the file manually (and somewhat
651 more efficient). Of course, because dot files are a Unixy concept, we're
654 =item STORE this, key, value
657 This method will be triggered every time an element in the tied hash is set
658 (written). It takes two arguments beyond its self reference: the index at
659 which we're trying to store something, and the value we're trying to put
662 Here in our DotFiles example, we'll be careful not to let
663 them try to overwrite the file unless they've called the clobber()
664 method on the original object reference returned by tie().
667 carp &whowasi if $DEBUG;
671 my $file = $self->{HOME} . "/.$dot";
672 my $user = $self->{USER};
674 croak "@{[&whowasi]}: $file not clobberable"
675 unless $self->{CLOBBER};
677 open(my $f, '>', $file) || croak "can't open $file: $!";
682 If they wanted to clobber something, they might say:
684 $ob = tie %daemon_dots, 'daemon';
686 $daemon_dots{signature} = "A true daemon\n";
688 Another way to lay hands on a reference to the underlying object is to
689 use the tied() function, so they might alternately have set clobber
692 tie %daemon_dots, 'daemon';
693 tied(%daemon_dots)->clobber(1);
695 The clobber method is simply:
699 $self->{CLOBBER} = @_ ? shift : 1;
702 =item DELETE this, key
705 This method is triggered when we remove an element from the hash,
706 typically by using the delete() function. Again, we'll
707 be careful to check whether they really want to clobber files.
710 carp &whowasi if $DEBUG;
714 my $file = $self->{HOME} . "/.$dot";
715 croak "@{[&whowasi]}: won't remove file $file"
716 unless $self->{CLOBBER};
717 delete $self->{LIST}->{$dot};
718 my $success = unlink($file);
719 carp "@{[&whowasi]}: can't unlink $file: $!" unless $success;
723 The value returned by DELETE becomes the return value of the call
724 to delete(). If you want to emulate the normal behavior of delete(),
725 you should return whatever FETCH would have returned for this key.
726 In this example, we have chosen instead to return a value which tells
727 the caller whether the file was successfully deleted.
732 This method is triggered when the whole hash is to be cleared, usually by
733 assigning the empty list to it.
735 In our example, that would remove all the user's dot files! It's such a
736 dangerous thing that they'll have to set CLOBBER to something higher than
740 carp &whowasi if $DEBUG;
742 croak "@{[&whowasi]}: won't remove all dot files for $self->{USER}"
743 unless $self->{CLOBBER} > 1;
745 foreach $dot ( keys %{$self->{LIST}}) {
750 =item EXISTS this, key
753 This method is triggered when the user uses the exists() function
754 on a particular hash. In our example, we'll look at the C<{LIST}>
755 hash element for this:
758 carp &whowasi if $DEBUG;
761 return exists $self->{LIST}->{$dot};
767 This method will be triggered when the user is going
768 to iterate through the hash, such as via a keys(), values(), or each() call.
771 carp &whowasi if $DEBUG;
773 my $a = keys %{$self->{LIST}}; # reset each() iterator
774 each %{$self->{LIST}}
777 FIRSTKEY is always called in scalar context and it should just
778 return the first key. values(), and each() in list context,
779 will call FETCH for the returned keys.
781 =item NEXTKEY this, lastkey
784 This method gets triggered during a keys(), values(), or each() iteration. It has a
785 second argument which is the last key that had been accessed. This is
786 useful if you're caring about ordering or calling the iterator from more
787 than one sequence, or not really storing things in a hash anywhere.
789 NEXTKEY is always called in scalar context and it should just
790 return the next key. values(), and each() in list context,
791 will call FETCH for the returned keys.
793 For our example, we're using a real hash so we'll do just the simple
794 thing, but we'll have to go through the LIST field indirectly.
797 carp &whowasi if $DEBUG;
799 return each %{ $self->{LIST} }
805 This is called when the hash is evaluated in scalar context. In order
806 to mimic the behaviour of untied hashes, this method should return a
807 false value when the tied hash is considered empty. If this method does
808 not exist, perl will make some educated guesses and return true when
809 the hash is inside an iteration. If this isn't the case, FIRSTKEY is
810 called, and the result will be a false value if FIRSTKEY returns the empty
811 list, true otherwise.
813 However, you should B<not> blindly rely on perl always doing the right
814 thing. Particularly, perl will mistakenly return true when you clear the
815 hash by repeatedly calling DELETE until it is empty. You are therefore
816 advised to supply your own SCALAR method when you want to be absolutely
817 sure that your hash behaves nicely in scalar context.
819 In our example we can just call C<scalar> on the underlying hash
820 referenced by C<$self-E<gt>{LIST}>:
823 carp &whowasi if $DEBUG;
825 return scalar %{ $self->{LIST} }
831 This is called when C<untie> occurs. See L<The C<untie> Gotcha> below.
836 This method is triggered when a tied hash is about to go out of
837 scope. You don't really need it unless you're trying to add debugging
838 or have auxiliary state to clean up. Here's a very simple function:
841 carp &whowasi if $DEBUG;
846 Note that functions such as keys() and values() may return huge lists
847 when used on large objects, like DBM files. You may prefer to use the
848 each() function to iterate over such. Example:
850 # print out history file offsets
852 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
853 while (($key,$val) = each %HIST) {
854 print $key, ' = ', unpack('L',$val), "\n";
858 =head2 Tying FileHandles
861 This is partially implemented now.
863 A class implementing a tied filehandle should define the following
864 methods: TIEHANDLE, at least one of PRINT, PRINTF, WRITE, READLINE, GETC,
865 READ, and possibly CLOSE, UNTIE and DESTROY. The class can also provide: BINMODE,
866 OPEN, EOF, FILENO, SEEK, TELL - if the corresponding perl operators are
869 When STDERR is tied, its PRINT method will be called to issue warnings
870 and error messages. This feature is temporarily disabled during the call,
871 which means you can use C<warn()> inside PRINT without starting a recursive
872 loop. And just like C<__WARN__> and C<__DIE__> handlers, STDERR's PRINT
873 method may be called to report parser errors, so the caveats mentioned under
874 L<perlvar/%SIG> apply.
876 All of this is especially useful when perl is embedded in some other
877 program, where output to STDOUT and STDERR may have to be redirected
878 in some special way. See nvi and the Apache module for examples.
880 When tying a handle, the first argument to C<tie> should begin with an
881 asterisk. So, if you are tying STDOUT, use C<*STDOUT>. If you have
882 assigned it to a scalar variable, say C<$handle>, use C<*$handle>.
883 C<tie $handle> ties the scalar variable C<$handle>, not the handle inside
886 In our example we're going to create a shouting handle.
892 =item TIEHANDLE classname, LIST
895 This is the constructor for the class. That means it is expected to
896 return a blessed reference of some sort. The reference can be used to
897 hold some internal information.
899 sub TIEHANDLE { print "<shout>\n"; my $i; bless \$i, shift }
901 =item WRITE this, LIST
904 This method will be called when the handle is written to via the
905 C<syswrite> function.
909 my($buf,$len,$offset) = @_;
910 print "WRITE called, \$buf=$buf, \$len=$len, \$offset=$offset";
913 =item PRINT this, LIST
916 This method will be triggered every time the tied handle is printed to
917 with the C<print()> or C<say()> functions. Beyond its self reference
918 it also expects the list that was passed to the print function.
920 sub PRINT { $r = shift; $$r++; print join($,,map(uc($_),@_)),$\ }
922 C<say()> acts just like C<print()> except $\ will be localized to C<\n> so
923 you need do nothing special to handle C<say()> in C<PRINT()>.
925 =item PRINTF this, LIST
928 This method will be triggered every time the tied handle is printed to
929 with the C<printf()> function.
930 Beyond its self reference it also expects the format and list that was
931 passed to the printf function.
936 print sprintf($fmt, @_);
939 =item READ this, LIST
942 This method will be called when the handle is read from via the C<read>
943 or C<sysread> functions.
948 my(undef,$len,$offset) = @_;
949 print "READ called, \$buf=$bufref, \$len=$len, \$offset=$offset";
950 # add to $$bufref, set $len to number of characters read
957 This method is called when the handle is read via C<E<lt>HANDLEE<gt>>
958 or C<readline HANDLE>.
960 As per L<C<readline>|perlfunc/readline>, in scalar context it should return
961 the next line, or C<undef> for no more data. In list context it should
962 return all remaining lines, or an empty list for no more data. The strings
963 returned should include the input record separator C<$/> (see L<perlvar>),
964 unless it is C<undef> (which means "slurp" mode).
969 return ("all remaining\n",
973 return "READLINE called " . ++$$r . " times\n";
980 This method will be called when the C<getc> function is called.
982 sub GETC { print "Don't GETC, Get Perl"; return "a"; }
987 This method will be called when the C<eof> function is called.
989 Starting with Perl 5.12, an additional integer parameter will be passed. It
990 will be zero if C<eof> is called without parameter; C<1> if C<eof> is given
991 a filehandle as a parameter, e.g. C<eof(FH)>; and C<2> in the very special
992 case that the tied filehandle is C<ARGV> and C<eof> is called with an empty
993 parameter list, e.g. C<eof()>.
995 sub EOF { not length $stringbuf }
1000 This method will be called when the handle is closed via the C<close>
1003 sub CLOSE { print "CLOSE called.\n" }
1008 As with the other types of ties, this method will be called when C<untie> happens.
1009 It may be appropriate to "auto CLOSE" when this occurs. See
1010 L<The C<untie> Gotcha> below.
1015 As with the other types of ties, this method will be called when the
1016 tied handle is about to be destroyed. This is useful for debugging and
1017 possibly cleaning up.
1019 sub DESTROY { print "</shout>\n" }
1023 Here's how to use our little example:
1026 print FOO "hello\n";
1028 print FOO $a, " plus ", $b, " equals ", $a + $b, "\n";
1034 You can define for all tie types an UNTIE method that will be called
1035 at untie(). See L<The C<untie> Gotcha> below.
1037 =head2 The C<untie> Gotcha
1040 If you intend making use of the object returned from either tie() or
1041 tied(), and if the tie's target class defines a destructor, there is a
1042 subtle gotcha you I<must> guard against.
1044 As setup, consider this (admittedly rather contrived) example of a
1045 tie; all it does is use a file to keep a log of the values assigned to
1056 my $filename = shift;
1057 my $handle = IO::File->new( "> $filename" )
1058 or die "Cannot open $filename: $!\n";
1060 print $handle "The Start\n";
1061 bless {FH => $handle, Value => 0}, $class;
1066 return $self->{Value};
1072 my $handle = $self->{FH};
1073 print $handle "$value\n";
1074 $self->{Value} = $value;
1079 my $handle = $self->{FH};
1080 print $handle "The End\n";
1086 Here is an example that makes use of this tie:
1092 tie $fred, 'Remember', 'myfile.txt';
1097 system "cat myfile.txt";
1099 This is the output when it is executed:
1107 So far so good. Those of you who have been paying attention will have
1108 spotted that the tied object hasn't been used so far. So lets add an
1109 extra method to the Remember class to allow comments to be included in
1110 the file; say, something like this:
1115 my $handle = $self->{FH};
1116 print $handle $text, "\n";
1119 And here is the previous example modified to use the C<comment> method
1120 (which requires the tied object):
1126 $x = tie $fred, 'Remember', 'myfile.txt';
1129 comment $x "changing...";
1132 system "cat myfile.txt";
1134 When this code is executed there is no output. Here's why:
1136 When a variable is tied, it is associated with the object which is the
1137 return value of the TIESCALAR, TIEARRAY, or TIEHASH function. This
1138 object normally has only one reference, namely, the implicit reference
1139 from the tied variable. When untie() is called, that reference is
1140 destroyed. Then, as in the first example above, the object's
1141 destructor (DESTROY) is called, which is normal for objects that have
1142 no more valid references; and thus the file is closed.
1144 In the second example, however, we have stored another reference to
1145 the tied object in $x. That means that when untie() gets called
1146 there will still be a valid reference to the object in existence, so
1147 the destructor is not called at that time, and thus the file is not
1148 closed. The reason there is no output is because the file buffers
1149 have not been flushed to disk.
1151 Now that you know what the problem is, what can you do to avoid it?
1152 Prior to the introduction of the optional UNTIE method the only way
1153 was the good old C<-w> flag. Which will spot any instances where you call
1154 untie() and there are still valid references to the tied object. If
1155 the second script above this near the top C<use warnings 'untie'>
1156 or was run with the C<-w> flag, Perl prints this
1159 untie attempted while 1 inner references still exist
1161 To get the script to work properly and silence the warning make sure
1162 there are no valid references to the tied object I<before> untie() is
1168 Now that UNTIE exists the class designer can decide which parts of the
1169 class functionality are really associated with C<untie> and which with
1170 the object being destroyed. What makes sense for a given class depends
1171 on whether the inner references are being kept so that non-tie-related
1172 methods can be called on the object. But in most cases it probably makes
1173 sense to move the functionality that would have been in DESTROY to the UNTIE
1176 If the UNTIE method exists then the warning above does not occur. Instead the
1177 UNTIE method is passed the count of "extra" references and can issue its own
1178 warning if appropriate. e.g. to replicate the no UNTIE case this method can
1183 my ($obj,$count) = @_;
1184 carp "untie attempted while $count inner references still exist" if $count;
1189 See L<DB_File> or L<Config> for some interesting tie() implementations.
1190 A good starting point for many tie() implementations is with one of the
1191 modules L<Tie::Scalar>, L<Tie::Array>, L<Tie::Hash>, or L<Tie::Handle>.
1195 The bucket usage information provided by C<scalar(%hash)> is not
1196 available. What this means is that using %tied_hash in boolean
1197 context doesn't work right (currently this always tests false,
1198 regardless of whether the hash is empty or hash elements).
1200 Localizing tied arrays or hashes does not work. After exiting the
1201 scope the arrays or the hashes are not restored.
1203 Counting the number of entries in a hash via C<scalar(keys(%hash))>
1204 or C<scalar(values(%hash)>) is inefficient since it needs to iterate
1205 through all the entries with FIRSTKEY/NEXTKEY.
1207 Tied hash/array slices cause multiple FETCH/STORE pairs, there are no
1208 tie methods for slice operations.
1210 You cannot easily tie a multilevel data structure (such as a hash of
1211 hashes) to a dbm file. The first problem is that all but GDBM and
1212 Berkeley DB have size limitations, but beyond that, you also have problems
1213 with how references are to be represented on disk. One
1214 module that does attempt to address this need is DBM::Deep. Check your
1215 nearest CPAN site as described in L<perlmodlib> for source code. Note
1216 that despite its name, DBM::Deep does not use dbm. Another earlier attempt
1217 at solving the problem is MLDBM, which is also available on the CPAN, but
1218 which has some fairly serious limitations.
1220 Tied filehandles are still incomplete. sysopen(), truncate(),
1221 flock(), fcntl(), stat() and -X can't currently be trapped.
1227 TIEHANDLE by Sven Verdoolaege <F<skimo@dns.ufsia.ac.be>> and Doug MacEachern <F<dougm@osf.org>>
1229 UNTIE by Nick Ing-Simmons <F<nick@ing-simmons.net>>
1231 SCALAR by Tassilo von Parseval <F<tassilo.von.parseval@rwth-aachen.de>>
1233 Tying Arrays by Casey West <F<casey@geeknest.com>>