perltie


SYNOPSIS
        tie VARIABLE, CLASSNAME, LIST

        $object = tied VARIABLE

        untie VARIABLE

DESCRIPTION
       Prior to release 5.0 of Perl, a programmer could use dbmopen() to
       connect an on-disk database in the standard Unix dbm(3x) format
       magically to a %HASH in their program.  However, their Perl was either
       built with one particular dbm library or another, but not both, and you
       couldn't extend this mechanism to other packages or types of variables.

       Now you can.

       The tie() function binds a variable to a class (package) that will
       provide the implementation for access methods for that variable.  Once
       this magic has been performed, accessing a tied variable automatically
       triggers method calls in the proper class.  The complexity of the class
       is hidden behind magic methods calls.  The method names are in ALL
       CAPS, which is a convention that Perl uses to indicate that they're
       called implicitly rather than explicitly--just like the BEGIN() and
       END() functions.

       In the tie() call, "VARIABLE" is the name of the variable to be
       enchanted.  "CLASSNAME" is the name of a class implementing objects of
       the correct type.  Any additional arguments in the "LIST" are passed to
       the appropriate constructor method for that class--meaning TIESCALAR(),
       TIEARRAY(), TIEHASH(), or TIEHANDLE().  (Typically these are arguments
       such as might be passed to the dbminit() function of C.) The object
       returned by the "new" method is also returned by the tie() function,
       which would be useful if you wanted to access other methods in
       "CLASSNAME". (You don't actually have to return a reference to a right
       "type" (e.g., HASH or "CLASSNAME") so long as it's a properly blessed
       object.)  You can also retrieve a reference to the underlying object
       using the tied() function.

       Unlike dbmopen(), the tie() function will not "use" or "require" a
       module for you--you need to do that explicitly yourself.

   Tying Scalars
       A class implementing a tied scalar should define the following methods:
       TIESCALAR, FETCH, STORE, and possibly UNTIE and/or DESTROY.

       Let's look at each in turn, using as an example a tie class for scalars
       that allows the user to do something like:

           tie $his_speed, 'Nice', getppid();
           tie $my_speed,  'Nice', $$;

       And now whenever either of those variables is accessed, its current
       system priority is retrieved and returned.  If those variables are set,

       TIESCALAR classname, LIST
           This is the constructor for the class.  That means it is expected
           to return a blessed reference to a new scalar (probably anonymous)
           that it's creating.  For example:

               sub TIESCALAR {
                   my $class = shift;
                   my $pid = shift || $$; # 0 means me

                   if ($pid !~ /^\d+$/) {
                       carp "Nice::Tie::Scalar got non-numeric pid $pid" if $^W;
                       return undef;
                   }

                   unless (kill 0, $pid) { # EPERM or ERSCH, no doubt
                       carp "Nice::Tie::Scalar got bad pid $pid: $!" if $^W;
                       return undef;
                   }

                   return bless \$pid, $class;
               }

           This tie class has chosen to return an error rather than raising an
           exception if its constructor should fail.  While this is how
           dbmopen() works, other classes may well not wish to be so
           forgiving.  It checks the global variable $^W to see whether to
           emit a bit of noise anyway.

       FETCH this
           This method will be triggered every time the tied variable is
           accessed (read).  It takes no arguments beyond its self reference,
           which is the object representing the scalar we're dealing with.
           Because in this case we're using just a SCALAR ref for the tied
           scalar object, a simple $$self allows the method to get at the real
           value stored there.  In our example below, that real value is the
           process ID to which we've tied our variable.

               sub FETCH {
                   my $self = shift;
                   confess "wrong type" unless ref $self;
                   croak "usage error" if @_;
                   my $nicety;
                   local($!) = 0;
                   $nicety = getpriority(PRIO_PROCESS, $$self);
                   if ($!) { croak "getpriority failed: $!" }
                   return $nicety;
               }

           This time we've decided to blow up (raise an exception) if the
           renice fails--there's no place for us to return an error otherwise,
           and it's probably the right thing to do.

       STORE this, value
                   if ($new_nicety < PRIO_MIN) {
                       carp sprintf
                         "WARNING: priority %d less than minimum system priority %d",
                             $new_nicety, PRIO_MIN if $^W;
                       $new_nicety = PRIO_MIN;
                   }

                   if ($new_nicety > PRIO_MAX) {
                       carp sprintf
                         "WARNING: priority %d greater than maximum system priority %d",
                             $new_nicety, PRIO_MAX if $^W;
                       $new_nicety = PRIO_MAX;
                   }

                   unless (defined setpriority(PRIO_PROCESS, $$self, $new_nicety)) {
                       confess "setpriority failed: $!";
                   }
               }

       UNTIE this
           This method will be triggered when the "untie" occurs. This can be
           useful if the class needs to know when no further calls will be
           made. (Except DESTROY of course.) See "The "untie" Gotcha" below
           for more details.

       DESTROY this
           This method will be triggered when the tied variable needs to be
           destructed.  As with other object classes, such a method is seldom
           necessary, because Perl deallocates its moribund object's memory
           for you automatically--this isn't C++, you know.  We'll use a
           DESTROY method here for debugging purposes only.

               sub DESTROY {
                   my $self = shift;
                   confess "wrong type" unless ref $self;
                   carp "[ Nice::DESTROY pid $$self ]" if $Nice::DEBUG;
               }

       That's about all there is to it.  Actually, it's more than all there is
       to it, because we've done a few nice things here for the sake of
       completeness, robustness, and general aesthetics.  Simpler TIESCALAR
       classes are certainly possible.

   Tying Arrays
       A class implementing a tied ordinary array should define the following
       methods: TIEARRAY, FETCH, STORE, FETCHSIZE, STORESIZE and perhaps UNTIE
       and/or DESTROY.

       FETCHSIZE and STORESIZE are used to provide $#array and equivalent
       "scalar(@array)" access.

       The methods POP, PUSH, SHIFT, UNSHIFT, SPLICE, DELETE, and EXISTS are
       required if the perl operator with the corresponding (but lowercase)
       name is to operate on the tied array. The Tie::Array class can be used
           tie @array, 'FixedElem_Array', 3;
           $array[0] = 'cat';  # ok.
           $array[1] = 'dogs'; # exception, length('dogs') > 3.

       The preamble code for the class is as follows:

           package FixedElem_Array;
           use Carp;
           use strict;

       TIEARRAY classname, LIST
           This is the constructor for the class.  That means it is expected
           to return a blessed reference through which the new array (probably
           an anonymous ARRAY ref) will be accessed.

           In our example, just to show you that you don't really have to
           return an ARRAY reference, we'll choose a HASH reference to
           represent our object.  A HASH works out well as a generic record
           type: the "{ELEMSIZE}" field will store the maximum element size
           allowed, and the "{ARRAY}" field will hold the true ARRAY ref.  If
           someone outside the class tries to dereference the object returned
           (doubtless thinking it an ARRAY ref), they'll blow up.  This just
           goes to show you that you should respect an object's privacy.

               sub TIEARRAY {
                 my $class    = shift;
                 my $elemsize = shift;
                 if ( @_ || $elemsize =~ /\D/ ) {
                   croak "usage: tie ARRAY, '" . __PACKAGE__ . "', elem_size";
                 }
                 return bless {
                   ELEMSIZE => $elemsize,
                   ARRAY    => [],
                 }, $class;
               }

       FETCH this, index
           This method will be triggered every time an individual element the
           tied array is accessed (read).  It takes one argument beyond its
           self reference: the index whose value we're trying to fetch.

               sub FETCH {
                 my $self  = shift;
                 my $index = shift;
                 return $self->{ARRAY}->[$index];
               }

           If a negative array index is used to read from an array, the index
           will be translated to a positive one internally by calling
           FETCHSIZE before being passed to FETCH.  You may disable this
           feature by assigning a true value to the variable $NEGATIVE_INDICES
           in the tied array class.

           As you may have noticed, the name of the FETCH method (et al.) is
           In our example, "undef" is really "$self->{ELEMSIZE}" number of
           spaces so we have a little more work to do here:

               sub STORE {
                 my $self = shift;
                 my( $index, $value ) = @_;
                 if ( length $value > $self->{ELEMSIZE} ) {
                   croak "length of $value is greater than $self->{ELEMSIZE}";
                 }
                 # fill in the blanks
                 $self->EXTEND( $index ) if $index > $self->FETCHSIZE();
                 # right justify to keep element size for smaller elements
                 $self->{ARRAY}->[$index] = sprintf "%$self->{ELEMSIZE}s", $value;
               }

           Negative indexes are treated the same as with FETCH.

       FETCHSIZE this
           Returns the total number of items in the tied array associated with
           object this. (Equivalent to "scalar(@array)").  For example:

               sub FETCHSIZE {
                 my $self = shift;
                 return scalar @{$self->{ARRAY}};
               }

       STORESIZE this, count
           Sets the total number of items in the tied array associated with
           object this to be count. If this makes the array larger then
           class's mapping of "undef" should be returned for new positions.
           If the array becomes smaller then entries beyond count should be
           deleted.

           In our example, 'undef' is really an element containing
           "$self->{ELEMSIZE}" number of spaces.  Observe:

               sub STORESIZE {
                 my $self  = shift;
                 my $count = shift;
                 if ( $count > $self->FETCHSIZE() ) {
                   foreach ( $count - $self->FETCHSIZE() .. $count ) {
                     $self->STORE( $_, '' );
                   }
                 } elsif ( $count < $self->FETCHSIZE() ) {
                   foreach ( 0 .. $self->FETCHSIZE() - $count - 2 ) {
                     $self->POP();
                   }
                 }
               }

       EXTEND this, count
           Informative call that array is likely to grow to have count
           entries.  Can be used to optimize allocation. This method need do
           nothing.
           Verify that the element at index key exists in the tied array this.

           In our example, we will determine that if an element consists of
           "$self->{ELEMSIZE}" spaces only, it does not exist:

               sub EXISTS {
                 my $self  = shift;
                 my $index = shift;
                 return 0 if ! defined $self->{ARRAY}->[$index] ||
                             $self->{ARRAY}->[$index] eq ' ' x $self->{ELEMSIZE};
                 return 1;
               }

       DELETE this, key
           Delete the element at index key from the tied array this.

           In our example, a deleted item is "$self->{ELEMSIZE}" spaces:

               sub DELETE {
                 my $self  = shift;
                 my $index = shift;
                 return $self->STORE( $index, '' );
               }

       CLEAR this
           Clear (remove, delete, ...) all values from the tied array
           associated with object this.  For example:

               sub CLEAR {
                 my $self = shift;
                 return $self->{ARRAY} = [];
               }

       PUSH this, LIST
           Append elements of LIST to the array.  For example:

               sub PUSH {
                 my $self = shift;
                 my @list = @_;
                 my $last = $self->FETCHSIZE();
                 $self->STORE( $last + $_, $list[$_] ) foreach 0 .. $#list;
                 return $self->FETCHSIZE();
               }

       POP this
           Remove last element of the array and return it.  For example:

               sub POP {
                 my $self = shift;
                 return pop @{$self->{ARRAY}};
               }

       SHIFT this
           Remove the first element of the array (shifting other elements
                 my $self = shift;
                 my @list = @_;
                 my $size = scalar( @list );
                 # make room for our list
                 @{$self->{ARRAY}}[ $size .. $#{$self->{ARRAY}} + $size ]
                  = @{$self->{ARRAY}};
                 $self->STORE( $_, $list[$_] ) foreach 0 .. $#list;
               }

       SPLICE this, offset, length, LIST
           Perform the equivalent of "splice" on the array.

           offset is optional and defaults to zero, negative values count back
           from the end of the array.

           length is optional and defaults to rest of the array.

           LIST may be empty.

           Returns a list of the original length elements at offset.

           In our example, we'll use a little shortcut if there is a LIST:

               sub SPLICE {
                 my $self   = shift;
                 my $offset = shift || 0;
                 my $length = shift || $self->FETCHSIZE() - $offset;
                 my @list   = ();
                 if ( @_ ) {
                   tie @list, __PACKAGE__, $self->{ELEMSIZE};
                   @list   = @_;
                 }
                 return splice @{$self->{ARRAY}}, $offset, $length, @list;
               }

       UNTIE this
           Will be called when "untie" happens. (See "The "untie" Gotcha"
           below.)

       DESTROY this
           This method will be triggered when the tied variable needs to be
           destructed.  As with the scalar tie class, this is almost never
           needed in a language that does its own garbage collection, so this
           time we'll just leave it out.

   Tying Hashes
       Hashes were the first Perl data type to be tied (see dbmopen()).  A
       class implementing a tied hash should define the following methods:
       TIEHASH is the constructor.  FETCH and STORE access the key and value
       pairs.  EXISTS reports whether a key is present in the hash, and DELETE
       deletes one.  CLEAR empties the hash by deleting all the key and value
       pairs.  FIRSTKEY and NEXTKEY implement the keys() and each() functions
       to iterate over all the keys. SCALAR is triggered when the tied hash is
       evaluated in scalar context. UNTIE is called when "untie" happens, and
       you a hash representing a particular user's dot files.  You index into
       the hash with the name of the file (minus the dot) and you get back
       that dot file's contents.  For example:

           use DotFiles;
           tie %dot, 'DotFiles';
           if ( $dot{profile} =~ /MANPATH/ ||
                $dot{login}   =~ /MANPATH/ ||
                $dot{cshrc}   =~ /MANPATH/    )
           {
               print "you seem to set your MANPATH\n";
           }

       Or here's another sample of using our tied class:

           tie %him, 'DotFiles', 'daemon';
           foreach $f ( keys %him ) {
               printf "daemon dot file %s is size %d\n",
                   $f, length $him{$f};
           }

       In our tied hash DotFiles example, we use a regular hash for the object
       containing several important fields, of which only the "{LIST}" field
       will be what the user thinks of as the real hash.

       USER whose dot files this object represents

       HOME where those dot files live

       CLOBBER
            whether we should try to change or remove those dot files

       LIST the hash of dot file names and content mappings

       Here's the start of Dotfiles.pm:

           package DotFiles;
           use Carp;
           sub whowasi { (caller(1))[3] . '()' }
           my $DEBUG = 0;
           sub debug { $DEBUG = @_ ? shift : 1 }

       For our example, we want to be able to emit debugging info to help in
       tracing during development.  We keep also one convenience function
       around internally to help print out warnings; whowasi() returns the
       function name that calls it.

       Here are the methods for the DotFiles tied hash.

       TIEHASH classname, LIST
           This is the constructor for the class.  That means it is expected
           to return a blessed reference through which the new object
           (probably but not necessarily an anonymous hash) will be accessed.

                   my $node = {
                       USER    => $user,
                       HOME    => $dir,
                       LIST    => {},
                       CLOBBER => 0,
                   };

                   opendir(DIR, $dir)
                           || croak "@{[&whowasi]}: can't opendir $dir: $!";
                   foreach $dot ( grep /^\./ && -f "$dir/$_", readdir(DIR)) {
                       $dot =~ s/^\.//;
                       $node->{LIST}{$dot} = undef;
                   }
                   closedir DIR;
                   return bless $node, $self;
               }

           It's probably worth mentioning that if you're going to filetest the
           return values out of a readdir, you'd better prepend the directory
           in question.  Otherwise, because we didn't chdir() there, it would
           have been testing the wrong file.

       FETCH this, key
           This method will be triggered every time an element in the tied
           hash is accessed (read).  It takes one argument beyond its self
           reference: the key whose value we're trying to fetch.

           Here's the fetch for our DotFiles example.

               sub FETCH {
                   carp &whowasi if $DEBUG;
                   my $self = shift;
                   my $dot = shift;
                   my $dir = $self->{HOME};
                   my $file = "$dir/.$dot";

                   unless (exists $self->{LIST}->{$dot} || -f $file) {
                       carp "@{[&whowasi]}: no $dot file" if $DEBUG;
                       return undef;
                   }

                   if (defined $self->{LIST}->{$dot}) {
                       return $self->{LIST}->{$dot};
                   } else {
                       return $self->{LIST}->{$dot} = `cat $dir/.$dot`;
                   }
               }

           It was easy to write by having it call the Unix cat(1) command, but
           it would probably be more portable to open the file manually (and
           somewhat more efficient).  Of course, because dot files are a Unixy
           concept, we're not that concerned.

       STORE this, key, value
                   my $dot = shift;
                   my $value = shift;
                   my $file = $self->{HOME} . "/.$dot";
                   my $user = $self->{USER};

                   croak "@{[&whowasi]}: $file not clobberable"
                       unless $self->{CLOBBER};

                   open(F, "> $file") || croak "can't open $file: $!";
                   print F $value;
                   close(F);
               }

           If they wanted to clobber something, they might say:

               $ob = tie %daemon_dots, 'daemon';
               $ob->clobber(1);
               $daemon_dots{signature} = "A true daemon\n";

           Another way to lay hands on a reference to the underlying object is
           to use the tied() function, so they might alternately have set
           clobber using:

               tie %daemon_dots, 'daemon';
               tied(%daemon_dots)->clobber(1);

           The clobber method is simply:

               sub clobber {
                   my $self = shift;
                   $self->{CLOBBER} = @_ ? shift : 1;
               }

       DELETE this, key
           This method is triggered when we remove an element from the hash,
           typically by using the delete() function.  Again, we'll be careful
           to check whether they really want to clobber files.

               sub DELETE   {
                   carp &whowasi if $DEBUG;

                   my $self = shift;
                   my $dot = shift;
                   my $file = $self->{HOME} . "/.$dot";
                   croak "@{[&whowasi]}: won't remove file $file"
                       unless $self->{CLOBBER};
                   delete $self->{LIST}->{$dot};
                   my $success = unlink($file);
                   carp "@{[&whowasi]}: can't unlink $file: $!" unless $success;
                   $success;
               }

           The value returned by DELETE becomes the return value of the call
           to delete().  If you want to emulate the normal behavior of

               sub CLEAR    {
                   carp &whowasi if $DEBUG;
                   my $self = shift;
                   croak "@{[&whowasi]}: won't remove all dot files for $self->{USER}"
                       unless $self->{CLOBBER} > 1;
                   my $dot;
                   foreach $dot ( keys %{$self->{LIST}}) {
                       $self->DELETE($dot);
                   }
               }

       EXISTS this, key
           This method is triggered when the user uses the exists() function
           on a particular hash.  In our example, we'll look at the "{LIST}"
           hash element for this:

               sub EXISTS   {
                   carp &whowasi if $DEBUG;
                   my $self = shift;
                   my $dot = shift;
                   return exists $self->{LIST}->{$dot};
               }

       FIRSTKEY this
           This method will be triggered when the user is going to iterate
           through the hash, such as via a keys() or each() call.

               sub FIRSTKEY {
                   carp &whowasi if $DEBUG;
                   my $self = shift;
                   my $a = keys %{$self->{LIST}};          # reset each() iterator
                   each %{$self->{LIST}}
               }

       NEXTKEY this, lastkey
           This method gets triggered during a keys() or each() iteration.  It
           has a second argument which is the last key that had been accessed.
           This is useful if you're carrying about ordering or calling the
           iterator from more than one sequence, or not really storing things
           in a hash anywhere.

           For our example, we're using a real hash so we'll do just the
           simple thing, but we'll have to go through the LIST field
           indirectly.

               sub NEXTKEY  {
                   carp &whowasi if $DEBUG;
                   my $self = shift;
                   return each %{ $self->{LIST} }
               }

       SCALAR this
           This is called when the hash is evaluated in scalar context. In
           context.

           In our example we can just call "scalar" on the underlying hash
           referenced by "$self->{LIST}":

               sub SCALAR {
                   carp &whowasi if $DEBUG;
                   my $self = shift;
                   return scalar %{ $self->{LIST} }
               }

       UNTIE this
           This is called when "untie" occurs.  See "The "untie" Gotcha"
           below.

       DESTROY this
           This method is triggered when a tied hash is about to go out of
           scope.  You don't really need it unless you're trying to add
           debugging or have auxiliary state to clean up.  Here's a very
           simple function:

               sub DESTROY  {
                   carp &whowasi if $DEBUG;
               }

       Note that functions such as keys() and values() may return huge lists
       when used on large objects, like DBM files.  You may prefer to use the
       each() function to iterate over such.  Example:

           # print out history file offsets
           use NDBM_File;
           tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
           while (($key,$val) = each %HIST) {
               print $key, ' = ', unpack('L',$val), "\n";
           }
           untie(%HIST);

   Tying FileHandles
       This is partially implemented now.

       A class implementing a tied filehandle should define the following
       methods: TIEHANDLE, at least one of PRINT, PRINTF, WRITE, READLINE,
       GETC, READ, and possibly CLOSE, UNTIE and DESTROY.  The class can also
       provide: BINMODE, OPEN, EOF, FILENO, SEEK, TELL - if the corresponding
       perl operators are used on the handle.

       When STDERR is tied, its PRINT method will be called to issue warnings
       and error messages.  This feature is temporarily disabled during the
       call, which means you can use "warn()" inside PRINT without starting a
       recursive loop.  And just like "__WARN__" and "__DIE__" handlers,
       STDERR's PRINT method may be called to report parser errors, so the
       caveats mentioned under "%SIG" in perlvar apply.

       All of this is especially useful when perl is embedded in some other
               sub TIEHANDLE { print "<shout>\n"; my $i; bless \$i, shift }

       WRITE this, LIST
           This method will be called when the handle is written to via the
           "syswrite" function.

               sub WRITE {
                   $r = shift;
                   my($buf,$len,$offset) = @_;
                   print "WRITE called, \$buf=$buf, \$len=$len, \$offset=$offset";
               }

       PRINT this, LIST
           This method will be triggered every time the tied handle is printed
           to with the "print()" or "say()" functions.  Beyond its self
           reference it also expects the list that was passed to the print
           function.

               sub PRINT { $r = shift; $$r++; print join($,,map(uc($_),@_)),$\ }

           "say()" acts just like "print()" except $\ will be localized to
           "\n" so you need do nothing special to handle "say()" in "PRINT()".

       PRINTF this, LIST
           This method will be triggered every time the tied handle is printed
           to with the "printf()" function.  Beyond its self reference it also
           expects the format and list that was passed to the printf function.

               sub PRINTF {
                   shift;
                   my $fmt = shift;
                   print sprintf($fmt, @_);
               }

       READ this, LIST
           This method will be called when the handle is read from via the
           "read" or "sysread" functions.

               sub READ {
                   my $self = shift;
                   my $bufref = \$_[0];
                   my(undef,$len,$offset) = @_;
                   print "READ called, \$buf=$bufref, \$len=$len, \$offset=$offset";
                   # add to $$bufref, set $len to number of characters read
                   $len;
               }

       READLINE this
           This method will be called when the handle is read from via
           <HANDLE>.  The method should return undef when there is no more
           data.

               sub READLINE { $r = shift; "READLINE called $$r times\n"; }

           As with the other types of ties, this method will be called when
           "untie" happens.  It may be appropriate to "auto CLOSE" when this
           occurs.  See "The "untie" Gotcha" below.

       DESTROY this
           As with the other types of ties, this method will be called when
           the tied handle is about to be destroyed. This is useful for
           debugging and possibly cleaning up.

               sub DESTROY { print "</shout>\n" }

       Here's how to use our little example:

           tie(*FOO,'Shout');
           print FOO "hello\n";
           $a = 4; $b = 6;
           print FOO $a, " plus ", $b, " equals ", $a + $b, "\n";
           print <FOO>;

   UNTIE this
       You can define for all tie types an UNTIE method that will be called at
       untie().  See "The "untie" Gotcha" below.

   The "untie" Gotcha
       If you intend making use of the object returned from either tie() or
       tied(), and if the tie's target class defines a destructor, there is a
       subtle gotcha you must guard against.

       As setup, consider this (admittedly rather contrived) example of a tie;
       all it does is use a file to keep a log of the values assigned to a
       scalar.

           package Remember;

           use strict;
           use warnings;
           use IO::File;

           sub TIESCALAR {
               my $class = shift;
               my $filename = shift;
               my $handle = IO::File->new( "> $filename" )
                                or die "Cannot open $filename: $!\n";

               print $handle "The Start\n";
               bless {FH => $handle, Value => 0}, $class;
           }

           sub FETCH {
               my $self = shift;
               return $self->{Value};
           }

           sub STORE {
           }

           1;

       Here is an example that makes use of this tie:

           use strict;
           use Remember;

           my $fred;
           tie $fred, 'Remember', 'myfile.txt';
           $fred = 1;
           $fred = 4;
           $fred = 5;
           untie $fred;
           system "cat myfile.txt";

       This is the output when it is executed:

           The Start
           1
           4
           5
           The End

       So far so good.  Those of you who have been paying attention will have
       spotted that the tied object hasn't been used so far.  So lets add an
       extra method to the Remember class to allow comments to be included in
       the file -- say, something like this:

           sub comment {
               my $self = shift;
               my $text = shift;
               my $handle = $self->{FH};
               print $handle $text, "\n";
           }

       And here is the previous example modified to use the "comment" method
       (which requires the tied object):

           use strict;
           use Remember;

           my ($fred, $x);
           $x = tie $fred, 'Remember', 'myfile.txt';
           $fred = 1;
           $fred = 4;
           comment $x "changing...";
           $fred = 5;
           untie $fred;
           system "cat myfile.txt";

       When this code is executed there is no output.  Here's why:

       The reason there is no output is because the file buffers have not been
       flushed to disk.

       Now that you know what the problem is, what can you do to avoid it?
       Prior to the introduction of the optional UNTIE method the only way was
       the good old "-w" flag. Which will spot any instances where you call
       untie() and there are still valid references to the tied object.  If
       the second script above this near the top "use warnings 'untie'" or was
       run with the "-w" flag, Perl prints this warning message:

           untie attempted while 1 inner references still exist

       To get the script to work properly and silence the warning make sure
       there are no valid references to the tied object before untie() is
       called:

           undef $x;
           untie $fred;

       Now that UNTIE exists the class designer can decide which parts of the
       class functionality are really associated with "untie" and which with
       the object being destroyed. What makes sense for a given class depends
       on whether the inner references are being kept so that non-tie-related
       methods can be called on the object. But in most cases it probably
       makes sense to move the functionality that would have been in DESTROY
       to the UNTIE method.

       If the UNTIE method exists then the warning above does not occur.
       Instead the UNTIE method is passed the count of "extra" references and
       can issue its own warning if appropriate. e.g. to replicate the no
       UNTIE case this method can be used:

           sub UNTIE
           {
            my ($obj,$count) = @_;
            carp "untie attempted while $count inner references still exist" if $count;
           }

SEE ALSO
       See DB_File or Config for some interesting tie() implementations.  A
       good starting point for many tie() implementations is with one of the
       modules Tie::Scalar, Tie::Array, Tie::Hash, or Tie::Handle.

BUGS
       The bucket usage information provided by "scalar(%hash)" is not
       available.  What this means is that using %tied_hash in boolean context
       doesn't work right (currently this always tests false, regardless of
       whether the hash is empty or hash elements).

       Localizing tied arrays or hashes does not work.  After exiting the
       scope the arrays or the hashes are not restored.

       Counting the number of entries in a hash via "scalar(keys(%hash))" or
       "scalar(values(%hash)") is inefficient since it needs to iterate
       attempt at solving the problem is MLDBM, which is also available on the
       CPAN, but which has some fairly serious limitations.

       Tied filehandles are still incomplete.  sysopen(), truncate(), flock(),
       fcntl(), stat() and -X can't currently be trapped.

AUTHOR
       Tom Christiansen

       TIEHANDLE by Sven Verdoolaege <skimo@dns.ufsia.ac.be> and Doug
       MacEachern <dougm@osf.org>

       UNTIE by Nick Ing-Simmons <nick@ing-simmons.net>

       SCALAR by Tassilo von Parseval <tassilo.von.parseval@rwth-aachen.de>

       Tying Arrays by Casey West <casey@geeknest.com>



perl v5.10.1                      2009-05-06                        PERLTIE(1)
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