


PERLTIE(1)       Perl Programmers Reference Guide      PERLTIE(1)


NAME
       perltie - how to hide an object class in a simple variable

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



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       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, then the process's priority is
       changed!

       We'll use Jarkko Hietaniemi <jhi@iki.fi>'s BSD::Resource
       class (not included) to access the PRIO_PROCESS, PRIO_MIN,
       and PRIO_MAX constants from your system, as well as the
       getpriority() and setpriority() system calls.  Here's the
       preamble of the class.

           package Nice;
           use Carp;
           use BSD::Resource;
           use strict;
           $Nice::DEBUG = 0 unless defined $Nice::DEBUG;

       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



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           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
           This method will be triggered every time the tied
           variable is set (assigned).  Beyond its self
           reference, it also expects one (and only one)
           argument--the new value the user is trying to assign.
           Don't worry about returning a value from STORE -- the
           semantic of assignment returning the assigned value is
           implemented with FETCH.

               sub STORE {
                   my $self = shift;
                   confess "wrong type" unless ref $self;
                   my $new_nicety = shift;
                   croak "usage error" if @_;

                   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",



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                             $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 as a base
       class to implement the first five of these in terms of the
       basic methods above.  The default implementations of
       DELETE and EXISTS in Tie::Array simply "croak".

       In addition EXTEND will be called when perl would have
       pre-extended allocation in a real array.



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       For this discussion, we'll implement an array whose
       elements are a fixed size at creation.  If you try to
       create an element larger than the fixed size, you'll take
       an exception.  For example:

           use FixedElem_Array;
           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.







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               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 the same for all accesses, even though the
           constructors differ in names (TIESCALAR vs TIEARRAY).
           While in theory you could have the same class
           servicing several tied types, in practice this becomes
           cumbersome, and it's easiest to keep them at simply
           one tie type per class.

       STORE this, index, value
           This method will be triggered every time an element in
           the tied array is set (written).  It takes two
           arguments beyond its self reference: the index at
           which we're trying to store something and the value
           we're trying to put there.

           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:







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               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.

           In our example, we want to make sure there are no
           blank ("undef") entries, so "EXTEND" will make use of
           "STORESIZE" to fill elements as needed:

               sub EXTEND {
                 my $self  = shift;
                 my $count = shift;
                 $self->STORESIZE( $count );
               }

       EXISTS this, key
           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:




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               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 down) and return it.  For example:




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

       UNSHIFT this, LIST
           Insert LIST elements at the beginning of the array,
           moving existing elements up to make room.  For
           example:

               sub UNSHIFT {
                 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



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           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
       DESTROY is called when the tied variable is garbage
       collected.

       If this seems like a lot, then feel free to inherit from
       merely the standard Tie::StdHash module for most of your
       methods, redefining only the interesting ones.  See
       Tie::Hash for details.

       Remember that Perl distinguishes between a key not
       existing in the hash, and the key existing in the hash but
       having a corresponding value of "undef".  The two
       possibilities can be tested with the "exists()" and
       "defined()" functions.

       Here's an example of a somewhat interesting tied hash
       class:  it gives 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



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       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.

           Here's the constructor:

               sub TIEHASH {
                   my $self = shift;
                   my $user = shift || $>;
                   my $dotdir = shift || '';
                   croak "usage: @{[&whowasi]} [USER [DOTDIR]]" if @_;
                   $user = getpwuid($user) if $user =~ /^\d+$/;
                   my $dir = (getpwnam($user))[7]
                           || croak "@{[&whowasi]}: no user $user";
                   $dir .= "/$dotdir" if $dotdir;

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



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                   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
           This method will be triggered every time an element in
           the tied hash is set (written).  It takes two
           arguments beyond its self reference: the index at



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           which we're trying to store something, and the value
           we're trying to put there.

           Here in our DotFiles example, we'll be careful not to
           let them try to overwrite the file unless they've
           called the clobber() method on the original object
           reference returned by tie().

               sub STORE {
                   carp &whowasi if $DEBUG;
                   my $self = shift;
                   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;



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                   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 delete(), you should return
           whatever FETCH would have returned for this key.  In
           this example, we have chosen instead to return a value
           which tells the caller whether the file was
           successfully deleted.

       CLEAR this
           This method is triggered when the whole hash is to be
           cleared, usually by assigning the empty list to it.

           In our example, that would remove all the user's dot
           files!  It's such a dangerous thing that they'll have
           to set CLOBBER to something higher than 1 to make it
           happen.

               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



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           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 order to mimic the behaviour of untied
           hashes, this method should return a false value when
           the tied hash is considered empty. If this method does
           not exist, perl will make some educated guesses and
           return true when the hash is inside an iteration. If
           this isn't the case, FIRSTKEY is called, and the
           result will be a false value if FIRSTKEY returns the
           empty list, true otherwise.

           However, you should not blindly rely on perl always
           doing the right thing. Particularly, perl will
           mistakenly return true when you clear the hash by
           repeatedly calling DELETE until it is empty. You are
           therefore advised to supply your own SCALAR method
           when you want to be absolutely sure that your hash
           behaves nicely in scalar 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} }
               }



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       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 program, where output to STDOUT and STDERR may
       have to be redirected in some special way.  See nvi and
       the Apache module for examples.

       In our example we're going to create a shouting handle.

           package Shout;



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       TIEHANDLE classname, LIST
           This is the constructor for the class.  That means it
           is expected to return a blessed reference of some
           sort. The reference can be used to hold some internal
           information.

               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;
               }



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       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"; }

       GETC this
           This method will be called when the "getc" function is
           called.

               sub GETC { print "Don't GETC, Get Perl"; return "a"; }

       CLOSE this
           This method will be called when the handle is closed
           via the "close" function.

               sub CLOSE { print "CLOSE called.\n" }

       UNTIE this
           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.




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           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 {
               my $self = shift;
               my $value = shift;
               my $handle = $self->{FH};
               print $handle "$value\n";
               $self->{Value} = $value;
           }

           sub DESTROY {
               my $self = shift;
               my $handle = $self->{FH};
               print $handle "The End\n";
               close $handle;
           }

           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:





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           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:

       When a variable is tied, it is associated with the object
       which is the return value of the TIESCALAR, TIEARRAY, or
       TIEHASH function.  This object normally has only one
       reference, namely, the implicit reference from the tied
       variable.  When untie() is called, that reference is
       destroyed.  Then, as in the first example above, the
       object's destructor (DESTROY) is called, which is normal
       for objects that have no more valid references; and thus
       the file is closed.

       In the second example, however, we have stored another
       reference to the tied object in $x.  That means that when
       untie() gets called there will still be a valid reference
       to the object in existence, so the destructor is not
       called at that time, and thus the file is not closed.  The
       reason there is no output is because the file buffers have
       not been flushed to disk.



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       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



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       restored.

       Counting the number of entries in a hash via
       "scalar(keys(%hash))" or "scalar(values(%hash)") is
       inefficient since it needs to iterate through all the
       entries with FIRSTKEY/NEXTKEY.

       Tied hash/array slices cause multiple FETCH/STORE pairs,
       there are no tie methods for slice operations.

       You cannot easily tie a multilevel data structure (such as
       a hash of hashes) to a dbm file.  The first problem is
       that all but GDBM and Berkeley DB have size limitations,
       but beyond that, you also have problems with how
       references are to be represented on disk.  One module that
       does attempt to address this need is DBM::Deep.  Check
       your nearest CPAN site as described in perlmodlib for
       source code.  Note that despite its name, DBM::Deep does
       not use dbm.  Another earlier 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>



















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