Provided by: perl-doc_5.18.2-2ubuntu1.7_all bug

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 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
           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",
                             $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.

       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.

               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:

               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:

               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:

               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 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 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,
                   };

                   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 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(my $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 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 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} }
               }

       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.

       When tying a handle, the first argument to "tie" should begin with an asterisk.  So, if
       you are tying STDOUT, use *STDOUT.  If you have assigned it to a scalar variable, say
       $handle, use *$handle.  "tie $handle" ties the scalar variable $handle, not the handle
       inside it.

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

           package Shout;

       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;
               }

       READLINE this
           This method is called when the handle is read via "<HANDLE>" or "readline HANDLE".

           As per "readline", in scalar context it should return the next line, or "undef" for no
           more data.  In list context it should return all remaining lines, or an empty list for
           no more data.  The strings returned should include the input record separator $/ (see
           perlvar), unless it is "undef" (which means "slurp" mode).

               sub READLINE {
                 my $r = shift;
                 if (wantarray) {
                   return ("all remaining\n",
                           "lines up\n",
                           "to eof\n");
                 } else {
                   return "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"; }

       EOF this
           This method will be called when the "eof" function is called.

           Starting with Perl 5.12, an additional integer parameter will be passed.  It will be
           zero if "eof" is called without parameter; 1 if "eof" is given a filehandle as a
           parameter, e.g. "eof(FH)"; and 2 in the very special case that the tied filehandle is
           "ARGV" and "eof" is called with an empty parameter list, e.g. "eof()".

               sub EOF { not length $stringbuf }

       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.

           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:

           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.

       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 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>