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

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

SSYYNNOOPPSSIISS
        tie VARIABLE, CLASSNAME, LIST

        $object = tied VARIABLE

        untie VARIABLE

DDEESSCCRRIIPPTTIIOONN
       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.

       TTyyiinngg SSccaallaarrss

       A class implementing a tied scalar should define the
       following methods: TIESCALAR, FETCH, STORE, and possibly
       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.

                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: $!";
                    }
                    return $new_nicety;
                }

       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.

       TTyyiinngg AArrrraayyss

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

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

       The methods POP, PUSH, SHIFT, UNSHIFT, SPLICE are required
       if the perl operator with the corresponding (but
       lowercase) name is to operate on the tied array. The
       TTiiee::::AArrrraayy class can be used as a base class to implement
       these in terms of the basic five methods above.

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

       This means that tied arrays are now complete. The example
       below needs upgrading to illustrate this. (The
       documentation in TTiiee::::AArrrraayy is more complete.)

       For this discussion, we'll implement an array whose
       indices are fixed at its creation.  If you try to access
       anything beyond those bounds, you'll take an exception.
       For example:

           require Bounded_Array;
           tie @ary, 'Bounded_Array', 2;
           $| = 1;
           for $i (0 .. 10) {
               print "setting index $i: ";
               $ary[$i] = 10 * $i;
               $ary[$i] = 10 * $i;
               print "value of elt $i now $ary[$i]\n";
           }

       The preamble code for the class is as follows:

           package Bounded_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
            {BOUND} field will store the maximum bound 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 $bound = shift;
                    confess "usage: tie(\@ary, 'Bounded_Array', max_subscript)"
                        if @_ || $bound =~ /\D/;
                    return bless {
                        BOUND => $bound,
                        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($self,$idx) = @_;
                  if ($idx > $self->{BOUND}) {
                    confess "Array OOB: $idx > $self->{BOUND}";
                  }
                  return $self->{ARRAY}[$idx];
                }

            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.  For example:

                sub STORE {
                  my($self, $idx, $value) = @_;
                  print "[STORE $value at $idx]\n" if _debug;
                  if ($idx > $self->{BOUND} ) {
                    confess "Array OOB: $idx > $self->{BOUND}";
                  }
                  return $self->{ARRAY}[$idx] = $value;
                }

       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.

       The code we presented at the top of the tied array class
       accesses many elements of the array, far more than we've
       set the bounds to.  Therefore, it will blow up once they
       try to access beyond the 2nd element of @ary, as the
       following output demonstrates:

           setting index 0: value of elt 0 now 0
           setting index 1: value of elt 1 now 10
           setting index 2: value of elt 2 now 20
           setting index 3: Array OOB: 3 > 2 at Bounded_Array.pm line 39
                   Bounded_Array::FETCH called at testba line 12

       TTyyiinngg HHaasshheess

       As the first Perl data type to be tied (see dbmopen()),
       hashes have the most complete and useful tie()
       implementation.  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.
       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::Hash module for most of your
       methods, redefining only the interesting ones.  See the
       Tie::Hash manpage 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(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} }
                }

       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);

       TTyyiinngg FFiilleeHHaannddlleess

       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
       and DESTROY.

       It 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;

       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() function.
            Beyond its self reference it also expects the list
            that was passed to the print function.

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

       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, @_)."\n";
                }

       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"; }

       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" }

       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>;

       TThhee uunnttiiee 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 IO::File;

           sub TIESCALAR {
               my $class = shift;
               my $filename = shift;
               my $handle = new IO::File "> $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?  Well, the good old -w flag will spot any
       instances where you call untie() and there are still valid
       references to the tied object.  If the second script above
       is 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;

SSEEEE AALLSSOO
       See the DB_File manpage or the Config manpage for some
       interesting tie() implementations.

BBUUGGSS
       Tied arrays are incomplete.  They are also distinctly
       lacking something for the $#ARRAY access (which is hard,
       as it's an lvalue), as well as the other obvious array
       functions, like push(), pop(), shift(), unshift(), and
       splice().

       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
       experimental module that does attempt to address this need
       partially is the MLDBM module.  Check your nearest CPAN
       site as described in the perlmodlib manpage for source
       code to MLDBM.

AAUUTTHHOORR
       Tom Christiansen

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

4/Mar/1999             perl 5.005, patch 03                     1