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, CLEAR 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(), values(), or each() call.

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

        FIRSTKEY is always called in scalar context and it should just
        return the first key. values(), and each() in list context, will
        call FETCH for the returned keys.

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

        NEXTKEY is always called in scalar context and it should just return
        the next key. values(), and each() in list context, will call FETCH
        for the returned keys.

        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} }
            }

        NOTE: In perl 5.25 the behavior of scalar %hash on an untied hash
        changed to return the count of keys. Prior to this it returned a
        string containing information about the bucket setup of the hash.
        See "bucket_ratio" in Hash::Util for a backwards compatibility path.

    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 normal return 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). [ This paragraph needs review in light of
    changes in 5.25 ]

    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>