MirOS Manual: perltie(1)


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

NAME

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

SYNOPSIS

      tie VARIABLE, CLASSNAME, LIST

      $object = tied VARIABLE

      untie VARIABLE

DESCRIPTION

     Prior to release 5.0 of Perl, a programmer could use dbmo-
     pen() to connect an on-disk database in the standard Unix
     dbm(3x) format magically to a %HASH in their program.  How-
     ever, 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, access-
     ing 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 implement-
     ing 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.

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     Tying Scalars

     A class implementing a tied scalar should define the follow-
     ing 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 get-
     priority() 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.

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         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 any-
         way.

     FETCH this
         This method will be triggered every time the tied vari-
         able 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 vari-
         able 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 return-
         ing 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;
                 }

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                 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 deallo-
         cates 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 possi-
     ble.

     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

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     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 ele-
     ment 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 some-
         one 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

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         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 inter-
         nally 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 try-
         ing 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

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         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 associ-
         ated 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.

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         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:

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             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, mov-
         ing 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:

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             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 dbmo-
     pen()).  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:

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         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.

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         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";

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                 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 try-
         ing 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";

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         Another way to lay hands on a reference to the underly-
         ing 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 hap-
         pen.

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             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} }
             }

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     SCALAR this
         This is called when the hash is evaluated in scalar con-
         text. 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 other-
         wise.

         However, you should not blindly rely on perl always
         doing the right thing. Particularly, perl will mistak-
         enly 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 underly-
         ing 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:

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         # 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 tem-
     porarily disabled during the call, which means you can use
     "warn()" inside PRINT without starting a recursive loop.
     And just like "__WARN__" and "__DIE__" handlers, STDERR's
     PRINT method may be called to report parser errors, so the
     caveats mentioned under "%SIG" in perlvar apply.

     All of this is especially useful when perl is embedded in
     some other program, where output to STDOUT and STDERR may
     have to be redirected in some special way.  See nvi and the
     Apache module for examples.

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

         package Shout;

     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";
             }

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     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, @_);
             }

     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" }

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     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) exam-
     ple 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 = new IO::File "> $filename"
                              or die "Cannot open $filename: $!\n";

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

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         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:

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         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 "com-
     ment" 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 refer-
     ence, 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 (DES-
     TROY) 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

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     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() implementa-
     tions. 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 exit-
     ing 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 ineffi-
     cient 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.

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     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 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(), trun-
     cate(), flock(), fcntl(), stat() and -X can't currently be
     trapped.

AUTHOR

     Tom Christiansen

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

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

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

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

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