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

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

SYNOPSIS
	tie VARIABLE, CLASSNAME, LIST

	$object	= tied VARIABLE

	untie VARIABLE

DESCRIPTION
       Prior to	release	5.0 of Perl, a programmer could	use dbmopen() to
       connect an on-disk database in the standard Unix	dbm(3x)	format
       magically to a %HASH in their program.  However,	their Perl was either
       built with one particular dbm library or	another, but not both, and you
       couldn't	extend this mechanism to other packages	or types of variables.

       Now you can.

       The tie() function binds	a variable to a	class (package)	that will
       provide the implementation for access methods for that variable.	 Once
       this magic has been performed, accessing	a tied variable	automatically
       triggers	method calls in	the proper class.  The complexity of the class
       is hidden behind	magic methods calls.  The method names are in ALL
       CAPS, which is a	convention that	Perl uses to indicate that they're
       called implicitly rather	than explicitly--just like the BEGIN() and
       END() functions.

       In the tie() call, "VARIABLE" is	the name of the	variable to be
       enchanted.  "CLASSNAME" is the name of a	class implementing objects of
       the correct type.  Any additional arguments in the "LIST" are passed to
       the appropriate constructor method for that class--meaning TIESCALAR(),
       TIEARRAY(), TIEHASH(), or TIEHANDLE().  (Typically these	are arguments
       such as might be	passed to the dbminit()	function of C.)	The object
       returned	by the "new" method is also returned by	the tie() function,
       which would be useful if	you wanted to access other methods in
       "CLASSNAME". (You don't actually	have to	return a reference to a	right
       "type" (e.g., HASH or "CLASSNAME") so long as it's a properly blessed
       object.)	 You can also retrieve a reference to the underlying object
       using the tied()	function.

       Unlike dbmopen(), the tie() function will not "use" or "require"	a
       module for you--you need	to do that explicitly yourself.

   Tying Scalars
       A class implementing a tied scalar should define	the following methods:
       TIESCALAR, FETCH, STORE,	and possibly 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->STORESIZE(	$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 there	is no reason to	implement this method, so we
	   leave it as a no-op.	This method is only relevant to	tied array
	   implementations where there is the possibility of having the
	   allocated size of the array be larger than is visible to a perl
	   programmer inspecting the size of the array.	Many tied array
	   implementations will	have no	reason to implement it.

	       sub EXTEND {
		 my $self  = shift;
		 my $count = shift;
		 # nothing to see here,	move along.
	       }

	   NOTE: It is generally an error to make this equivalent to
	   STORESIZE.  Perl may	from time to time call EXTEND without wanting
	   to actually change the array	size directly. Any tied	array should
	   function correctly if this method is	a no-op, even if perhaps they
	   might not be	as efficient as	they would if this method was
	   implemented.

       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, and	in 5.28	onwards, by "keys" in boolean
       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,	and in
	   5.28	onwards, by "keys" in boolean context. In order	to mimic the
	   behaviour of	untied hashes, this method must	return a value which
	   when	used as	boolean, indicates whether 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>

perl v5.32.0			  2020-06-14			    PERLTIE(1)

NAME | SYNOPSIS | DESCRIPTION | SEE ALSO | BUGS | AUTHOR

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