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RedHat 9 (Linux i386) - man page for perltie (redhat section 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 dbmopen() to connect an on-disk data-
       base 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 implementa-
       tion 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 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.  While this is how dbmopen() works, other classes may well
	   not wish to be so forgiving.  It checks the global variable $^W to see whether to emit
	   a bit of noise anyway.

       FETCH this
	   This method will be triggered every time the tied variable is accessed (read).  It
	   takes no arguments beyond its self reference, which is the object representing the
	   scalar we're dealing with.  Because in this case we're using just a SCALAR ref for the
	   tied scalar object, a simple $$self allows the method to get at the real value stored
	   there.  In our example below, that real value is the process ID to which we've tied
	   our variable.

	       sub FETCH {
		   my $self = shift;
		   confess "wrong type" unless ref $self;
		   croak "usage error" if @_;
		   my $nicety;
		   local($!) = 0;
		   $nicety = getpriority(PRIO_PROCESS, $$self);
		   if ($!) { croak "getpriority failed: $!" }
		   return $nicety;
	       }

	   This time we've decided to blow up (raise an exception) if the renice fails--there's
	   no place for us to return an error otherwise, and it's probably the right thing to do.

       STORE this, value
	   This method will be triggered every time the tied variable is set (assigned).  Beyond
	   its self reference, it also expects one (and only one) argument--the new value the
	   user is trying to assign.

	       sub STORE {
		   my $self = shift;
		   confess "wrong type" unless ref $self;
		   my $new_nicety = shift;
		   croak "usage error" if @_;

		   if ($new_nicety < PRIO_MIN) {
		       carp sprintf
			 "WARNING: priority %d less than minimum system priority %d",
			     $new_nicety, PRIO_MIN if $^W;
		       $new_nicety = PRIO_MIN;
		   }

		   if ($new_nicety > PRIO_MAX) {
		       carp sprintf
			 "WARNING: priority %d greater than maximum system priority %d",
			     $new_nicety, PRIO_MAX if $^W;
		       $new_nicety = PRIO_MAX;
		   }

		   unless (defined setpriority(PRIO_PROCESS, $$self, $new_nicety)) {
		       confess "setpriority failed: $!";
		   }
		   return $new_nicety;
	       }

       UNTIE this
	   This method will be triggered when the "untie" occurs. This can be useful if the class
	   needs to know when no further calls will be made. (Except DESTROY of course.) See "The
	   "untie" Gotcha" below for more details.

       DESTROY this
	   This method will be triggered when the tied variable needs to be destructed.  As with
	   other object classes, such a method is seldom necessary, because Perl deallocates its
	   moribund object's memory for you automatically--this isn't C++, you know.  We'll use a
	   DESTROY method here for debugging purposes only.

	       sub DESTROY {
		   my $self = shift;
		   confess "wrong type" unless ref $self;
		   carp "[ Nice::DESTROY pid $$self ]" if $Nice::DEBUG;
	       }

       That's about all there is to it.  Actually, it's more than all there is to it, because
       we've done a few nice things here for the sake of completeness, robustness, and general
       aesthetics.  Simpler TIESCALAR classes are certainly possible.

       Tying Arrays

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

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

       The methods POP, PUSH, SHIFT, UNSHIFT, SPLICE, DELETE, and EXISTS are required if the perl
       operator with the corresponding (but lowercase) name is to operate on the tied array. The
       Tie::Array class can be used as a base class to implement the first five of these in terms
       of the basic methods above.  The default implementations of DELETE and EXISTS in
       Tie::Array simply "croak".

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

       For this discussion, we'll implement an array whose elements are a fixed size at creation.
       If you try to create an element larger than the fixed size, you'll take an exception.  For
       example:

	   use FixedElem_Array;
	   tie @array, 'FixedElem_Array', 3;
	   $array[0] = 'cat';  # ok.
	   $array[1] = 'dogs'; # exception, length('dogs') > 3.

       The preamble code for the class is as follows:

	   package FixedElem_Array;
	   use Carp;
	   use strict;

       TIEARRAY classname, LIST
	   This is the constructor for the class.  That means it is expected to return a blessed
	   reference through which the new array (probably an anonymous ARRAY ref) will be
	   accessed.

	   In our example, just to show you that you don't really have to return an ARRAY refer-
	   ence, 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 pri-
	   vacy.

	       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 (writ-
	   ten).  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
	   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.  UNTIE is called when "untie" happens, and DESTROY is called when the tied vari-
       able 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 repre-
       senting a particular user's dot files.  You index into the hash with the name of the file
       (minus the dot) and you get back that dot file's contents.  For example:

	   use DotFiles;
	   tie %dot, 'DotFiles';
	   if ( $dot{profile} =~ /MANPATH/ ||
		$dot{login}   =~ /MANPATH/ ||
		$dot{cshrc}   =~ /MANPATH/    )
	   {
	       print "you seem to set your MANPATH\n";
	   }

       Or here's another sample of using our tied class:

	   tie %him, 'DotFiles', 'daemon';
	   foreach $f ( keys %him ) {
	       printf "daemon dot file %s is size %d\n",
		   $f, length $him{$f};
	   }

       In our tied hash DotFiles example, we use a regular hash for the object containing several
       important fields, of which only the "{LIST}" field will be what the user thinks of as the
       real hash.

       USER whose dot files this object represents

       HOME where those dot files live

       CLOBBER
	    whether we should try to change or remove those dot files

       LIST the hash of dot file names and content mappings

       Here's the start of Dotfiles.pm:

	   package DotFiles;
	   use Carp;
	   sub whowasi { (caller(1))[3] . '()' }
	   my $DEBUG = 0;
	   sub debug { $DEBUG = @_ ? shift : 1 }

       For our example, we want to be able to emit debugging info to help in tracing during
       development.  We keep also one convenience function around internally to help print out
       warnings; whowasi() returns the function name that calls it.

       Here are the methods for the DotFiles tied hash.

       TIEHASH classname, LIST
	   This is the constructor for the class.  That means it is expected to return a blessed
	   reference through which the new object (probably but not necessarily an anonymous
	   hash) will be accessed.

	   Here's the constructor:

	       sub TIEHASH {
		   my $self = shift;
		   my $user = shift || $>;
		   my $dotdir = shift || '';
		   croak "usage: @{[&whowasi]} [USER [DOTDIR]]" if @_;
		   $user = getpwuid($user) if $user =~ /^\d+$/;
		   my $dir = (getpwnam($user))[7]
			   || croak "@{[&whowasi]}: no user $user";
		   $dir .= "/$dotdir" if $dotdir;

		   my $node = {
		       USER    => $user,
		       HOME    => $dir,
		       LIST    => {},
		       CLOBBER => 0,
		   };

		   opendir(DIR, $dir)
			   || croak "@{[&whowasi]}: can't opendir $dir: $!";
		   foreach $dot ( grep /^\./ && -f "$dir/$_", readdir(DIR)) {
		       $dot =~ s/^\.//;
		       $node->{LIST}{$dot} = undef;
		   }
		   closedir DIR;
		   return bless $node, $self;
	       }

	   It's probably worth mentioning that if you're going to filetest the return values out
	   of a readdir, you'd better prepend the directory in question.  Otherwise, because we
	   didn't chdir() there, it would have been testing the wrong file.

       FETCH this, key
	   This method will be triggered every time an element in the tied hash is accessed
	   (read).  It takes one argument beyond its self reference: the key whose value we're
	   trying to fetch.

	   Here's the fetch for our DotFiles example.

	       sub FETCH {
		   carp &whowasi if $DEBUG;
		   my $self = shift;
		   my $dot = shift;
		   my $dir = $self->{HOME};
		   my $file = "$dir/.$dot";

		   unless (exists $self->{LIST}->{$dot} || -f $file) {
		       carp "@{[&whowasi]}: no $dot file" if $DEBUG;
		       return undef;
		   }

		   if (defined $self->{LIST}->{$dot}) {
		       return $self->{LIST}->{$dot};
		   } else {
		       return $self->{LIST}->{$dot} = `cat $dir/.$dot`;
		   }
	       }

	   It was easy to write by having it call the Unix cat(1) command, but it would probably
	   be more portable to open the file manually (and somewhat more efficient).  Of course,
	   because dot files are a Unixy concept, we're not that concerned.

       STORE this, key, value
	   This method will be triggered every time an element in the tied hash is set (written).
	   It takes two arguments beyond its self reference: the index at which we're trying to
	   store something, and the value we're trying to put there.

	   Here in our DotFiles example, we'll be careful not to let them try to overwrite the
	   file unless they've called the clobber() method on the original object reference
	   returned by tie().

	       sub STORE {
		   carp &whowasi if $DEBUG;
		   my $self = shift;
		   my $dot = shift;
		   my $value = shift;
		   my $file = $self->{HOME} . "/.$dot";
		   my $user = $self->{USER};

		   croak "@{[&whowasi]}: $file not clobberable"
		       unless $self->{CLOBBER};

		   open(F, "> $file") || croak "can't open $file: $!";
		   print F $value;
		   close(F);
	       }

	   If they wanted to clobber something, they might say:

	       $ob = tie %daemon_dots, 'daemon';
	       $ob->clobber(1);
	       $daemon_dots{signature} = "A true daemon\n";

	   Another way to lay hands on a reference to the underlying object is to use the tied()
	   function, so they might alternately have set clobber using:

	       tie %daemon_dots, 'daemon';
	       tied(%daemon_dots)->clobber(1);

	   The clobber method is simply:

	       sub clobber {
		   my $self = shift;
		   $self->{CLOBBER} = @_ ? shift : 1;
	       }

       DELETE this, key
	   This method is triggered when we remove an element from the hash, typically by using
	   the delete() function.  Again, we'll be careful to check whether they really want to
	   clobber files.

	       sub DELETE   {
		   carp &whowasi if $DEBUG;

		   my $self = shift;
		   my $dot = shift;
		   my $file = $self->{HOME} . "/.$dot";
		   croak "@{[&whowasi]}: won't remove file $file"
		       unless $self->{CLOBBER};
		   delete $self->{LIST}->{$dot};
		   my $success = unlink($file);
		   carp "@{[&whowasi]}: can't unlink $file: $!" unless $success;
		   $success;
	       }

	   The value returned by DELETE becomes the return value of the call to delete().  If you
	   want to emulate the normal behavior of delete(), you should return whatever FETCH
	   would have returned for this key.  In this example, we have chosen instead to return a
	   value which tells the caller whether the file was successfully deleted.

       CLEAR this
	   This method is triggered when the whole hash is to be cleared, usually by assigning
	   the empty list to it.

	   In our example, that would remove all the user's dot files!	It's such a dangerous
	   thing that they'll have to set CLOBBER to something higher than 1 to make it happen.

	       sub CLEAR    {
		   carp &whowasi if $DEBUG;
		   my $self = shift;
		   croak "@{[&whowasi]}: won't remove all dot files for $self->{USER}"
		       unless $self->{CLOBBER} > 1;
		   my $dot;
		   foreach $dot ( keys %{$self->{LIST}}) {
		       $self->DELETE($dot);
		   }
	       }

       EXISTS this, key
	   This method is triggered when the user uses the exists() function on a particular
	   hash.  In our example, we'll look at the "{LIST}" hash element for this:

	       sub EXISTS   {
		   carp &whowasi if $DEBUG;
		   my $self = shift;
		   my $dot = shift;
		   return exists $self->{LIST}->{$dot};
	       }

       FIRSTKEY this
	   This method will be triggered when the user is going to iterate through the hash, such
	   as via a keys() or each() call.

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

       NEXTKEY this, lastkey
	   This method gets triggered during a keys() or each() iteration.  It has a second argu-
	   ment 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} }
	       }

       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 cor-
       responding perl operators are used on the handle.

       It is especially useful when perl is embedded in some other program, where output to STD-
       OUT and STDERR may have to be redirected in some special way. See nvi and the Apache mod-
       ule for examples.

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

	   package Shout;

       TIEHANDLE classname, LIST
	   This is the constructor for the class.  That means it is expected to return a blessed
	   reference of some sort. The reference can be used to hold some internal information.

	       sub TIEHANDLE { print "<shout>\n"; my $i; bless \$i, shift }

       WRITE this, LIST
	   This method will be called when the handle is written to via the "syswrite" function.

	       sub WRITE {
		   $r = shift;
		   my($buf,$len,$offset) = @_;
		   print "WRITE called, \$buf=$buf, \$len=$len, \$offset=$offset";
	       }

       PRINT this, LIST
	   This method will be triggered every time the tied handle is printed to with the
	   "print()" function.	Beyond its self reference it also expects the list that was
	   passed to the print function.

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

       PRINTF this, LIST
	   This method will be triggered every time the tied handle is printed to with the
	   "printf()" function.  Beyond its self reference it also expects the format and list
	   that was passed to the printf function.

	       sub PRINTF {
		   shift;
		   my $fmt = shift;
		   print sprintf($fmt, @_)."\n";
	       }

       READ this, LIST
	   This method will be called when the handle is read from via the "read" or "sysread"
	   functions.

	       sub READ {
		   my $self = shift;
		   my $bufref = \$_[0];
		   my(undef,$len,$offset) = @_;
		   print "READ called, \$buf=$bufref, \$len=$len, \$offset=$offset";
		   # add to $$bufref, set $len to number of characters read
		   $len;
	       }

       READLINE this
	   This method will be called when the handle is read from via <HANDLE>.  The method
	   should return undef when there is no more data.

	       sub READLINE { $r = shift; "READLINE called $$r times\n"; }

       GETC this
	   This method will be called when the "getc" function is called.

	       sub GETC { print "Don't GETC, Get Perl"; return "a"; }

       CLOSE this
	   This method will be called when the handle is closed via the "close" function.

	       sub CLOSE { print "CLOSE called.\n" }

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

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

	   sub FETCH {
	       my $self = shift;
	       return $self->{Value};
	   }

	   sub STORE {
	       my $self = shift;
	       my $value = shift;
	       my $handle = $self->{FH};
	       print $handle "$value\n";
	       $self->{Value} = $value;
	   }

	   sub DESTROY {
	       my $self = shift;
	       my $handle = $self->{FH};
	       print $handle "The End\n";
	       close $handle;
	   }

	   1;

       Here is an example that makes use of this tie:

	   use strict;
	   use Remember;

	   my $fred;
	   tie $fred, 'Remember', 'myfile.txt';
	   $fred = 1;
	   $fred = 4;
	   $fred = 5;
	   untie $fred;
	   system "cat myfile.txt";

       This is the output when it is executed:

	   The Start
	   1
	   4
	   5
	   The End

       So far so good.	Those of you who have been paying attention will have spotted that the
       tied object hasn't been used so far.  So lets add an extra method to the Remember class to
       allow comments to be included in the file -- say, something like this:

	   sub comment {
	       my $self = shift;
	       my $text = shift;
	       my $handle = $self->{FH};
	       print $handle $text, "\n";
	   }

       And here is the previous example modified to use the "comment" method (which requires the
       tied object):

	   use strict;
	   use Remember;

	   my ($fred, $x);
	   $x = tie $fred, 'Remember', 'myfile.txt';
	   $fred = 1;
	   $fred = 4;
	   comment $x "changing...";
	   $fred = 5;
	   untie $fred;
	   system "cat myfile.txt";

       When this code is executed there is no output.  Here's why:

       When a variable is tied, it is associated with the object which is the return value of the
       TIESCALAR, TIEARRAY, or TIEHASH function.  This object normally has only one reference,
       namely, the implicit reference from the tied variable.  When untie() is called, that ref-
       erence 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 intro-
       duction 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
       You cannot easily tie a multilevel data structure (such as a hash of hashes) to a dbm
       file.  The first problem is that all but GDBM and Berkeley DB have size limitations, but
       beyond that, you also have problems with how references are to be represented on disk.
       One experimental module that does attempt to address this need partially is the MLDBM mod-
       ule.  Check your nearest CPAN site as described in perlmodlib for source code to MLDBM.

       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>

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

perl v5.8.0				    2003-02-18				       PERLTIE(1)


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