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

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

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

NNAAMMEE #

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

SSYYNNOOPPSSIISS #

  tie VARIABLE, CLASSNAME, LIST

  $object = tied VARIABLE

  untie VARIABLE

DDEESSCCRRIIPPTTIIOONN #

 Prior to release 5.0 of Perl, a programmer could use ddbbmmooppeenn(()) to connect
 an on-disk database in the standard Unix ddbbmm(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 ttiiee(()) 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 BBEEGGIINN(()) and EENNDD(())
 functions.

 In the ttiiee(()) 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 TTIIEESSCCAALLAARR(()),
 TTIIEEAARRRRAAYY(()), TTIIEEHHAASSHH(()), or TTIIEEHHAANNDDLLEE(()).  (Typically these are arguments
 such as might be passed to the ddbbmmiinniitt(()) function of C.) The object
 returned by the "new" method is also returned by the ttiiee(()) 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 ttiieedd(()) function.

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

TTyyiinngg SSccaallaarrss A class implementing a tied scalar should define the following methods: TIESCALAR, FETCH, STORE, and possibly 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 <_j_h_i_@_i_k_i_._f_i>'s BSD::Resource class (not
 included) to access the PRIO_PROCESS, PRIO_MIN, and PRIO_MAX constants
 from your system, as well as the ggeettpprriioorriittyy(()) and sseettpprriioorriittyy(()) 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
     ddbbmmooppeenn(()) 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.

TTyyiinngg AArrrraayyss 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 TTiiee::::AArrrraayy 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
 TTiiee::::AArrrraayy 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 _r_e_a_l_l_y 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 _t_h_i_s. (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 _t_h_i_s to be _c_o_u_n_t. 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 _c_o_u_n_t 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.
         }

     NNOOTTEE:: 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 _k_e_y exists in the tied array _t_h_i_s.

     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 _k_e_y from the tied array _t_h_i_s.

     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 _t_h_i_s.  For example:

         sub CLEAR {
           my $self = shift;
           return $self->{ARRAY} = [];
         }

 PUSH this, LIST
     Append elements of _L_I_S_T 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.

     _o_f_f_s_e_t is optional and defaults to zero, negative values count back
     from the end of the array.

     _l_e_n_g_t_h is optional and defaults to rest of the array.

     _L_I_S_T may be empty.

     Returns a list of the original _l_e_n_g_t_h elements at _o_f_f_s_e_t.

     In our example, we'll use a little shortcut if there is a _L_I_S_T:

         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.

TTyyiinngg HHaasshheess Hashes were the first Perl data type to be tied (see ddbbmmooppeenn(())). 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 kkeeyyss(()) and eeaacchh(()) 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 _D_o_t_f_i_l_e_s_._p_m:

     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; wwhhoowwaassii(()) 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 $class = 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, $class;
         }

     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 cchhddiirr(()) 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 ccaatt(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 cclloobbbbeerr(()) method on the
     original object reference returned by ttiiee(()).

         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 ttiieedd(()) 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 ddeelleettee(()) 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
     ddeelleettee(()).  If you want to emulate the normal behavior of ddeelleettee(()),
     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 eexxiissttss(()) 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 kkeeyyss(()), vvaalluueess(()), or eeaacchh(()) 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.  vvaalluueess(()), and eeaacchh(()) in list context, will call FETCH
     for the returned keys.

 NEXTKEY this, lastkey
     This method gets triggered during a kkeeyyss(()), vvaalluueess(()), or eeaacchh(())
     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.  vvaalluueess(()), and eeaacchh(()) 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}->%*
         }

     If the object underlying your tied hash isn't a real hash and you
     don't have "each" available, then you should return "undef" or the
     empty list once you've reached the end of your list of keys. See
     "each's own documentation" for more details.

 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 nnoott 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 kkeeyyss(()) and vvaalluueess(()) may return huge lists
 when used on large objects, like DBM files.  You may prefer to use the
 eeaacchh(()) function to iterate over such.  Example:

     # print out history file offsets
     use NDBM_File;
     tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
     while (($key,$val) = each %HIST) {
         print $key, ' = ', unpack('L',$val), "\n";
     }
     untie(%HIST);

TTyyiinngg FFiilleeHHaannddlleess This is partially implemented now.

 A class implementing a tied filehandle should define the following
 methods: TIEHANDLE, at least one of PRINT, PRINTF, WRITE, READLINE, GETC,
 READ, and possibly CLOSE, 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>;

UUNNTTIIEE tthhiiss You can define for all tie types an UNTIE method that will be called at uunnttiiee(()). See “The “untie” Gotcha” below.

TThhee “"uunnttiiee"” GGoottcchhaa If you intend making use of the object returned from either ttiiee(()) or ttiieedd(()), and if the tie’s target class defines a destructor, there is a subtle gotcha you _m_u_s_t 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 v5.36;
     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 uunnttiiee(()) 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 uunnttiiee(()) 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
 uunnttiiee(()) 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 _b_e_f_o_r_e uunnttiiee(()) 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;
  }

SSEEEE AALLSSOO #

 See DB_File or Config for some interesting ttiiee(()) implementations.  A good
 starting point for many ttiiee(()) implementations is with one of the modules
 Tie::Scalar, Tie::Array, Tie::Hash, or Tie::Handle.

BBUUGGSS #

 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.  ssyyssooppeenn(()), ttrruunnccaattee(()), fflloocckk(()),
 ffccnnttll(()), ssttaatt(()) and -X can't currently be trapped.

AAUUTTHHOORR #

 Tom Christiansen

 TIEHANDLE by Sven Verdoolaege <_s_k_i_m_o_@_d_n_s_._u_f_s_i_a_._a_c_._b_e> and Doug MacEachern
 <_d_o_u_g_m_@_o_s_f_._o_r_g>

 UNTIE by Nick Ing-Simmons <_n_i_c_k_@_i_n_g_-_s_i_m_m_o_n_s_._n_e_t>

 SCALAR by Tassilo von Parseval <_t_a_s_s_i_l_o_._v_o_n_._p_a_r_s_e_v_a_l_@_r_w_t_h_-_a_a_c_h_e_n_._d_e>

 Tying Arrays by Casey West <_c_a_s_e_y_@_g_e_e_k_n_e_s_t_._c_o_m>

perl v5.36.3 2023-02-15 PERLTIE(1)