Tie::Hash(3pm) Perl Programmers Reference Guide Tie::Hash(3pm)

NAME
       Tie::Hash, Tie::StdHash, Tie::ExtraHash - base class definitions for
       tied hashes

SYNOPSIS
           package NewHash;
           require Tie::Hash;

           @ISA = (Tie::Hash);

           sub DELETE { ... }          # Provides needed method
           sub CLEAR { ... }           # Overrides inherited method

           package NewStdHash;
           require Tie::Hash;

           @ISA = (Tie::StdHash);

           # All methods provided by default, define only those needing overrides
           # Accessors access the storage in %{$[0]};
           # TIEHASH should return a reference to the actual storage
           sub DELETE { ... }

           package NewExtraHash;
           require Tie::Hash;

           @ISA = (Tie::ExtraHash);

           # All methods provided by default, define only those needing overrides
           # Accessors access the storage in %{$[0][0]};
           # TIEHASH should return an array reference with the first element being
           # the reference to the actual storage
           sub DELETE {
             $[0][1]->('del', $[0][0], $[1]); # Call the report writer
             delete $[0][0]->{$[1]};           #  $[0]->SUPER::DELETE($[1])
           }

           package main;

           tie %newhash, 'NewHash';
           tie %newstdhash, 'NewStdHash';
           tie %newextrahash, 'NewExtraHash',
               sub {warn "Doing \U$[1]\E of $[2].\n"};

DESCRIPTION
       This module provides some skeletal methods for hash-tying classes. See
       perltie for a list of the functions required in order to tie a hash to
       a package. The basic TTiiee::::HHaasshh package provides a "new" method, as well
       as methods "TIEHASH", "EXISTS" and "CLEAR". The TTiiee::::SSttddHHaasshh and
       TTiiee::::EExxttrraaHHaasshh packages provide most methods for hashes described in
       perltie (the exceptions are "UNTIE" and "DESTROY").  They cause tied
       hashes to behave exactly like standard hashes, and allow for selective
       overwriting of methods.  TTiiee::::HHaasshh grandfathers the "new" method: it is
       used if "TIEHASH" is not defined in the case a class forgets to include
       a "TIEHASH" method.

       For developers wishing to write their own tied hashes, the required
       methods are briefly defined below. See the perltie section for more
       detailed descriptive, as well as example code:

       TIEHASH classname, LIST
           The method invoked by the command "tie %hash, classname". Asso-
           ciates a new hash instance with the specified class. "LIST" would
           represent additional arguments (along the lines of AnyDBMFile and
           compatriots) needed to complete the association.

       STORE this, key, value
           Store datum value into key for the tied hash this.

       FETCH this, key
           Retrieve the datum in key for the tied hash this.

       FIRSTKEY this
           Return the first key in the hash.

       NEXTKEY this, lastkey
           Return the next key in the hash.

       EXISTS this, key
           Verify that key exists with the tied hash this.

           The TTiiee::::HHaasshh implementation is a stub that simply croaks.

       DELETE this, key
           Delete the key key from the tied hash this.

       CLEAR this
           Clear all values from the tied hash this.

       SCALAR this
           Returns what evaluating the hash in scalar context yields.

           TTiiee::::HHaasshh does not implement this method (but TTiiee::::SSttddHHaasshh and
           TTiiee::::EExxttrraaHHaasshh do).

IInnhheerriittiinngg ffrroomm TTiiee::::SSttddHHaasshh
       The accessor methods assume that the actual storage for the data in the
       tied hash is in the hash referenced by "tied(%tiedhash)".  Thus over-
       written "TIEHASH" method should return a hash reference, and the
       remaining methods should operate on the hash referenced by the first
       argument:

         package ReportHash;
         our @ISA = 'Tie::StdHash';

         sub TIEHASH  {
           my $storage = bless {}, shift;
           warn "New ReportHash created, stored in $storage.\n";
           $storage
         }
         sub STORE    {
           warn "Storing data with key $[1] at $[0].\n";
           $[0]{$[1]} = $[2]
         }

IInnhheerriittiinngg ffrroomm TTiiee::::EExxttrraaHHaasshh
       The accessor methods assume that the actual storage for the data in the
       tied hash is in the hash referenced by "(tied(%tiedhash))->[0]".  Thus
       overwritten "TIEHASH" method should return an array reference with the
       first element being a hash reference, and the remaining methods should
       operate on the hash "%{ $[0]->[0] }":

         package ReportHash;
         our @ISA = 'Tie::ExtraHash';

         sub TIEHASH  {
           my $class = shift;
           my $storage = bless [{}, @], $class;
           warn "New ReportHash created, stored in $storage.\n";
           $storage;
         }
         sub STORE    {
           warn "Storing data with key $[1] at $[0].\n";
           $[0][0]{$[1]} = $[2]
         }

       The default "TIEHASH" method stores "extra" arguments to tie() starting
       from offset 1 in the array referenced by "tied(%tiedhash)"; this is the
       same storage algorithm as in TIEHASH subroutine above.  Hence, a typi-
       cal package inheriting from TTiiee::::EExxttrraaHHaasshh does not need to overwrite
       this method.

""SSCCAALLAARR"", "UNTIE" and "DESTROY"
       The methods "UNTIE" and "DESTROY" are not defined in TTiiee::::HHaasshh,
       TTiiee::::SSttddHHaasshh, or TTiiee::::EExxttrraaHHaasshh.  Tied hashes do not require presence
       of these methods, but if defined, the methods will be called in proper
       time, see perltie.

       "SCALAR" is only defined in TTiiee::::SSttddHHaasshh and TTiiee::::EExxttrraaHHaasshh.

       If needed, these methods should be defined by the package inheriting
       from TTiiee::::HHaasshh, TTiiee::::SSttddHHaasshh, or TTiiee::::EExxttrraaHHaasshh. See "SCALAR" in pertie
       to find out what happens when "SCALAR" does not exist.

MMOORREE IINNFFOORRMMAATTIIOONN
       The packages relating to various DBM-related implementations (DBFile,
       NDBMFile, etc.) show examples of general tied hashes, as does the Con-
       fig module. While these do not utilize TTiiee::::HHaasshh, they serve as good
       working examples.



perl v5.8.8                       2001-09-21                    Tie::Hash(3pm)