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


NAME
       Tie::Hash, Tie::StdHash, Tie::ExtraHash - base class defi-
       nitions 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 %new_hash, 'NewHash';
           tie %new_std_hash, 'NewStdHash';
           tie %new_extra_hash, '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 Tie::Hash
       package provides a "new" method, as well as methods
       "TIEHASH", "EXISTS" and "CLEAR". The Tie::StdHash and
       Tie::ExtraHash 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



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


       methods.  Tie::Hash grandfathers the "new" method: it is
       used if "TIEHASH" is not defined in the case a class for-
       gets 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, class-
           name". Associates a new hash instance with the speci-
           fied class. "LIST" would represent additional argu-
           ments (along the lines of AnyDBM_File 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 Tie::Hash 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.

           Tie::Hash does not implement this method (but
           Tie::StdHash and Tie::ExtraHash do).

Inheriting from Tie::StdHash
       The accessor methods assume that the actual storage for
       the data in the tied hash is in the hash referenced by
       "tied(%tiedhash)".  Thus overwritten "TIEHASH" method
       should return a hash reference, and the remaining methods
       should operate on the hash referenced by the first argu-
       ment:



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


         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]
         }

Inheriting from Tie::ExtraHash
       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 typical package
       inheriting from Tie::ExtraHash does not need to overwrite
       this method.

"SCALAR", "UNTIE" and "DESTROY"
       The methods "UNTIE" and "DESTROY" are not defined in
       Tie::Hash, Tie::StdHash, or Tie::ExtraHash.  Tied hashes
       do not require presense of these methods, but if defined,
       the methods will be called in proper time, see perltie.

       "SCALAR" is only defined in Tie::StdHash and Tie::Extra-
       Hash.

       If needed, these methods should be defined by the package
       inheriting from Tie::Hash, Tie::StdHash, or Tie::Extra-
       Hash. See "SCALAR" in pertie to find out what happens when



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


       "SCALAR" does not exist.

MORE INFORMATION
       The packages relating to various DBM-related implementa-
       tions (DB_File, NDBM_File, etc.) show examples of general
       tied hashes, as does the Config module. While these do not
       utilize Tie::Hash, they serve as good working examples.


















































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