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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 = qw(Tie::Hash);

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


           package NewStdHash;
           require Tie::Hash;

           @ISA = qw(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 = qw(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 methods.  Tie::Hash 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".
           Associates a new hash instance with the specified class. "LIST"
           would represent additional arguments (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
       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]
         }


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 presence
       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::ExtraHash.

       If needed, these methods should be defined by the package inheriting
       from Tie::Hash, Tie::StdHash, or Tie::ExtraHash. See "SCALAR" in
       perltie to find out what happens when "SCALAR" does not exist.


MORE INFORMATION

       The packages relating to various DBM-related implementations (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.



perl v5.24.0                      2016-02-05                    Tie::Hash(3pm)

perl 5.24 - Generated Sat Nov 26 07:40:46 CST 2016
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