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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 v5.16.3				    2013-03-04				   Tie::Hash(3pm)
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