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

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

NNAAMMEE #

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

SSYYNNOOPPSSIISS #

     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"};

DDEESSCCRRIIPPTTIIOONN #

 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 has legacy support for 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 _v_a_l_u_e into _k_e_y for the tied hash _t_h_i_s.

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

 FIRSTKEY this
     Return the first key in the hash.

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

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

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

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

 CLEAR this
     Clear all values from the tied hash _t_h_i_s.

 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 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]
   }

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 ttiiee(()) 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 TTiiee::::EExxttrraaHHaasshh does not need to overwrite this
 method.

“"SSCCAALLAARR"”,, “"UUNNTTIIEE"” aanndd “"DDEESSTTRROOYY"” 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 perltie to
 find out what happens when "SCALAR" does not exist.

MMOORREE IINNFFOORRMMAATTIIOONN #

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

perl v5.36.3 2023-02-15 Tie::Hash(3p)