lib/perl5/5.6.1/Tie/Hash.pm

   1 package Tie::Hash;
   2
   3 =head1 NAME
   4
   5 Tie::Hash, Tie::StdHash - base class definitions for tied hashes
   6
   7 =head1 SYNOPSIS
   8
   9     package NewHash;
  10     require Tie::Hash;
  11
  12     @ISA = (Tie::Hash);
  13
  14     sub DELETE { ... }          # Provides needed method
  15     sub CLEAR { ... }           # Overrides inherited method
  16
  17
  18     package NewStdHash;
  19     require Tie::Hash;
  20
  21     @ISA = (Tie::StdHash);
  22
  23     # All methods provided by default, define only those needing overrides
  24     sub DELETE { ... }
  25
  26
  27     package main;
  28
  29     tie %new_hash, 'NewHash';
  30     tie %new_std_hash, 'NewStdHash';
  31
  32 =head1 DESCRIPTION
  33
  34 This module provides some skeletal methods for hash-tying classes. See
  35 L<perltie> for a list of the functions required in order to tie a hash
  36 to a package. The basic B<Tie::Hash> package provides a C<new> method, as well
  37 as methods C<TIEHASH>, C<EXISTS> and C<CLEAR>. The B<Tie::StdHash> package
  38 provides most methods required for hashes in L<perltie>. It inherits from
  39 B<Tie::Hash>, and causes tied hashes to behave exactly like standard hashes,
  40 allowing for selective overloading of methods. The C<new> method is provided
  41 as grandfathering in the case a class forgets to include a C<TIEHASH> method.
  42
  43 For developers wishing to write their own tied hashes, the required methods
  44 are briefly defined below. See the L<perltie> section for more detailed
  45 descriptive, as well as example code:
  46
  47 =over
  48
  49 =item TIEHASH classname, LIST
  50
  51 The method invoked by the command C<tie %hash, classname>. Associates a new
  52 hash instance with the specified class. C<LIST> would represent additional
  53 arguments (along the lines of L<AnyDBM_File> and compatriots) needed to
  54 complete the association.
  55
  56 =item STORE this, key, value
  57
  58 Store datum I<value> into I<key> for the tied hash I<this>.
  59
  60 =item FETCH this, key
  61
  62 Retrieve the datum in I<key> for the tied hash I<this>.
  63
  64 =item FIRSTKEY this
  65
  66 Return the (key, value) pair for the first key in the hash.
  67
  68 =item NEXTKEY this, lastkey
  69
  70 Return the next key for the hash.
  71
  72 =item EXISTS this, key
  73
  74 Verify that I<key> exists with the tied hash I<this>.
  75
  76 The B<Tie::Hash> implementation is a stub that simply croaks.
  77
  78 =item DELETE this, key
  79
  80 Delete the key I<key> from the tied hash I<this>.
  81
  82 =item CLEAR this
  83
  84 Clear all values from the tied hash I<this>.
  85
  86 =back
  87
  88 =head1 CAVEATS
  89
  90 The L<perltie> documentation includes a method called C<DESTROY> as
  91 a necessary method for tied hashes. Neither B<Tie::Hash> nor B<Tie::StdHash>
  92 define a default for this method. This is a standard for class packages,
  93 but may be omitted in favor of a simple default.
  94
  95 =head1 MORE INFORMATION
  96
  97 The packages relating to various DBM-related implementations (F<DB_File>,
  98 F<NDBM_File>, etc.) show examples of general tied hashes, as does the
  99 L<Config> module. While these do not utilize B<Tie::Hash>, they serve as
 100 good working examples.
 101
 102 =cut
 103
 104 use Carp;
 105 use warnings::register;
 106
 107 sub new {
 108     my $pkg = shift;
 109     $pkg->TIEHASH(@_);
 110 }
 111
 112 # Grandfather "new"
 113
 114 sub TIEHASH {
 115     my $pkg = shift;
 116     if (defined &{"${pkg}::new"}) {
 117         warnings::warnif("WARNING: calling ${pkg}->new since ${pkg}->TIEHASH is missing");
 118         $pkg->new(@_);
 119     }
 120     else {
 121         croak "$pkg doesn't define a TIEHASH method";
 122     }
 123 }
 124
 125 sub EXISTS {
 126     my $pkg = ref $_[0];
 127     croak "$pkg doesn't define an EXISTS method";
 128 }
 129
 130 sub CLEAR {
 131     my $self = shift;
 132     my $key = $self->FIRSTKEY(@_);
 133     my @keys;
 134
 135     while (defined $key) {
 136         push @keys, $key;
 137         $key = $self->NEXTKEY(@_, $key);
 138     }
 139     foreach $key (@keys) {
 140         $self->DELETE(@_, $key);
 141     }
 142 }
 143
 144 # The Tie::StdHash package implements standard perl hash behaviour.
 145 # It exists to act as a base class for classes which only wish to
 146 # alter some parts of their behaviour.
 147
 148 package Tie::StdHash;
 149 @ISA = qw(Tie::Hash);
 150
 151 sub TIEHASH  { bless {}, $_[0] }
 152 sub STORE    { $_[0]->{$_[1]} = $_[2] }
 153 sub FETCH    { $_[0]->{$_[1]} }
 154 sub FIRSTKEY { my $a = scalar keys %{$_[0]}; each %{$_[0]} }
 155 sub NEXTKEY  { each %{$_[0]} }
 156 sub EXISTS   { exists $_[0]->{$_[1]} }
 157 sub DELETE   { delete $_[0]->{$_[1]} }
 158 sub CLEAR    { %{$_[0]} = () }
 159
 160 1;