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   1  package Tie::Hash;
   2  
   3  our $VERSION = '1.02';
   4  
   5  =head1 NAME
   6  
   7  Tie::Hash, Tie::StdHash, Tie::ExtraHash - base class definitions for tied hashes
   8  
   9  =head1 SYNOPSIS
  10  
  11      package NewHash;
  12      require Tie::Hash;
  13  
  14      @ISA = qw(Tie::Hash);
  15  
  16      sub DELETE { ... }        # Provides needed method
  17      sub CLEAR { ... }        # Overrides inherited method
  18  
  19  
  20      package NewStdHash;
  21      require Tie::Hash;
  22  
  23      @ISA = qw(Tie::StdHash);
  24  
  25      # All methods provided by default, define only those needing overrides
  26      # Accessors access the storage in %{$_[0]};
  27      # TIEHASH should return a reference to the actual storage
  28      sub DELETE { ... }
  29  
  30      package NewExtraHash;
  31      require Tie::Hash;
  32  
  33      @ISA = qw(Tie::ExtraHash);
  34  
  35      # All methods provided by default, define only those needing overrides
  36      # Accessors access the storage in %{$_[0][0]};
  37      # TIEHASH should return an array reference with the first element being
  38      # the reference to the actual storage 
  39      sub DELETE { 
  40        $_[0][1]->('del', $_[0][0], $_[1]); # Call the report writer
  41        delete $_[0][0]->{$_[1]};          #  $_[0]->SUPER::DELETE($_[1])
  42      }
  43  
  44  
  45      package main;
  46  
  47      tie %new_hash, 'NewHash';
  48      tie %new_std_hash, 'NewStdHash';
  49      tie %new_extra_hash, 'NewExtraHash',
  50      sub {warn "Doing \U$_[1]\E of $_[2].\n"};
  51  
  52  =head1 DESCRIPTION
  53  
  54  This module provides some skeletal methods for hash-tying classes. See
  55  L<perltie> for a list of the functions required in order to tie a hash
  56  to a package. The basic B<Tie::Hash> package provides a C<new> method, as well
  57  as methods C<TIEHASH>, C<EXISTS> and C<CLEAR>. The B<Tie::StdHash> and
  58  B<Tie::ExtraHash> packages
  59  provide most methods for hashes described in L<perltie> (the exceptions
  60  are C<UNTIE> and C<DESTROY>).  They cause tied hashes to behave exactly like standard hashes,
  61  and allow for selective overwriting of methods.  B<Tie::Hash> grandfathers the
  62  C<new> method: it is used if C<TIEHASH> is not defined
  63  in the case a class forgets to include a C<TIEHASH> method.
  64  
  65  For developers wishing to write their own tied hashes, the required methods
  66  are briefly defined below. See the L<perltie> section for more detailed
  67  descriptive, as well as example code:
  68  
  69  =over 4
  70  
  71  =item TIEHASH classname, LIST
  72  
  73  The method invoked by the command C<tie %hash, classname>. Associates a new
  74  hash instance with the specified class. C<LIST> would represent additional
  75  arguments (along the lines of L<AnyDBM_File> and compatriots) needed to
  76  complete the association.
  77  
  78  =item STORE this, key, value
  79  
  80  Store datum I<value> into I<key> for the tied hash I<this>.
  81  
  82  =item FETCH this, key
  83  
  84  Retrieve the datum in I<key> for the tied hash I<this>.
  85  
  86  =item FIRSTKEY this
  87  
  88  Return the first key in the hash.
  89  
  90  =item NEXTKEY this, lastkey
  91  
  92  Return the next key in the hash.
  93  
  94  =item EXISTS this, key
  95  
  96  Verify that I<key> exists with the tied hash I<this>.
  97  
  98  The B<Tie::Hash> implementation is a stub that simply croaks.
  99  
 100  =item DELETE this, key
 101  
 102  Delete the key I<key> from the tied hash I<this>.
 103  
 104  =item CLEAR this
 105  
 106  Clear all values from the tied hash I<this>.
 107  
 108  =item SCALAR this
 109  
 110  Returns what evaluating the hash in scalar context yields.
 111  
 112  B<Tie::Hash> does not implement this method (but B<Tie::StdHash>
 113  and B<Tie::ExtraHash> do).
 114  
 115  =back
 116  
 117  =head1 Inheriting from B<Tie::StdHash>
 118  
 119  The accessor methods assume that the actual storage for the data in the tied
 120  hash is in the hash referenced by C<tied(%tiedhash)>.  Thus overwritten
 121  C<TIEHASH> method should return a hash reference, and the remaining methods
 122  should operate on the hash referenced by the first argument:
 123  
 124    package ReportHash;
 125    our @ISA = 'Tie::StdHash';
 126  
 127    sub TIEHASH  {
 128      my $storage = bless {}, shift;
 129      warn "New ReportHash created, stored in $storage.\n";
 130      $storage
 131    }
 132    sub STORE    {
 133      warn "Storing data with key $_[1] at $_[0].\n";
 134      $_[0]{$_[1]} = $_[2]
 135    }
 136  
 137  
 138  =head1 Inheriting from B<Tie::ExtraHash>
 139  
 140  The accessor methods assume that the actual storage for the data in the tied
 141  hash is in the hash referenced by C<(tied(%tiedhash))-E<gt>[0]>.  Thus overwritten
 142  C<TIEHASH> method should return an array reference with the first
 143  element being a hash reference, and the remaining methods should operate on the
 144  hash C<< %{ $_[0]->[0] } >>:
 145  
 146    package ReportHash;
 147    our @ISA = 'Tie::ExtraHash';
 148  
 149    sub TIEHASH  {
 150      my $class = shift;
 151      my $storage = bless [{}, @_], $class;
 152      warn "New ReportHash created, stored in $storage.\n";
 153      $storage;
 154    }
 155    sub STORE    {
 156      warn "Storing data with key $_[1] at $_[0].\n";
 157      $_[0][0]{$_[1]} = $_[2]
 158    }
 159  
 160  The default C<TIEHASH> method stores "extra" arguments to tie() starting
 161  from offset 1 in the array referenced by C<tied(%tiedhash)>; this is the
 162  same storage algorithm as in TIEHASH subroutine above.  Hence, a typical
 163  package inheriting from B<Tie::ExtraHash> does not need to overwrite this
 164  method.
 165  
 166  =head1 C<SCALAR>, C<UNTIE> and C<DESTROY>
 167  
 168  The methods C<UNTIE> and C<DESTROY> are not defined in B<Tie::Hash>,
 169  B<Tie::StdHash>, or B<Tie::ExtraHash>.  Tied hashes do not require
 170  presence of these methods, but if defined, the methods will be called in
 171  proper time, see L<perltie>.
 172  
 173  C<SCALAR> is only defined in B<Tie::StdHash> and B<Tie::ExtraHash>.
 174  
 175  If needed, these methods should be defined by the package inheriting from
 176  B<Tie::Hash>, B<Tie::StdHash>, or B<Tie::ExtraHash>. See L<pertie/"SCALAR">
 177  to find out what happens when C<SCALAR> does not exist.
 178  
 179  =head1 MORE INFORMATION
 180  
 181  The packages relating to various DBM-related implementations (F<DB_File>,
 182  F<NDBM_File>, etc.) show examples of general tied hashes, as does the
 183  L<Config> module. While these do not utilize B<Tie::Hash>, they serve as
 184  good working examples.
 185  
 186  =cut
 187  
 188  use Carp;
 189  use warnings::register;
 190  
 191  sub new {
 192      my $pkg = shift;
 193      $pkg->TIEHASH(@_);
 194  }
 195  
 196  # Grandfather "new"
 197  
 198  sub TIEHASH {
 199      my $pkg = shift;
 200      if (defined &{"$pkg}::new"}) {
 201      warnings::warnif("WARNING: calling $pkg}->new since $pkg}->TIEHASH is missing");
 202      $pkg->new(@_);
 203      }
 204      else {
 205      croak "$pkg doesn't define a TIEHASH method";
 206      }
 207  }
 208  
 209  sub EXISTS {
 210      my $pkg = ref $_[0];
 211      croak "$pkg doesn't define an EXISTS method";
 212  }
 213  
 214  sub CLEAR {
 215      my $self = shift;
 216      my $key = $self->FIRSTKEY(@_);
 217      my @keys;
 218  
 219      while (defined $key) {
 220      push @keys, $key;
 221      $key = $self->NEXTKEY(@_, $key);
 222      }
 223      foreach $key (@keys) {
 224      $self->DELETE(@_, $key);
 225      }
 226  }
 227  
 228  # The Tie::StdHash package implements standard perl hash behaviour.
 229  # It exists to act as a base class for classes which only wish to
 230  # alter some parts of their behaviour.
 231  
 232  package Tie::StdHash;
 233  # @ISA = qw(Tie::Hash);        # would inherit new() only
 234  
 235  sub TIEHASH  { bless {}, $_[0] }
 236  sub STORE    { $_[0]->{$_[1]} = $_[2] }
 237  sub FETCH    { $_[0]->{$_[1]} }
 238  sub FIRSTKEY { my $a = scalar keys %{$_[0]}; each %{$_[0]} }
 239  sub NEXTKEY  { each %{$_[0]} }
 240  sub EXISTS   { exists $_[0]->{$_[1]} }
 241  sub DELETE   { delete $_[0]->{$_[1]} }
 242  sub CLEAR    { %{$_[0]} = () }
 243  sub SCALAR   { scalar %{$_[0]} }
 244  
 245  package Tie::ExtraHash;
 246  
 247  sub TIEHASH  { my $p = shift; bless [{}, @_], $p }
 248  sub STORE    { $_[0][0]{$_[1]} = $_[2] }
 249  sub FETCH    { $_[0][0]{$_[1]} }
 250  sub FIRSTKEY { my $a = scalar keys %{$_[0][0]}; each %{$_[0][0]} }
 251  sub NEXTKEY  { each %{$_[0][0]} }
 252  sub EXISTS   { exists $_[0][0]->{$_[1]} }
 253  sub DELETE   { delete $_[0][0]->{$_[1]} }
 254  sub CLEAR    { %{$_[0][0]} = () }
 255  sub SCALAR   { scalar %{$_[0][0]} }
 256  
 257  1;