7 Tie::Hash, Tie::StdHash, Tie::ExtraHash - base class definitions for tied hashes
16 sub DELETE { ... } # Provides needed method
17 sub CLEAR { ... } # Overrides inherited method
23 @ISA = (Tie::StdHash);
25 # All methods provided by default, define only those needing overrides
26 # Accessors access the storage in %{$_[0]};
27 # TIEHASH should return a reference to the actual storage
33 @ISA = (Tie::ExtraHash);
35 # All methods provided by default, define only those needing overrides
36 # Accessors access the storage in %{$_[0][0]};
37 # TIEHASH should return an array reference with the first element being
38 # the reference to the actual storage
40 $_[0][1]->('del', $_[0][0], $_[1]); # Call the report writer
41 delete $_[0][0]->{$_[1]}; # $_[0]->SUPER::DELETE($_[1]) }
46 tie %new_hash, 'NewHash';
47 tie %new_std_hash, 'NewStdHash';
48 tie %new_extra_hash, 'NewExtraHash',
49 sub {warn "Doing \U$_[1]\E of $_[2].\n"};
53 This module provides some skeletal methods for hash-tying classes. See
54 L<perltie> for a list of the functions required in order to tie a hash
55 to a package. The basic B<Tie::Hash> package provides a C<new> method, as well
56 as methods C<TIEHASH>, C<EXISTS> and C<CLEAR>. The B<Tie::StdHash> and
57 B<Tie::ExtraHash> packages
58 provide most methods for hashes described in L<perltie> (the exceptions
59 are C<UNTIE> and C<DESTROY>). They cause tied hashes to behave exactly like standard hashes,
60 and allow for selective overwriting of methods. B<Tie::Hash> grandfathers the
61 C<new> method: it is used if C<TIEHASH> is not defined
62 in the case a class forgets to include a C<TIEHASH> method.
64 For developers wishing to write their own tied hashes, the required methods
65 are briefly defined below. See the L<perltie> section for more detailed
66 descriptive, as well as example code:
70 =item TIEHASH classname, LIST
72 The method invoked by the command C<tie %hash, classname>. Associates a new
73 hash instance with the specified class. C<LIST> would represent additional
74 arguments (along the lines of L<AnyDBM_File> and compatriots) needed to
75 complete the association.
77 =item STORE this, key, value
79 Store datum I<value> into I<key> for the tied hash I<this>.
83 Retrieve the datum in I<key> for the tied hash I<this>.
87 Return the first key in the hash.
89 =item NEXTKEY this, lastkey
91 Return the next key in the hash.
93 =item EXISTS this, key
95 Verify that I<key> exists with the tied hash I<this>.
97 The B<Tie::Hash> implementation is a stub that simply croaks.
99 =item DELETE this, key
101 Delete the key I<key> from the tied hash I<this>.
105 Clear all values from the tied hash I<this>.
109 =head1 Inheriting from B<Tie::StdHash>
111 The accessor methods assume that the actual storage for the data in the tied
112 hash is in the hash referenced by C<tied(%tiedhash)>. Thus overwritten
113 C<TIEHASH> method should return a hash reference, and the remaining methods
114 should operate on the hash referenced by the first argument:
117 our @ISA = 'Tie::StdHash';
120 my $storage = bless {}, shift;
121 warn "New ReportHash created, stored in $storage.\n";
125 warn "Storing data with key $_[1] at $_[0].\n";
130 =head1 Inheriting from B<Tie::ExtraHash>
132 The accessor methods assume that the actual storage for the data in the tied
133 hash is in the hash referenced by C<(tied(%tiedhash))[0]>. Thus overwritten
134 C<TIEHASH> method should return an array reference with the first
135 element being a hash reference, and the remaining methods should operate on the
136 hash C<< %{ $_[0]->[0] } >>:
139 our @ISA = 'Tie::ExtraHash';
143 my $storage = bless [{}, @_], $class;
144 warn "New ReportHash created, stored in $storage.\n";
148 warn "Storing data with key $_[1] at $_[0].\n";
149 $_[0][0]{$_[1]} = $_[2]
152 The default C<TIEHASH> method stores "extra" arguments to tie() starting
153 from offset 1 in the array referenced by C<tied(%tiedhash)>; this is the
154 same storage algorithm as in TIEHASH subroutine above. Hence, a typical
155 package inheriting from B<Tie::ExtraHash> does not need to overwrite this
158 =head1 C<UNTIE> and C<DESTROY>
160 The methods C<UNTIE> and C<DESTROY> are not defined in B<Tie::Hash>,
161 B<Tie::StdHash>, or B<Tie::ExtraHash>. Tied hashes do not require
162 presense of these methods, but if defined, the methods will be called in
163 proper time, see L<perltie>.
165 If needed, these methods should be defined by the package inheriting from
166 B<Tie::Hash>, B<Tie::StdHash>, or B<Tie::ExtraHash>.
168 =head1 MORE INFORMATION
170 The packages relating to various DBM-related implementations (F<DB_File>,
171 F<NDBM_File>, etc.) show examples of general tied hashes, as does the
172 L<Config> module. While these do not utilize B<Tie::Hash>, they serve as
173 good working examples.
178 use warnings::register;
189 if (defined &{"${pkg}::new"}) {
190 warnings::warnif("WARNING: calling ${pkg}->new since ${pkg}->TIEHASH is missing");
194 croak "$pkg doesn't define a TIEHASH method";
200 croak "$pkg doesn't define an EXISTS method";
205 my $key = $self->FIRSTKEY(@_);
208 while (defined $key) {
210 $key = $self->NEXTKEY(@_, $key);
212 foreach $key (@keys) {
213 $self->DELETE(@_, $key);
217 # The Tie::StdHash package implements standard perl hash behaviour.
218 # It exists to act as a base class for classes which only wish to
219 # alter some parts of their behaviour.
221 package Tie::StdHash;
222 # @ISA = qw(Tie::Hash); # would inherit new() only
224 sub TIEHASH { bless {}, $_[0] }
225 sub STORE { $_[0]->{$_[1]} = $_[2] }
226 sub FETCH { $_[0]->{$_[1]} }
227 sub FIRSTKEY { my $a = scalar keys %{$_[0]}; each %{$_[0]} }
228 sub NEXTKEY { each %{$_[0]} }
229 sub EXISTS { exists $_[0]->{$_[1]} }
230 sub DELETE { delete $_[0]->{$_[1]} }
231 sub CLEAR { %{$_[0]} = () }
233 package Tie::ExtraHash;
235 sub TIEHASH { my $p = shift; bless [{}, @_], $p }
236 sub STORE { $_[0][0]{$_[1]} = $_[2] }
237 sub FETCH { $_[0][0]{$_[1]} }
238 sub FIRSTKEY { my $a = scalar keys %{$_[0][0]}; each %{$_[0][0]} }
239 sub NEXTKEY { each %{$_[0][0]} }
240 sub EXISTS { exists $_[0][0]->{$_[1]} }
241 sub DELETE { delete $_[0][0]->{$_[1]} }
242 sub CLEAR { %{$_[0][0]} = () }