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 %{$_[0]};
27 # TIEHANDLE should return a reference to the actual storage
33 @ISA = (Tie::ExtraHash);
35 # All methods provided by default, define only those needing overrides
36 # Accessors access the storage in %{$_[0][0]};
37 # TIEHANDLE 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<TIEHANDLE> 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<TIEHANDLE> 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::StdHash';
142 my $storage = bless {}, shift;
143 warn "New ReportHash created, stored in $storage.\n";
147 warn "Storing data with key $_[1] at $_[0].\n";
148 $_[0][0]{$_[1]} = $_[2]
151 The default C<TIEHANDLE> method stores "extra" arguments to tie() starting
152 from offset 1 in the array referenced by C<tied(%tiedhash)>; this is the
153 same storage algorithm as in TIEHASH subroutine above. Hence, a typical
154 package inheriting from B<Tie::ExtraHash> does not need to overwrite this
157 =head1 C<UNTIE> and C<DESTROY>
159 The methods C<UNTIE> and C<DESTROY> are not defined in B<Tie::Hash>,
160 B<Tie::StdHash>, or B<Tie::ExtraHash>. Tied hashes do not require
161 presense of these methods, but if defined, the methods will be called in
162 proper time, see L<perltie>.
164 If needed, these methods should be defined by the package inheriting from
165 B<Tie::Hash>, B<Tie::StdHash>, or B<Tie::ExtraHash>.
167 =head1 MORE INFORMATION
169 The packages relating to various DBM-related implementations (F<DB_File>,
170 F<NDBM_File>, etc.) show examples of general tied hashes, as does the
171 L<Config> module. While these do not utilize B<Tie::Hash>, they serve as
172 good working examples.
177 use warnings::register;
188 if (defined &{"${pkg}::new"}) {
189 warnings::warnif("WARNING: calling ${pkg}->new since ${pkg}->TIEHASH is missing");
193 croak "$pkg doesn't define a TIEHASH method";
199 croak "$pkg doesn't define an EXISTS method";
204 my $key = $self->FIRSTKEY(@_);
207 while (defined $key) {
209 $key = $self->NEXTKEY(@_, $key);
211 foreach $key (@keys) {
212 $self->DELETE(@_, $key);
216 # The Tie::StdHash package implements standard perl hash behaviour.
217 # It exists to act as a base class for classes which only wish to
218 # alter some parts of their behaviour.
220 package Tie::StdHash;
221 # @ISA = qw(Tie::Hash); # would inherit new() only
223 sub TIEHASH { bless {}, $_[0] }
224 sub STORE { $_[0]->{$_[1]} = $_[2] }
225 sub FETCH { $_[0]->{$_[1]} }
226 sub FIRSTKEY { my $a = scalar keys %{$_[0]}; each %{$_[0]} }
227 sub NEXTKEY { each %{$_[0]} }
228 sub EXISTS { exists $_[0]->{$_[1]} }
229 sub DELETE { delete $_[0]->{$_[1]} }
230 sub CLEAR { %{$_[0]} = () }
232 package Tie::ExtraHash;
234 sub TIEHASH { my $p = shift; bless [{}, @_], $p }
235 sub STORE { $_[0][0]{$_[1]} = $_[2] }
236 sub FETCH { $_[0][0]{$_[1]} }
237 sub FIRSTKEY { my $a = scalar keys %{$_[0][0]}; each %{$_[0][0]} }
238 sub NEXTKEY { each %{$_[0][0]} }
239 sub EXISTS { exists $_[0][0]->{$_[1]} }
240 sub DELETE { delete $_[0][0]->{$_[1]} }
241 sub CLEAR { %{$_[0][0]} = () }