1Tie::Hash(3pm) Perl Programmers Reference Guide Tie::Hash(3pm)
2
6 Tie::Hash, Tie::StdHash, Tie::ExtraHash - base class definitions for
7 tied hashes
8
10 package NewHash;
11 require Tie::Hash;
12 @ISA = qw(Tie::Hash);
14 sub DELETE { ... } # Provides needed method
16 sub CLEAR { ... } # Overrides inherited method
17 package NewStdHash;
20 require Tie::Hash;
21 @ISA = qw(Tie::StdHash);
23 # All methods provided by default, define only those needing overrides
25 # Accessors access the storage in %{$_[0]};
26 # TIEHASH should return a reference to the actual storage
27 sub DELETE { ... }
28 package NewExtraHash;
30 require Tie::Hash;
31 @ISA = qw(Tie::ExtraHash);
33 # All methods provided by default, define only those needing overrides
35 # Accessors access the storage in %{$_[0][0]};
36 # TIEHASH should return an array reference with the first element being
37 # the reference to the actual storage
38 sub DELETE {
39 $_[0][1]->('del', $_[0][0], $_[1]); # Call the report writer
40 delete $_[0][0]->{$_[1]}; # $_[0]->SUPER::DELETE($_[1])
41 }
42 package main;
45 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"};
50
52 This module provides some skeletal methods for hash-tying classes. See
53 perltie for a list of the functions required in order to tie a hash to
54 a package. The basic Tie::Hash package provides a "new" method, as well
55 as methods "TIEHASH", "EXISTS" and "CLEAR". The Tie::StdHash and
56 Tie::ExtraHash packages provide most methods for hashes described in
57 perltie (the exceptions are "UNTIE" and "DESTROY"). They cause tied
58 hashes to behave exactly like standard hashes, and allow for selective
59 overwriting of methods. Tie::Hash grandfathers the "new" method: it is
60 used if "TIEHASH" is not defined in the case a class forgets to include
61 a "TIEHASH" method.
62 For developers wishing to write their own tied hashes, the required
64 methods are briefly defined below. See the perltie section for more
65 detailed descriptive, as well as example code:
66 TIEHASH classname, LIST
68 The method invoked by the command "tie %hash, classname".
69 Associates a new hash instance with the specified class. "LIST"
70 would represent additional arguments (along the lines of
71 AnyDBM_File and compatriots) needed to complete the association.
72 STORE this, key, value
74 Store datum value into key for the tied hash this.
75 FETCH this, key
77 Retrieve the datum in key for the tied hash this.
78 FIRSTKEY this
80 Return the first key in the hash.
81 NEXTKEY this, lastkey
83 Return the next key in the hash.
84 EXISTS this, key
86 Verify that key exists with the tied hash this.
87 The Tie::Hash implementation is a stub that simply croaks.
89 DELETE this, key
91 Delete the key key from the tied hash this.
92 CLEAR this
94 Clear all values from the tied hash this.
95 SCALAR this
97 Returns what evaluating the hash in scalar context yields.
98 Tie::Hash does not implement this method (but Tie::StdHash and
100 Tie::ExtraHash do).
101
103 The accessor methods assume that the actual storage for the data in the
104 tied hash is in the hash referenced by "tied(%tiedhash)". Thus
105 overwritten "TIEHASH" method should return a hash reference, and the
106 remaining methods should operate on the hash referenced by the first
107 argument:
108 package ReportHash;
110 our @ISA = 'Tie::StdHash';
111 sub TIEHASH {
113 my $storage = bless {}, shift;
114 warn "New ReportHash created, stored in $storage.\n";
115 $storage
116 }
117 sub STORE {
118 warn "Storing data with key $_[1] at $_[0].\n";
119 $_[0]{$_[1]} = $_[2]
120 }
121
123 The accessor methods assume that the actual storage for the data in the
124 tied hash is in the hash referenced by "(tied(%tiedhash))->[0]". Thus
125 overwritten "TIEHASH" method should return an array reference with the
126 first element being a hash reference, and the remaining methods should
127 operate on the hash "%{ $_[0]->[0] }":
128 package ReportHash;
130 our @ISA = 'Tie::ExtraHash';
131 sub TIEHASH {
133 my $class = shift;
134 my $storage = bless [{}, @_], $class;
135 warn "New ReportHash created, stored in $storage.\n";
136 $storage;
137 }
138 sub STORE {
139 warn "Storing data with key $_[1] at $_[0].\n";
140 $_[0][0]{$_[1]} = $_[2]
141 }
142 The default "TIEHASH" method stores "extra" arguments to tie() starting
144 from offset 1 in the array referenced by "tied(%tiedhash)"; this is the
145 same storage algorithm as in TIEHASH subroutine above. Hence, a
146 typical package inheriting from Tie::ExtraHash does not need to
147 overwrite this method.
148
150 The methods "UNTIE" and "DESTROY" are not defined in Tie::Hash,
151 Tie::StdHash, or Tie::ExtraHash. Tied hashes do not require presence
152 of these methods, but if defined, the methods will be called in proper
153 time, see perltie.
154 "SCALAR" is only defined in Tie::StdHash and Tie::ExtraHash.
156 If needed, these methods should be defined by the package inheriting
158 from Tie::Hash, Tie::StdHash, or Tie::ExtraHash. See "SCALAR" in
159 perltie to find out what happens when "SCALAR" does not exist.
160
162 The packages relating to various DBM-related implementations (DB_File,
163 NDBM_File, etc.) show examples of general tied hashes, as does the
164 Config module. While these do not utilize Tie::Hash, they serve as good
165 working examples.
166perl v5.12.4 2011-06-07 Tie::Hash(3pm)