1Tie::Hash(3pm) Perl Programmers Reference Guide Tie::Hash(3pm)
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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
25 # only those needing overrides
26 # Accessors access the storage in %{$_[0]};
27 # TIEHASH should return a reference to the actual storage
28 sub DELETE { ... }
29 package NewExtraHash;
31 require Tie::Hash;
32 @ISA = qw(Tie::ExtraHash);
34 # All methods provided by default, define
36 # only those needing overrides
37 # Accessors access the storage in %{$_[0][0]};
38 # TIEHASH should return an array reference with the first element
39 # being the reference to the actual storage
40 sub DELETE {
41 $_[0][1]->('del', $_[0][0], $_[1]); # Call the report writer
42 delete $_[0][0]->{$_[1]}; # $_[0]->SUPER::DELETE($_[1])
43 }
44 package main;
47 tie %new_hash, 'NewHash';
49 tie %new_std_hash, 'NewStdHash';
50 tie %new_extra_hash, 'NewExtraHash',
51 sub {warn "Doing \U$_[1]\E of $_[2].\n"};
52
54 This module provides some skeletal methods for hash-tying classes. See
55 perltie for a list of the functions required in order to tie a hash to
56 a package. The basic Tie::Hash package provides a "new" method, as well
57 as methods "TIEHASH", "EXISTS" and "CLEAR". The Tie::StdHash and
58 Tie::ExtraHash packages provide most methods for hashes described in
59 perltie (the exceptions are "UNTIE" and "DESTROY"). They cause tied
60 hashes to behave exactly like standard hashes, and allow for selective
61 overwriting of methods. Tie::Hash grandfathers the "new" method: it is
62 used if "TIEHASH" is not defined in the case a class forgets to include
63 a "TIEHASH" method.
64 For developers wishing to write their own tied hashes, the required
66 methods are briefly defined below. See the perltie section for more
67 detailed descriptive, as well as example code:
68 TIEHASH classname, LIST
70 The method invoked by the command "tie %hash, classname".
71 Associates a new hash instance with the specified class. "LIST"
72 would represent additional arguments (along the lines of
73 AnyDBM_File and compatriots) needed to complete the association.
74 STORE this, key, value
76 Store datum value into key for the tied hash this.
77 FETCH this, key
79 Retrieve the datum in key for the tied hash this.
80 FIRSTKEY this
82 Return the first key in the hash.
83 NEXTKEY this, lastkey
85 Return the next key in the hash.
86 EXISTS this, key
88 Verify that key exists with the tied hash this.
89 The Tie::Hash implementation is a stub that simply croaks.
91 DELETE this, key
93 Delete the key key from the tied hash this.
94 CLEAR this
96 Clear all values from the tied hash this.
97 SCALAR this
99 Returns what evaluating the hash in scalar context yields.
100 Tie::Hash does not implement this method (but Tie::StdHash and
102 Tie::ExtraHash do).
103
105 The accessor methods assume that the actual storage for the data in the
106 tied hash is in the hash referenced by "tied(%tiedhash)". Thus
107 overwritten "TIEHASH" method should return a hash reference, and the
108 remaining methods should operate on the hash referenced by the first
109 argument:
110 package ReportHash;
112 our @ISA = 'Tie::StdHash';
113 sub TIEHASH {
115 my $storage = bless {}, shift;
116 warn "New ReportHash created, stored in $storage.\n";
117 $storage
118 }
119 sub STORE {
120 warn "Storing data with key $_[1] at $_[0].\n";
121 $_[0]{$_[1]} = $_[2]
122 }
123
125 The accessor methods assume that the actual storage for the data in the
126 tied hash is in the hash referenced by "(tied(%tiedhash))->[0]". Thus
127 overwritten "TIEHASH" method should return an array reference with the
128 first element being a hash reference, and the remaining methods should
129 operate on the hash "%{ $_[0]->[0] }":
130 package ReportHash;
132 our @ISA = 'Tie::ExtraHash';
133 sub TIEHASH {
135 my $class = shift;
136 my $storage = bless [{}, @_], $class;
137 warn "New ReportHash created, stored in $storage.\n";
138 $storage;
139 }
140 sub STORE {
141 warn "Storing data with key $_[1] at $_[0].\n";
142 $_[0][0]{$_[1]} = $_[2]
143 }
144 The default "TIEHASH" method stores "extra" arguments to tie() starting
146 from offset 1 in the array referenced by "tied(%tiedhash)"; this is the
147 same storage algorithm as in TIEHASH subroutine above. Hence, a
148 typical package inheriting from Tie::ExtraHash does not need to
149 overwrite this method.
150
152 The methods "UNTIE" and "DESTROY" are not defined in Tie::Hash,
153 Tie::StdHash, or Tie::ExtraHash. Tied hashes do not require presence
154 of these methods, but if defined, the methods will be called in proper
155 time, see perltie.
156 "SCALAR" is only defined in Tie::StdHash and Tie::ExtraHash.
158 If needed, these methods should be defined by the package inheriting
160 from Tie::Hash, Tie::StdHash, or Tie::ExtraHash. See "SCALAR" in
161 perltie to find out what happens when "SCALAR" does not exist.
162
164 The packages relating to various DBM-related implementations (DB_File,
165 NDBM_File, etc.) show examples of general tied hashes, as does the
166 Config module. While these do not utilize Tie::Hash, they serve as good
167 working examples.
168perl v5.34.0 2021-10-18 Tie::Hash(3pm)