1autovivification(3) User Contributed Perl Documentation autovivification(3)
2
6 autovivification - Lexically disable autovivification.
7
9 Version 0.18
10
12 no autovivification;
13 my $hashref;
15 my $a = $hashref->{key_a}; # $hashref stays undef
17 if (exists $hashref->{option}) { # Still undef
19 ...
20 }
21 delete $hashref->{old}; # Still undef again
23 $hashref->{new} = $value; # Vivifies to { new => $value }
25
27 When an undefined variable is dereferenced, it gets silently upgraded
28 to an array or hash reference (depending of the type of the
29 dereferencing). This behaviour is called autovivification and usually
30 does what you mean (e.g. when you store a value) but it may be
31 unnatural or surprising because your variables gets populated behind
32 your back. This is especially true when several levels of
33 dereferencing are involved, in which case all levels are vivified up to
34 the last, or when it happens in intuitively read-only constructs like
35 "exists".
36 This pragma lets you disable autovivification for some constructs and
38 optionally throws a warning or an error when it would have happened.
39
41 "unimport"
42 no autovivification; # defaults to qw<fetch exists delete>
43 no autovivification qw<fetch store exists delete>;
44 no autovivification warn => @categories;
45 no autovivification strict => @categories;
46 Magically called when "no autovivification @opts" is encountered.
48 Enables the features given in @opts, which can be :
49 • 'fetch'
51 Turns off autovivification for rvalue dereferencing expressions,
53 such as :
54 $value = $arrayref->[$idx]
56 $value = $hashref->{$key}
57 keys %$hashref
58 values %$hashref
59 Starting from perl 5.11, it also covers "keys" and "values" on
61 array references :
62 keys @$arrayref
64 values @$arrayref
65 When the expression would have autovivified, "undef" is returned
67 for a plain fetch, while "keys" and "values" return 0 in scalar
68 context and the empty list in list context.
69 • 'exists'
71 Turns off autovivification for dereferencing expressions that are
73 parts of an "exists", such as :
74 exists $arrayref->[$idx]
76 exists $hashref->{$key}
77 '' is returned when the expression would have autovivified.
79 • 'delete'
81 Turns off autovivification for dereferencing expressions that are
83 parts of a "delete", such as :
84 delete $arrayref->[$idx]
86 delete $hashref->{$key}
87 "undef" is returned when the expression would have autovivified.
89 • 'store'
91 Turns off autovivification for lvalue dereferencing expressions,
93 such as :
94 $arrayref->[$idx] = $value
96 $hashref->{$key} = $value
97 for ($arrayref->[$idx]) { ... }
98 for ($hashref->{$key}) { ... }
99 function($arrayref->[$idx])
100 function($hashref->{$key})
101 An exception is thrown if vivification is needed to store the
103 value, which means that effectively you can only assign to levels
104 that are already defined. In the example, this would require
105 $arrayref (resp. $hashref) to already be an array (resp. hash)
106 reference.
107 • 'warn'
109 Emits a warning when an autovivification is avoided for the
111 categories specified in @opts.
112 Note that "no autovivification 'warn'" currently does nothing by
114 itself, in particular it does not make the default categories warn.
115 This behaviour may change in a future version of this pragma.
116 • 'strict'
118 Throws an exception when an autovivification is avoided for the
120 categories specified in @opts.
121 Note that "no autovivification 'strict'" currently does nothing by
123 itself, in particular it does not make the default categories die.
124 This behaviour may change in a future version of this pragma.
125 Each call to "unimport" adds the specified features to the ones already
127 in use in the current lexical scope.
128 When @opts is empty, it defaults to "qw<fetch exists delete>".
130 "import"
132 use autovivification; # default Perl behaviour
133 use autovivification qw<fetch store exists delete>;
134 Magically called when "use autovivification @opts" is encountered.
136 Disables the features given in @opts, which can be the same as for
137 "unimport".
138 Each call to "import" removes the specified features to the ones
140 already in use in the current lexical scope.
141 When @opts is empty, it defaults to restoring the original Perl
143 autovivification behaviour.
144
146 "A_THREADSAFE"
147 True if and only if the module could have been built with thread-safety
148 features enabled. This constant only has a meaning when your perl is
149 threaded, otherwise it will always be false.
150 "A_FORKSAFE"
152 True if and only if this module could have been built with fork-safety
153 features enabled. This constant will always be true, except on Windows
154 where it is false for perl 5.10.0 and below.
155
157 Using this pragma will cause a slight global slowdown of any subsequent
158 compilation phase that happens anywere in your code - even outside of
159 the scope of use of "no autovivification" - which may become noticeable
160 if you rely heavily on numerous calls to "eval STRING".
161 The pragma doesn't apply when one dereferences the returned value of an
163 array or hash slice, as in "@array[$id]->{member}" or
164 @hash{$key}->{member}. This syntax is valid Perl, yet it is
165 discouraged as the slice is here useless since the dereferencing
166 enforces scalar context. If warnings are turned on, Perl will complain
167 about one-element slices.
168 Autovivifications that happen in code "eval"'d during the global
170 destruction phase of a spawned thread or pseudo-fork (the processes
171 used internally for the "fork" emulation on Windows) are not reported.
172
174 perl 5.8.3.
175 A C compiler. This module may happen to build with a C++ compiler as
177 well, but don't rely on it, as no guarantee is made in this regard.
178 XSLoader (standard since perl 5.6.0).
180
182 perlref.
183
185 Vincent Pit, "<perl at profvince.com>", <http://www.profvince.com>.
186 You can contact me by mail or on "irc.perl.org" (vincent).
188
190 Please report any bugs or feature requests to "bug-autovivification at
191 rt.cpan.org", or through the web interface at
192 <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=autovivification>. I
193 will be notified, and then you'll automatically be notified of progress
194 on your bug as I make changes.
195
197 You can find documentation for this module with the perldoc command.
198 perldoc autovivification
200
202 Matt S. Trout asked for it.
203
205 Copyright 2009,2010,2011,2012,2013,2014,2015,2017 Vincent Pit, all
206 rights reserved.
207 This program is free software; you can redistribute it and/or modify it
209 under the same terms as Perl itself.
210perl v5.34.0 2022-01-21 autovivification(3)