1autovivification(3) User Contributed Perl Documentation autovivification(3)
2
6 autovivification - Lexically disable autovivification.
7
9 Version 0.09
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's sometimes
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 @opts"
42 Magically called when "no autovivification @opts" is encountered.
43 Enables the features given in @opts, which can be :
44 · 'fetch'
46 Turns off autovivification for rvalue dereferencing expressions,
48 such as :
49 $value = $arrayref->[$idx]
51 $value = $hashref->{$key}
52 keys %$hashref
53 values %$hashref
54 Starting from perl 5.11, it also covers "keys" and "values" on
56 array references :
57 keys @$arrayref
59 values @$arrayref
60 When the expression would have autovivified, "undef" is returned
62 for a plain fetch, while "keys" and "values" return 0 in scalar
63 context and the empty list in list context.
64 · 'exists'
66 Turns off autovivification for dereferencing expressions that are
68 parts of an "exists", such as :
69 exists $arrayref->[$idx]
71 exists $hashref->{$key}
72 '' is returned when the expression would have autovivified.
74 · 'delete'
76 Turns off autovivification for dereferencing expressions that are
78 parts of a "delete", such as :
79 delete $arrayref->[$idx]
81 delete $hashref->{$key}
82 "undef" is returned when the expression would have autovivified.
84 · 'store'
86 Turns off autovivification for lvalue dereferencing expressions,
88 such as :
89 $arrayref->[$idx] = $value
91 $hashref->{$key} = $value
92 for ($arrayref->[$idx]) { ... }
93 for ($hashref->{$key}) { ... }
94 function($arrayref->[$idx])
95 function($hashref->{$key})
96 An exception is thrown if vivification is needed to store the
98 value, which means that effectively you can only assign to levels
99 that are already defined In the example, this would require
100 $arrayref (resp. $hashref) to already be an array (resp. hash)
101 reference.
102 · 'warn'
104 Emits a warning when an autovivification is avoided.
106 · 'strict'
108 Throws an exception when an autovivification is avoided.
110 Each call to "unimport" adds the specified features to the ones already
112 in use in the current lexical scope.
113 When @opts is empty, it defaults to "qw<fetch exists delete>".
115 "import @opts"
117 Magically called when "use autovivification @opts" is encountered.
118 Disables the features given in @opts, which can be the same as for
119 "unimport".
120 Each call to "import" removes the specified features to the ones
122 already in use in the current lexical scope.
123 When @opts is empty, it defaults to restoring the original Perl
125 autovivification behaviour.
126
128 "A_THREADSAFE"
129 True iff the module could have been built with thread-safety features
130 enabled. This constant only has a meaning with your perl is threaded ;
131 otherwise, it'll always be false.
132 "A_FORKSAFE"
134 True iff this module could have been built with fork-safety features
135 enabled. This will always be true except on Windows where it's false
136 for perl 5.10.0 and below .
137
139 The pragma doesn't apply when one dereferences the returned value of an
140 array or hash slice, as in "@array[$id]->{member}" or
141 @hash{$key}->{member}. This syntax is valid Perl, yet it's discouraged
142 as the slice is here useless since the dereferencing enforces scalar
143 context. If warnings are turned on, Perl will complain about one-
144 element slices.
145
147 perl 5.8.3.
148 A C compiler. This module may happen to build with a C++ compiler as
150 well, but don't rely on it, as no guarantee is made in this regard.
151 XSLoader (standard since perl 5.006).
153
155 perlref.
156
158 Vincent Pit, "<perl at profvince.com>", <http://www.profvince.com>.
159 You can contact me by mail or on "irc.perl.org" (vincent).
161
163 Please report any bugs or feature requests to "bug-autovivification at
164 rt.cpan.org", or through the web interface at
165 <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=autovivification>. I
166 will be notified, and then you'll automatically be notified of progress
167 on your bug as I make changes.
168
170 You can find documentation for this module with the perldoc command.
171 perldoc autovivification
173 Tests code coverage report is available at
175 <http://www.profvince.com/perl/cover/autovivification>.
176
178 Matt S. Trout asked for it.
179
181 Copyright 2009,2010,2011 Vincent Pit, all rights reserved.
182 This program is free software; you can redistribute it and/or modify it
184 under the same terms as Perl itself.
185perl v5.12.3 2011-01-05 autovivification(3)