2 autovivification - Lexically disable autovivification.
12 my $a = $hashref->{key_a}; # $hashref stays undef
14 if (exists $hashref->{option}) { # Still undef
18 delete $hashref->{old}; # Still undef again
20 $hashref->{new} = $value; # Vivifies to { new => $value }
23 When an undefined variable is dereferenced, it gets silently upgraded to
24 an array or hash reference (depending of the type of the dereferencing).
25 This behaviour is called *autovivification* and usually does what you
26 mean (e.g. when you store a value) but it's sometimes unnatural or
27 surprising because your variables gets populated behind your back. This
28 is especially true when several levels of dereferencing are involved, in
29 which case all levels are vivified up to the last, or when it happens in
30 intuitively read-only constructs like "exists".
32 This pragma lets you disable autovivification for some constructs and
33 optionally throws a warning or an error when it would have happened.
37 Magically called when "no autovivification" is encountered. Enables the
38 features given in @opts, which can be :
42 Turn off autovivification for rvalue dereferencing expressions, such
43 as "$value = $hashref->{key}[$idx]{$field}", "keys
44 %{$hashref->{key}}" or "values %{$hashref->{key}}". Starting from
45 perl 5.11, it also covers "keys" and "values" on array references.
46 When the expression would have autovivified, "undef" is returned for
47 a plain fetch, while "keys" and "values" return 0 in scalar context
48 and the empty list in list context.
52 Turn off autovivification for dereferencing expressions that are
53 parts of an "exists", such as "exists
54 $hashref->{key}[$idx]{$field}". '' is returned when the expression
55 would have autovivified.
59 Turn off autovivification for dereferencing expressions that are
60 parts of a "delete", such as "delete $hashref->{key}[$idx]{$field}".
61 "undef" is returned when the expression would have autovivified.
65 Turn off autovivification for lvalue dereferencing expressions, such
66 as "$hashref->{key}[$idx]{$field} = $value" or "for
67 ($hashref->{key}[$idx]{$field}) { ... }". An exception is thrown if
68 vivification is needed to store the value, which means that
69 effectively you can only assign to levels that are already defined
70 (in the example, this would require "$hashref->{key}[$idx]" to
71 already be a hash reference).
75 Emit a warning when an autovivification is avoided.
79 Throw an exception when an autovivification is avoided.
81 Each call to "unimport" adds the specified features to the ones already
82 in use in the current lexical scope.
84 When @opts is empty, it defaults to "qw/fetch exists delete/".
87 Magically called when "use autovivification" is encountered. Disables
88 the features given in @opts, which can be the same as for "unimport".
90 Each call to "import" removes the specified features to the ones already
91 in use in the current lexical scope.
93 When @opts is empty, it defaults to restoring the original Perl
94 autovivification behaviour.
98 True iff the module could have been built with thread-safety features
99 enabled. This constant only has a meaning with your perl is threaded ;
100 otherwise, it'll always be false.
103 True iff this module could have been built with fork-safety features
104 enabled. This will always be true except on Windows where it's false for
105 perl 5.10.0 and below .
108 The pragma doesn't apply when one dereferences the returned value of an
109 array or hash slice, as in "@array[$id]->{member}" or
110 @hash{$key}->{member}. This syntax is valid Perl, yet it's discouraged
111 as the slice is here useless since the dereferencing enforces scalar
112 context. If warnings are turned on, Perl will complain about one-element
118 XSLoader (standard since perl 5.006).
124 Vincent Pit, "<perl at profvince.com>", <http://www.profvince.com>.
126 You can contact me by mail or on "irc.perl.org" (vincent).
129 Please report any bugs or feature requests to "bug-autovivification at
130 rt.cpan.org", or through the web interface at
131 <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=autovivification>. I
132 will be notified, and then you'll automatically be notified of progress
133 on your bug as I make changes.
136 You can find documentation for this module with the perldoc command.
138 perldoc autovivification
140 Tests code coverage report is available at
141 <http://www.profvince.com/perl/cover/autovivification>.
144 Matt S. Trout asked for it.
147 Copyright 2009,2010 Vincent Pit, all rights reserved.
149 This program is free software; you can redistribute it and/or modify it
150 under the same terms as Perl itself.