1 package autovivification;
10 autovivification - Lexically disable autovivification.
29 my $a = $hashref->{key_a}; # $hashref stays undef
31 if (exists $hashref->{option}) { # Still undef
35 delete $hashref->{old}; # Still undef again
37 $hashref->{new} = $value; # Vivifies to { new => $value }
41 When an undefined variable is dereferenced, it gets silently upgraded to an array or hash reference (depending of the type of the dereferencing).
42 This behaviour is called I<autovivification> and usually does what you mean (e.g. when you store a value) but it may be unnatural or surprising because your variables gets populated behind your back.
43 This is especially true when several levels of dereferencing are involved, in which case all levels are vivified up to the last, or when it happens in intuitively read-only constructs like C<exists>.
45 This pragma lets you disable autovivification for some constructs and optionally throws a warning or an error when it would have happened.
51 XSLoader::load(__PACKAGE__, $VERSION);
58 no autovivification; # defaults to qw<fetch exists delete>
59 no autovivification qw<fetch store exists delete>;
60 no autovivification warn => @categories;
61 no autovivification strict => @categories;
63 Magically called when C<no autovivification @opts> is encountered.
64 Enables the features given in C<@opts>, which can be :
72 Turns off autovivification for rvalue dereferencing expressions, such as :
74 $value = $arrayref->[$idx]
75 $value = $hashref->{$key}
79 Starting from perl C<5.11>, it also covers C<keys> and C<values> on array references :
84 When the expression would have autovivified, C<undef> is returned for a plain fetch, while C<keys> and C<values> return C<0> in scalar context and the empty list in list context.
90 Turns off autovivification for dereferencing expressions that are parts of an C<exists>, such as :
92 exists $arrayref->[$idx]
93 exists $hashref->{$key}
95 C<''> is returned when the expression would have autovivified.
101 Turns off autovivification for dereferencing expressions that are parts of a C<delete>, such as :
103 delete $arrayref->[$idx]
104 delete $hashref->{$key}
106 C<undef> is returned when the expression would have autovivified.
112 Turns off autovivification for lvalue dereferencing expressions, such as :
114 $arrayref->[$idx] = $value
115 $hashref->{$key} = $value
116 for ($arrayref->[$idx]) { ... }
117 for ($hashref->{$key}) { ... }
118 function($arrayref->[$idx])
119 function($hashref->{$key})
121 An exception is thrown if vivification is needed to store the value, which means that effectively you can only assign to levels that are already defined.
122 In the example, this would require C<$arrayref> (resp. C<$hashref>) to already be an array (resp. hash) reference.
128 Emits a warning when an autovivification is avoided for the categories specified in C<@opts>.
130 Note that C<no autovivification 'warn'> currently does nothing by itself, in particular it does not make the default categories warn.
131 This behaviour may change in a future version of this pragma.
137 Throws an exception when an autovivification is avoided for the categories specified in C<@opts>.
139 Note that C<no autovivification 'strict'> currently does nothing by itself, in particular it does not make the default categories die.
140 This behaviour may change in a future version of this pragma.
144 Each call to C<unimport> B<adds> the specified features to the ones already in use in the current lexical scope.
146 When C<@opts> is empty, it defaults to C<< qw<fetch exists delete> >>.
151 strict => A_HINT_STRICT,
153 fetch => A_HINT_FETCH|A_HINT_KEYS|A_HINT_VALUES,
154 store => A_HINT_STORE,
155 exists => A_HINT_EXISTS,
156 delete => A_HINT_DELETE,
161 my $hint = _detag($^H{+(__PACKAGE__)}) || 0;
162 @_ = qw<fetch exists delete> unless @_;
163 $hint |= $bits{$_} for grep exists $bits{$_}, @_;
165 $^H{+(__PACKAGE__)} = _tag($hint);
171 use autovivification; # default Perl behaviour
172 use autovivification qw<fetch store exists delete>;
174 Magically called when C<use autovivification @opts> is encountered.
175 Disables the features given in C<@opts>, which can be the same as for L</unimport>.
177 Each call to C<import> B<removes> the specified features to the ones already in use in the current lexical scope.
179 When C<@opts> is empty, it defaults to restoring the original Perl autovivification behaviour.
187 $hint = _detag($^H{+(__PACKAGE__)}) || 0;
188 $hint &= ~$bits{$_} for grep exists $bits{$_}, @_;
191 $^H{+(__PACKAGE__)} = _tag($hint);
197 =head2 C<A_THREADSAFE>
199 True if and only if the module could have been built with thread-safety features enabled.
200 This constant only has a meaning when your perl is threaded, otherwise it will always be false.
204 True if and only if this module could have been built with fork-safety features enabled.
205 This constant will always be true, except on Windows where it is false for perl 5.10.0 and below.
209 Using this pragma will cause a slight global slowdown of any subsequent compilation phase that happens anywere in your code - even outside of the scope of use of C<no autovivification> - which may become noticeable if you rely heavily on numerous calls to C<eval STRING>.
211 The pragma doesn't apply when one dereferences the returned value of an array or hash slice, as in C<< @array[$id]->{member} >> or C<< @hash{$key}->{member} >>.
212 This syntax is valid Perl, yet it is discouraged as the slice is here useless since the dereferencing enforces scalar context.
213 If warnings are turned on, Perl will complain about one-element slices.
215 Autovivifications that happen in code C<eval>'d during the global destruction phase of a spawned thread or pseudo-fork (the processes used internally for the C<fork> emulation on Windows) are not reported.
222 This module may happen to build with a C++ compiler as well, but don't rely on it, as no guarantee is made in this regard.
224 L<XSLoader> (standard since perl 5.6.0).
232 Vincent Pit, C<< <perl at profvince.com> >>, L<http://www.profvince.com>.
234 You can contact me by mail or on C<irc.perl.org> (vincent).
238 Please report any bugs or feature requests to C<bug-autovivification at rt.cpan.org>, or through the web interface at L<http://rt.cpan.org/NoAuth/ReportBug.html?Queue=autovivification>.
239 I will be notified, and then you'll automatically be notified of progress on your bug as I make changes.
243 You can find documentation for this module with the perldoc command.
245 perldoc autovivification
247 =head1 ACKNOWLEDGEMENTS
249 Matt S. Trout asked for it.
251 =head1 COPYRIGHT & LICENSE
253 Copyright 2009,2010,2011,2012,2013,2014,2015,2017 Vincent Pit, all rights reserved.
255 This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
259 1; # End of autovivification