NAME

       autovivification - Lexically disable autovivification.

VERSION

       Version 0.12

SYNOPSIS

           no autovivification;

           my $hashref;

           my $a = $hashref->{key_a};       # $hashref stays undef

           if (exists $hashref->{option}) { # Still undef
            ...
           }

           delete $hashref->{old};          # Still undef again

           $hashref->{new} = $value;        # Vivifies to { new => $value }

DESCRIPTION

       When an undefined variable is dereferenced, it gets silently upgraded to an array or hash
       reference (depending of the type of the dereferencing).  This behaviour is called
       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.  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 "exists".

       This pragma lets you disable autovivification for some constructs and optionally throws a
       warning or an error when it would have happened.

METHODS

   "unimport"
           no autovivification; # defaults to qw<fetch exists delete>
           no autovivification qw<fetch store exists delete>;
           no autovivification 'warn';
           no autovivification 'strict';

       Magically called when "no autovivification @opts" is encountered.  Enables the features
       given in @opts, which can be :

       •   'fetch'

           Turns off autovivification for rvalue dereferencing expressions, such as :

               $value = $arrayref->[$idx]
               $value = $hashref->{$key}
               keys %$hashref
               values %$hashref

           Starting from perl 5.11, it also covers "keys" and "values" on array references :

               keys @$arrayref
               values @$arrayref

           When the expression would have autovivified, "undef" is returned for a plain fetch,
           while "keys" and "values" return 0 in scalar context and the empty list in list
           context.

       •   'exists'

           Turns off autovivification for dereferencing expressions that are parts of an
           "exists", such as :

               exists $arrayref->[$idx]
               exists $hashref->{$key}

           '' is returned when the expression would have autovivified.

       •   'delete'

           Turns off autovivification for dereferencing expressions that are parts of a "delete",
           such as :

               delete $arrayref->[$idx]
               delete $hashref->{$key}

           "undef" is returned when the expression would have autovivified.

       •   'store'

           Turns off autovivification for lvalue dereferencing expressions, such as :

               $arrayref->[$idx] = $value
               $hashref->{$key} = $value
               for ($arrayref->[$idx]) { ... }
               for ($hashref->{$key}) { ... }
               function($arrayref->[$idx])
               function($hashref->{$key})

           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.  In the example,
           this would require $arrayref (resp. $hashref) to already be an array (resp. hash)
           reference.

       •   'warn'

           Emits a warning when an autovivification is avoided.

       •   'strict'

           Throws an exception when an autovivification is avoided.

       Each call to "unimport" adds the specified features to the ones already in use in the
       current lexical scope.

       When @opts is empty, it defaults to "qw<fetch exists delete>".

   "import"
           use autovivification; # default Perl behaviour
           use autovivification qw<fetch store exists delete>;

       Magically called when "use autovivification @opts" is encountered.  Disables the features
       given in @opts, which can be the same as for "unimport".

       Each call to "import" removes the specified features to the ones already in use in the
       current lexical scope.

       When @opts is empty, it defaults to restoring the original Perl autovivification
       behaviour.

CONSTANTS

   "A_THREADSAFE"
       True if and only if the module could have been built with thread-safety features enabled.
       This constant only has a meaning when your perl is threaded, otherwise it will always be
       false.

   "A_FORKSAFE"
       True if and only if this module could have been built with fork-safety features enabled.
       This constant will always be true, except on Windows where it is false for perl 5.10.0 and
       below.

CAVEATS

       The pragma doesn't apply when one dereferences the returned value of an array or hash
       slice, as in "@array[$id]->{member}" or @hash{$key}->{member}.  This syntax is valid Perl,
       yet it is discouraged as the slice is here useless since the dereferencing enforces scalar
       context.  If warnings are turned on, Perl will complain about one-element slices.

DEPENDENCIES

       perl 5.8.3.

       A C compiler.  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.

       XSLoader (standard since perl 5.6.0).

SEE ALSO

       perlref.

AUTHOR

       Vincent Pit, "<perl at profvince.com>", <http://www.profvince.com>.

       You can contact me by mail or on "irc.perl.org" (vincent).

BUGS

       Please report any bugs or feature requests to "bug-autovivification at rt.cpan.org", or
       through the web interface at
       <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=autovivification>.  I will be notified,
       and then you'll automatically be notified of progress on your bug as I make changes.

SUPPORT

       You can find documentation for this module with the perldoc command.

           perldoc autovivification

       Tests code coverage report is available at
       <http://www.profvince.com/perl/cover/autovivification>.

ACKNOWLEDGEMENTS

       Matt S. Trout asked for it.

COPYRIGHT & LICENSE

       Copyright 2009,2010,2011,2012,2013 Vincent Pit, all rights reserved.

       This program is free software; you can redistribute it and/or modify it under the same
       terms as Perl itself.