NAME
   autovivification - Lexically disable autovivification.

VERSION
   Version 0.18

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   => @categories;
       no autovivification strict => @categories;

   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 for the
       categories specified in @opts.

       Note that "no autovivification 'warn'" currently does nothing by
       itself, in particular it does not make the default categories warn.
       This behaviour may change in a future version of this pragma.

   *   'strict'

       Throws an exception when an autovivification is avoided for the
       categories specified in @opts.

       Note that "no autovivification 'strict'" currently does nothing by
       itself, in particular it does not make the default categories die.
       This behaviour may change in a future version of this pragma.

   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
   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 "no autovivification" - which may become noticeable
   if you rely heavily on numerous calls to "eval STRING".

   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.

   Autovivifications that happen in code "eval"'d during the global
   destruction phase of a spawned thread or pseudo-fork (the processes used
   internally for the "fork" emulation on Windows) are not reported.

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

ACKNOWLEDGEMENTS
   Matt S. Trout asked for it.

COPYRIGHT & LICENSE
   Copyright 2009,2010,2011,2012,2013,2014,2015,2017 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.

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