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autovivification(3)   User Contributed Perl Documentation  autovivification(3)

NAME
       autovivification - Lexically disable autovivification.

VERSION
       Version 0.09

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's sometimes
       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 @opts"
       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 @opts"
       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 iff the module could have been built with thread-safety features
       enabled.	 This constant only has a meaning with your perl is threaded ;
       otherwise, it'll always be false.

   "A_FORKSAFE"
       True iff this module could have been built with fork-safety features
       enabled.	 This will always be true except on Windows where it's 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's 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 5.006).

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 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.

perl v5.14.1			  2011-01-05		   autovivification(3)
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