autovivification(3)

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.