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autovivification-0.06
River stage two • 24 direct dependents • 67 total dependents
29 ++
/ autovivification

NAME

autovivification - Lexically disable autovivification.

VERSION

Version 0.06

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 is encountered. Enables the features given in @opts, which can be :

  • 'fetch'

    Turn off autovivification for rvalue dereferencing expressions, such as $value = $hashref->{key}[$idx]{$field}, keys %{$hashref->{key}} or values %{$hashref->{key}}. Starting from perl 5.11, it also covers keys and values on array references. 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'

    Turn off autovivification for dereferencing expressions that are parts of an exists, such as exists $hashref->{key}[$idx]{$field}. '' is returned when the expression would have autovivified.

  • 'delete'

    Turn off autovivification for dereferencing expressions that are parts of a delete, such as delete $hashref->{key}[$idx]{$field}. undef is returned when the expression would have autovivified.

  • 'store'

    Turn off autovivification for lvalue dereferencing expressions, such as $hashref->{key}[$idx]{$field} = $value or for ($hashref->{key}[$idx]{$field}) { ... }. 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 $hashref->{key}[$idx] to already be a hash reference).

  • 'warn'

    Emit a warning when an autovivification is avoided.

  • 'strict'

    Throw 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 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.

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 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.