autovivification(3pm)
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 => $value }
DESCRIPTION
When an undefined variable is dereferenced, it gets silently upgraded
to an array or hash reference (depending of the type of the
dereferencing). This behaviour is called autovivification and usually
does what you mean (e.g. when you store a value) but it's sometimes
unnatural or surprising because your variables gets populated behind
your back. This is especially true when several levels of
dereferencing are involved, in which case all levels are vivified up to
the last, or when it happens in intuitively read-only constructs like
"exists".
This pragma lets you disable autovivification for some constructs and
optionally throws a warning or an error when it would have happened.
METHODS
- "unimport @opts"
- Magically called when "no autovivification" is encountered. Enables
the features given in @opts, which can be : - o 'fetch'
Turn off autovivification for rvalue dereferencing expressions,
such as "$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. - o '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. - o 'delete'
Turn off autovivification for dereferencing expressions that are
parts of a "delete", such as "delete
$hashref->{key}[$idx]{$field}". "undef" is returned when the
expression would have autovivified. - o 'store'
Turn off autovivification for lvalue dereferencing expressions,
such as "$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). - o 'warn'
Emit a warning when an autovivification is avoided. - o '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 oneelement 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.