NAME
Perl::Critic::Policy::Variables::RequireNoAutovivification - Say no autovivification where a read through an element would create it.
VERSION
version 0.001
Perl::Critic::Policy::Variables::RequireNoAutovivification
A read through an element creates that element when it is missing. Perl calls this autovivification. $h{a}{b} looks up b in the hash that $h{a} refers to. When $h{a} is not there, perl makes it an empty hash first, and leaves it there after the read. exists $h{a}{b} and delete $h{a}{b} do the same. The next exists $h{a}, keys %h or serialization of %h then sees a key that nobody stored.
The autovivification pragma from CPAN stops it. Say no autovivification; in the scope of the read:
my $port = $config{server}{port}; # reported
no autovivification;
my $port = $config{server}{port}; # OK!
if ( exists $seen{$user}{$host} ) { ... } # reported
my @names = keys %{ $self->{children} }; # reported
no autovivification;
if ( exists $seen{$user}{$host} ) { ... } # OK!
my @names = keys %{ $self->{children} }; # OK!Only a read through an element is reported. A read of one level, $h{$k}, creates nothing in %h, so a file without a read through an element needs no pragma. A store through an element creates the levels on purpose, and is not reported:
$count{$user}{$host}++; # OK!
push @{ $children{$parent} }, $child; # OK!PROHIBITED
Each of these, without a no autovivification in scope that covers it:
$h{a}{b} # two levels
$h{a}->{b} # the same, with the arrow
$r->{a}[0] # a reference, then two levels
$_[0]{a} # an element of @_ is the caller's variable
$$r{a}{b} # $r->{a}{b}
${ $h{a} }{b} # a block dereference of an element
@{ $h{a} }{qw(b c)} # a slice of an element
$h{a}->@{qw(b c)} # a postfix slice
$o->m->{a}{b} # through what a method returned
@{ $h{a}{b} } # a read through an element, inside a cast
keys %{ $h{a} } # keys or values of an element
keys $h{a}->%* # with a postfix cast
exists $h{a}{b} # needs the 'exists' category
delete $h{a}{b} # needs the 'delete' categoryALLOWED
$h{a} # one level
$r->{a} # one level, through a reference
@{ $h{a} } # strict refs dies here, it does not vivify
exists $h{a} # one level
f()->{a} # one level of what a sub returned
( f() )[0]{k} # a list slice is a copy
$h{a}{b} = 1 # an assignment
$h{a}{b}++ # and any other change
push @{ $h{a}{b} }, 1 # the first argument of push, pop and the like
\$h{a}{b} # a reference
local $h{a}{b}
f( $h{a}{b} ) # an argument of a sub or method, see CAVEATS
$h{a}{b}->m # the invocant of a method
for ( @{ $h{a}{b} } ) {} # a foreach list, and the list of map and grep
$h{a}{b} =~ s/x// # a substitution or transliteration
chomp $h{a}{b} # chomp, chop, undef, sprintfSCOPE AND CATEGORIES
no autovivification is lexical, like no strict. It covers the rest of the block that it is in, from the statement after it, and every block inside that. A no autovivification inside a sub covers only that sub.
Each read needs one category. A read, and keys or values, needs fetch. exists needs exists, and delete needs delete. Without arguments, no autovivification turns on all three. With arguments, it turns on only the categories it names, so these do not cover a read:
no autovivification qw(store); # store only
no autovivification 'warn'; # warn and strict alone turn on nothing
no autovivification (); # empty parens do not call unimportwarn and strict with categories, such as no autovivification strict => 'fetch', turn those categories on. Each no autovivification adds to the categories in scope. use autovivification takes away the ones it names, or all of them without arguments, until the end of its block.
The policy does not ask for store. Under store, a store through a missing level dies instead of creating it. That changes what a correct program does, where the other categories only stop a read from writing.
CAVEATS
Perl passes the arguments of a sub or method, a foreach list, and the list of map and grep as aliases, so it creates the levels above them as it does for a store. The default categories do not stop that, so the policy does not report it. Copy the value into a variable first:
my $port = $config{server}{port};
connect_to($port);An argument that the policy cannot read, such as no autovivification @categories, counts as no arguments.
The policy does not read inside strings. "$h{a}{b}" creates $h{a} as well.
A read of one level through a reference, $r->{a}, makes $r a reference to an empty hash when $r is undefined. The policy does not report it: the variable that changes is the one that the code reads through, and the code is about to find out what it holds.
CONFIGURATION
equivalent_modules names modules that turn the default categories on in the scope of the code that uses them, the way some bundles of pragmas do. A use of one of them counts as no autovivification;.
[Variables::RequireNoAutovivification]
equivalent_modules = My::KitMETHODS
supported_parameters
default_severity
default_themes
applies_to
violates
BUGS
Please report any bugs or feature requests on the bugtracker website https://github.com/teodesian/perl-critic-policy-requirenoautovivification/issues
When submitting a bug or request, please include a test-file or a patch to an existing test-file that illustrates the bug or desired feature.
AUTHORS
Current Maintainers:
George S. Baugh <george@troglodyne.net>
CONTRIBUTOR
troglodyne-bot <morpheus@troglodyne.net>
COPYRIGHT AND LICENSE
Copyright (c) 2026 Troglodyne LLC
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.