Re: Strange behaviour of interpreter when dereferencing an undefined value

References in lvalue context autovivify.

%{ $a } in lvalue context is effectively %{ $a //= {} }

@{ $a } in lvalue context is effectively @{ $a //= [] }

For example,

>perl -MData::Dumper -E"my $ref; $ref->{a}{b} = 'c'; print Dumper $x;"
$VAR1 = {
          'a' => {
                   'b' => 'c'
                 }
        };
[download]

As you can see, $x->{a}{b} in lvalue context is effectively ( ( $x //= {} )->{a} //= {} )->{b}.

You can control the behaviour using the autovivification module. (Don't file a bug report.)

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Re^2: Strange behaviour of interpreter when dereferencing an undefined value by kejv2 (Acolyte) on Nov 03, 2014 at 20:43 UTC
I can't see how this could be related to my examples since in `perl -we'use strict; sub foo {}; my $a; foo( %{ $a } )'` [download] the expression `%{ $a }` isn't in lvalue context. Also autovivification doesn't appear only in lvalue context: `perl -MData::Dumper -we'use strict; my $a; $a->{b}{c}; print Dumper $a +' $VAR1 = { 'b' => {} };` [download]	[reply] [d/l] [select]
Re^3: Strange behaviour of interpreter when dereferencing an undefined value by ikegami (Patriarch) on Nov 04, 2014 at 03:38 UTC
the expression `%{ $a }` isn't in lvalue context. Yes, it is. `$ perl -E'sub f { $_[0] = 'abc' } f($x); say $x' abc $ perl -E'sub f { $_[1] = 'abc' } $h{a}=undef; f(%h); say $h{a}' abc` [download] Perl arguments are always passed by reference. Also autovivification doesn't appear only in lvalue context I didn't say otherwise.	[reply] [d/l] [select]