http://www.perlmonks.org?node_id=1063106


in reply to Unexpected warning

You get that warning on that line if $something is undefined. Although in that case you also ought to get another warning as well, about using an uninitialized value in a hash element on the calling line.

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Re^2: Unexpected warning
by LanX (Saint) on Nov 18, 2013 at 15:00 UTC
    > You get that warning on that line if $something is undefined.

    yep I can reproduce this. =)

    Seems like in the context of the call warnings were disabled.

      DB<132> use warnings; sub tst {my $a=shift, my $b =shift}

  DB<133> %h=()

  DB<134> $bla=""
 => ""

  DB<135> tst $h{$bla}
 => (undef, undef)

  DB<136> undef $bla
 => undef

  DB<137> tst $h{$bla}
Use of uninitialized value in scalar assignment at (eval 81)[multi_per
+l5db.pl:644] line 2.

    [download]

    Cheers Rolf

    ( addicted to the Perl Programming Language)

[reply]
[d/l]
      I can reproduce it myself too now: 
      tst(undef);        # doesn't warn from tst
tst($foo{undef});  # doesn't warn from tst
tst($foo{$undef}); # warns from tst

      [download]
      I would say this is a bug in perl, or I am missing something?
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[d/l]

        The undef value is subtly different from using an undefined variable, so that may be causing the problem.

        tst($foo{undef});  # doesn't warn from tst

        This uses the undef value as a key into the %foo hash; the key presumably doesn't exist so Perl returns undef. This is working as I would expect.

        tst($foo{$undef});  # warns from tst

        This looks up the value of $undef, which presumably isn't defined, so Perl should flag an error at that statement, not inside tst. If that is the case, then it, too, is working as I would expect.

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[d/l]
[select]

      I get no warnings from the code below: 

      use strict;
use warnings;
sub test { my $x = shift; }
my %hash = ( a => "a" );
test $hash{"b"};

      [download]
[reply]
[d/l]
        The key must be undef.

        lanx@nc10-ubuntu:~$ perl
use strict;
{ use warnings;
  sub test { my $x = shift; }
}
my %hash = ( a => "a" );
my $key;
test $hash{$key}
Use of uninitialized value in scalar assignment at - line 3.

lanx@nc10-ubuntu:~$ perl -version

This is perl, v5.10.0 built for i486-linux-gnu-thread-multi

        [download]

        I agree with salva that this is kind of a bug in lexical warnings.

        Cheers Rolf

        ( addicted to the Perl Programming Language)

[reply]
[d/l]
Re^2: Unexpected warning
by samwyse (Scribe) on Nov 18, 2013 at 17:11 UTC
    This should be pretty definitive... 
    use strict;
use warnings;

sub tst {
    warn "entering tst()\n";
    my $arg = shift;  # line 6
    warn "leaving tst()\n\n";
}

my %h=();
tst $h{''};
tst $h{undef};  # line 12
my $bla='';
tst $h{$bla};  # line 14
$bla=undef;
tst $h{$bla};  # line 16
undef $bla;
tst $h{$bla};  # line 19

    [download]
    And here's the results: 
    entering tst()
leaving tst()

entering tst()
leaving tst()

entering tst()
leaving tst()

Use of uninitialized value $bla in hash element at perlmonks-1063097.p
+l line 16.
entering tst()
Use of uninitialized value in scalar assignment at perlmonks-1063097.p
+l line 6.
leaving tst()

Use of uninitialized value $bla in hash element at perlmonks-1063097.p
+l line 18.
entering tst()
Use of uninitialized value in scalar assignment at perlmonks-1063097.p
+l line 6.
leaving tst()

    [download]

    Turning off 'strict' has no effect, while turning off 'warnings' warnings gets rid of the messages.

    I'm actually surprised by a couple of aspects of this. First, I expected $bla=undef; tst $h{$bla} to behave the same as tst $h{undef}; it didn't. Instead, $bla=undef seems to have the same effect as undef $bla. Second, it seems that lines 16 and 19 are generating a value other than a simple undef to pass into the subroutine, otherwise the invocation in line 12 would have also generated an error in line 6.

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[d/l]
[select]
Re^2: Unexpected warning
by hdb (Monsignor) on Nov 18, 2013 at 15:01 UTC

    I get no warnings from this code below: 

    use strict;
use warnings;
my @array;
my $x = shift @array;
my $y = shift @array;

    [download]
[reply]
[d/l]

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