:lvalue, How I Hate Thee

[jk2addict]

I had reason to use the perl functionality built into a vendors software package today. The package comes with 5.8.3, and it's own set of perl modules to do software related tasks like tweaking data records.

At one point, I have to figure out why this didn't work:

    $valobj->value = undef;

but this did:

    $valobj->value = '';

After looking into the source code, the object and it's methods use the :lvalue attribute.

    sub value : lvalue {
        my $this = shift;
        $this->{VALUE};
    }

So, that doesn't work if lvalue is undef, but does work if lvalue is an empty string. Is that a 'feature' of the lvalue attribute, or something about the method?

I wouldn't expect this traditional setup to work:

    sub value {
        my ($self, $value) = @_;
        if (defined $value) {
            $self->{value} = $value;
        };
    };
    $valobj->value(undef);

But I would've expected all the work behind :lvalue to fix that.

Updated: Yes, I've read the pod. It's experimental. Good thing they're using it. The pod doesn't really mention what would happen in the case of setting undef though.

-=Chris

[BrowserUk]

At one point, I have to figure out why this didn't work:

Define "didn't work"? In what way didn't it work?

This compiles and runs without error:

#! perl -slw
use strict;

package thing;
sub new { bless {}, $_[0]; }
sub value :lvalue { my $this = shift; $this->{VALUE};}

package main;

my $thing = thing->new;
$thing->value = undef;

[jk2addict]

Didn't work, as in $self->{VALUE} maintained it's original value of 'Yes', instead of being set to undef.

What version ot perl was that under?

[BrowserUk]

5.8.4, but I think you're right that there is something else involved here.

#! perl -slw
use strict;

package thing;

sub new {
    bless { VALUE => 'YES' }, $_[0];
}

sub value :lvalue {
    my $this = shift;
    $this->{VALUE};
}
package main;

print $];

my $thing = thing->new;

print $thing->value;
$thing->value = undef;
print $thing->value;
__END__
P:\test>junk2
5.008004
YES
Use of uninitialized value in print at P:\test\junk2.pl line 22.

[jk2addict]

It works on one machine, but not the other. One is perl 5.8.3 and the other is 5.8.6, but it feels like something else is afoot....