pike_uk has asked for the wisdom of the Perl Monks concerning the following question:

Hi, If you undef a var, or assign that var to something esle, then how does this happen? 
## the test script

use Report::testob;


my $testo;

$x=0;

while($x<10){


undef $testo;

$testo="ooof";

$testo=testob->new();


$testo->printblah();

$testo="yess";


$x++;

}


#####

# the "object"

package testob;


sub new {

    my $class=shift;
    my $self={@_};

    bless($self,$class);
    return $self;    
}


sub printblah{

 my $self=shift;

 $blah++;
$self->{blah}++;
print "blah is $blah \n";
print "self blah is ".$self->{blah}."\n";
}


1;

#####

# the output

blah is 1
self blah is 1
blah is 2
self blah is 1
blah is 3
self blah is 1
blah is 4
self blah is 1
blah is 5
self blah is 1
blah is 6
self blah is 1
blah is 7
self blah is 1
blah is 8
self blah is 1
blah is 9
self blah is 1
blah is 10
self blah is 1

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## how does this happen? I thought that assigning $testob to a string or calling undef would remove any reference to the memory location of the object, but it is clear here that $blah in the "object" retains this as does $testob?? please enlighten me! thanks in advance.

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Re: Undef doesnt work?This has got me stumped
by broquaint (Abbot) on Feb 09, 2004 at 10:20 UTC
    but it is clear here that $blah in the "object" retains this as does $testob??
    You are in fact modifying the package variable $blah, and like all package variables, are visible throughout the execution of the program. Whereas a new instance of $testob is being created every time as indicated by the value of $self->{blah} always being at 1 post-increment. See. perlobj for more info on objects in perl.
    HTH

    _________
    broquaint

[reply]
      Thanks for the reply. I understand the package variable bit, thanks, but when (or how) does this get destroyed? Does the package reference survive until the script dies?
[reply]
        Correct, package variables live until perl exits, or the variable is explicitly destroyed. If you want shorter lived variables then you'll want to be using lexical variables who's life-span is tied to the length of the surrounding lexical scope (unless they're referenced from a higher scope). For more information on lexical variables see. Lexical scoping like a fox.
        HTH

        _________
        broquaint

[reply]
Re: Undef doesnt work?This has got me stumped
by borisz (Canon) on Feb 09, 2004 at 10:19 UTC
    It works as expected. self->{blah} is only one on every iteration. But you increment also the global var blah every time you call  printblah. try to start your script with use strict; to note such errors.
    Boris
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Re: Undef doesnt work?This has got me stumped
by Corion (Patriarch) on Feb 09, 2004 at 10:22 UTC

    I'm not exactly sure what you expect to happen, but I'm trying to interpret what you see:

    You're not using strict, which is always a bad idea. In your case, it is a bad idea, because it allows you to be lax about where $blah is defined and used. Perl thinks you want $blah to be used as a global variable, that is why you see the value of $blah increasing in each call to printblah.

    In testob::new(), you never give $self->{blah} a value, so it will be 1 after you increment it, which is then what is printed. If you want to hold on to $testo in your main loop, you would need to keep it declared on the outside of the loop, or turn your testob::new() subroutine into a subroutine that only returns one and the same object (fancy speaking people call this a "Singleton"):

    
{ my $singleton;
sub new {
  if (! $singleton) {
    my $class=shift;
    $singleton = {@_};

    bless($singleton,$class);
  };
  return $singleton;    
};

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