Weaken references in Perl

melo@simplicidade.org (Pedro Melo) — Sat, 02 Jul 2005 17:43:07 +0000

In case you need circular structures in Perl, you should known about Scalar::Util and it’s weaken function.

See this example:

package T;

sub DESTROY {
  my $self = shift;
  print STDERR "Bye bye: $self->{name}\n";
}

package main;
use Scalar::Util qw( weaken );

{
  my $a = bless { name => 'a' }, 'T';
  my $b = bless { name => 'b' }, 'T';

  $a->{b} = $b;
  $b->{a} = $a;
}
print "Should see a destroy 'a' and 'b', but you wont...\n";

{
  my $c = bless { name => 'c' }, 'T';
  my $d = bless { name => 'd' }, 'T';

  $c->{d} = weaken($d);
  $d->{c} = weaken($c);
}
print "Should see a destroy 'c' and 'd'!\n";
print "Now you'll see a destroy 'a' and 'b'\n";

The output is this:

Should see a destroy 'a' and 'b', but you wont...
Bye bye: d
Bye bye: c
Should see a destroy 'c' and 'd'!
Now you'll see a destroy 'a' and 'b'
Bye bye: b
Bye bye: a

The problem is that in the first block, although the $a and $b are no longer in scope, each one holds a reference to the other, and that prevents them both from being destroyed.

In the second block, each one of $c and $d takes a weak reference to each other. Weak references don’t increment the reference count internal to all Perl variables, so at the end of the block, they are correctly destroyed.

You’ve warned.

References on Notes

Weaken references in Perl