Re: The latest & greatest in 5.8.x?

What else is great? Here's a couple of my favorites:

Weak references. Ever had interdependent objects? Perl 5.005:
```
package A;
sub new { 
  my $class = shift;
  my $this = bless {}, $class;
  $this->{-twin} = new B(-twin => $this);
  return $this;
}
package B;
sub new {
  my $class = shift;
  bless {@_}, $class;
}
$obj = new A;
undef $obj;
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```
The A object formerly referenced by $obj will not go away until the program exits, even after $obj itself is undefed. Why? Because there's a B object that references it. And that B object won't go away because that A object references it too. So it's like the A and B are pointing at each other, keeping each other alive (this is called a circular reference.)
Perl 5.6 added some magic called a 'weak reference'.
```
package A;
sub new { 
  my $class = shift;
  my $this = bless {}, $class;
  $this->{-twin} = new B(-twin => $this);
  return $this;
}
package B;
sub new {
  my $class = shift;
  my $this = bless {@_}, $class;
  weaken $this->{-twin};
  return $this;
}
$obj = new A;
undef $obj;
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```
See the 'weaken'? That specific reference is now "weak", and does not actually add to the reference count (so the only reference to the A object is $obj). When $obj is then undefined, the reference count goes to zero, and any weak references are automatically set to undef by Perl. It's perfect for when you need backreferences that should not keep their referents alive.

Builtin version of IO::Scalar

$bar = "this\nthat\n";
open(FH, '<', \$bar);
$this = <FH>; # reads "this\n"
$that = <FH>; # reads "that\n"
open(FH, '>', \$bar);
print FH "this\n";
print FH "that\n";
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This makes it very easy to convert a function that displays text to STDOUT into one returning its output as a string:

# $foo = capture { print 'hi'; }
# $foo will be filled as if someone had done $foo = `perl -e { print '
+hi'; }` or something like that
# if the sub prints directly to STDOUT, like:  print STDOUT 'hi'; then
+ the output will not be captured
sub capture(&) {
    my ($oldsel, $fh, $ret);
    open($fh, '>', \$ret) or die "can't capture: $!";
    $oldsel = select($fh);
    &{$_[0]};
    select $oldsel;
    close($fh);
    $ret;
}
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--Stevie-O

$"=$,,$_=q>|\p4<6 8p<M/_|<('=>
.q>.<4-KI<l|2$<6%s!<qn#F<>;$,
.=pack'N*',"@{[unpack'C*',$_]
}"for split/</;$_=$,,y[A-Z a-z]
         {}cd;print lc
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Comment on Re: The latest & greatest in 5.8.x? Select or Download Code

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Re: Re: The latest & greatest in 5.8.x? by drewbie (Chaplain) on Apr 21, 2004 at 17:31 UTC
Oh yes, I had forgotten about weak references. I personally don't have a use for them (yet), but I've seen a couple modules that use it which I couldn't install for that reason. A built-in IO::Scalar is awesome! I use that pretty regularly, especially in unit tests. In fact, our co-branding code uses it extensively. Anything that makes it faster/easier/better to capture STDOUT will definitely be a win for us.	[reply]

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Re: Re: The latest & greatest in 5.8.x?
by drewbie (Chaplain) on Apr 21, 2004 at 17:31 UTC

A built-in IO::Scalar is awesome! I use that pretty regularly, especially in unit tests. In fact, our co-branding code uses it extensively. Anything that makes it faster/easier/better to capture STDOUT will definitely be a win for us.

[reply]