Passing pointers around

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

I want to do the following:

#!/usr/misc/bin/perl5 -w
use strict;

my $blahblah = 5;
my $ref_to_blahblah = \$blahblah;

print "$$ref_to_blahblah\n";
fubar($ref_to_blahblah);
print "$$ref_to_blahblah\n";

sub fubar {
   $$_[0] = 4;
   print "$$_[0]\n";
}
[download]

Right now, the output is:
5
4
5
I want to have fubar() change $blahblah, so that the ouput is 5-4-4. I've looked around and I can't find a way to do this. Help!

Comment on Passing pointers around Download Code

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Re: Passing pointers around by extremely (Priest) on Jan 15, 2001 at 02:37 UTC
Change fubar() to this: `sub fubar { ${$_[0]} = 4; print "${$_[0]}\n"; }` [download] Unfortunately for you, this is one of those cases where Perl's Do-What-I-Mean nature happens to do what someone else meant. In effect, what you did was set `${$_}[0] = 4;` which, since $_ isn't declared but is special winds up creating an anonymous array in $_ which has a single item in it, 4. All that without complaining since $_ has so much magic surrounding it. =) The problem there is that perl binds "$" stronger than "[]" so it deref's $_ rather than deref'ing @_'s first item. HTH, BTW this sort of question really belongs in Seekers of Perl Wisdom...	[reply] [d/l] [select]
Re: Passing pointers around by a (Friar) on Jan 15, 2001 at 11:24 UTC
This seems to work: `sub fubar { my $ref = shift; ${$ref} = 4; print "$$ref\n"; }` [download] and I'm assuming you're using ref's to scalars as a learning experience. I dunno if the $$_&lbr;0] (sorry, forgot the code for left bracket) is putting another layer of ref in there. Likewise: `feebar(\$blahblah); sub feebar { my $ref = shift; $$ref = 4; print "$$ref\n"; }` [download] works too. More questions? Buy Object Oriented Perl (by D. Conway) As good a 30 bucks as you can spend on perl manuals (not the best, but as good as ... ;-). a	[reply] [d/l] [select]
Re: Passing pointers around by repson (Chaplain) on Jan 15, 2001 at 07:03 UTC
How about `my $blahblah = 5; print "$blahblah\n"; fubar($blahblah); print "$blahblah\n"; sub fubar { $_[0] = 4; print "$_[0]\n"; }` [download]	[reply] [d/l]