FAQ 7.14 What is variable suicide and how can I prevent it?
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From: PerlFAQ Server on 10 May 2010 18:00 This is an excerpt from the latest version perlfaq7.pod, which comes with the standard Perl distribution. These postings aim to reduce the number of repeated questions as well as allow the community to review and update the answers. The latest version of the complete perlfaq is at http://faq.perl.org . -------------------------------------------------------------------- 7.14: What is variable suicide and how can I prevent it? This problem was fixed in perl 5.004_05, so preventing it means upgrading your version of perl. ;) Variable suicide is when you (temporarily or permanently) lose the value of a variable. It is caused by scoping through my() and local() interacting with either closures or aliased foreach() iterator variables and subroutine arguments. It used to be easy to inadvertently lose a variable's value this way, but now it's much harder. Take this code: my $f = 'foo'; sub T { while ($i++ < 3) { my $f = $f; $f .= "bar"; print $f, "\n" } } T; print "Finally $f\n"; If you are experiencing variable suicide, that "my $f" in the subroutine doesn't pick up a fresh copy of the $f whose value is <foo>. The output shows that inside the subroutine the value of $f leaks through when it shouldn't, as in this output: foobar foobarbar foobarbarbar Finally foo The $f that has "bar" added to it three times should be a new $f "my $f" should create a new lexical variable each time through the loop. The expected output is: foobar foobar foobar Finally foo -------------------------------------------------------------------- The perlfaq-workers, a group of volunteers, maintain the perlfaq. They are not necessarily experts in every domain where Perl might show up, so please include as much information as possible and relevant in any corrections. The perlfaq-workers also don't have access to every operating system or platform, so please include relevant details for corrections to examples that do not work on particular platforms. Working code is greatly appreciated. If you'd like to help maintain the perlfaq, see the details in perlfaq.pod. |