Question

In perl 5.8.5, if I do the following, I don’t get an error:

use strict;

my $a = undef;
foreach my $el (@$a) {
  ...whatever
}

What’s going on here? Printing out the output of ref($a) shows that $a changes to become a valid array reference at some point. But I never explicitly set $a to anything.

Seems kind of odd that the contents of a variable could change without me doing anything.

Thoughts, anyone?

EDIT: Yes, I know all about auto-vivification. I always thought that there had to be a assignment somewhere along the way to trigger it, not just a reference.

Answer 1

Read Uri Guttman’s article on autovivification.

There is nothing odd about it once you know about it and saves a lot of awkwardness.

Perl first evaluates a dereference expression and sees that the current reference value is undefined. It notes the type of dereference (scalar, array or hash) and allocates an anonymous reference of that type. Perl then stores that new reference value where the undefined value was stored. Then the dereference operation in progress is continued. If you do a nested dereference expression, then each level from top to bottom can cause its own autovivication.