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Re: autovivication and referencing

by sundialsvc4 (Abbot)
on Nov 20, 2013 at 14:11 UTC ( #1063532=note: print w/ replies, xml ) Need Help??

in reply to autovivication and referencing

Also keep in mind that the first scenario specifies a reference to a variable, other_hash.   So, you now have two or more references to the same chunk of storage.   (Fortran calls that EQUIVALENCE.)

Data::Dumper makes this clear.   Consider this:

use strict; use warnings; use Data::Dumper; my @a = (1, 2); my $b = { "c"=>\@a }; print Dumper(\@a, $b); $b->{"c"}[0]=4; print Dumper(\@a, $b);'
$VAR1 = [ 1, 2 ]; $VAR2 = { 'c' => $VAR1 }; $VAR1 = [ 4, 2 ]; $VAR2 = { 'c' => $VAR1 };

The dumper knows that the hash-key points to the same block of storage that the array-variable does, so it refers to it in the second case by its name.   And you can see how a change, made by means of the hash-key, changed the value in the array variable, because the two are the same.   This is a very easy mistake to make in a production setting.

Also note that “references” will work in this way, whether-or-not a particular reference comes into existence by means of the “autovivification” feature.

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