Question

Assume I have a muti-level hash:

$class->{‘key1’}->{‘key2’}->{$key3}->{‘string’}->{$key5},
where $class->{‘key1’}->{‘key2’}->{$key3}->{‘string’}->{$key5}
equals to some integer number.
$key3 can be class name like "music", "english"…
$key5 can be student name like "mary", "luke"…

Will the following operation release all the memory under level $key3="music"?
i.e. will the memory assigned to $key5 be released?

$current_class = $class->{‘key1’}->{‘key2’}->{"music"};
$current_class = undef;

Update:

thanks both. My understanding between delete and undef is: delete will remove the entry of key='music'

so $class->{'key1'}->{'key2'}->{"music"} will not exist. while undef will set value of $class->{'key1'}->{'key2'}->{"music"} as undef. but entry of key='music' is still there but the value of it will be undef so $class->{'key1'}->{'key2'}->{"music"} = undef.

so I should delete the entry, is it correct?

but do you mean

undef $class->{‘key1’}->{‘key2’}->{"music"};

and

$class->{‘key1’}->{‘key2’}->{"music"} = undef;

are different?

Answer 1

Perl has reference counting garbage collector.

$ cat test.pl

#!/usr/bin/perl
use strict;
package Cat;
sub new { my ($class,$name) = @_; print "cat $name here\n"; bless \$name, $class; }
sub DESTROY { my $self = shift; print "  cat $$self gone\n"; }

package main;
my $ref->{aaa}->{bbb}->{ccc} = new Cat 'Alfred';
print  "111111\n";
$ref->{aaa} = 'qweqwe';
print  "222222\n";

$ ./test.pl

cat Alfred here
111111
  cat Alfred gone
222222