reference as key to hash

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

You get to see a piece of perl code which (condensed) reads:

my $v;
# ...
$v = $hash{$foo}->{$bar};

# ... some lines later the same location
# has to be retrieved to check for changes
# to the multilevel hash

$ref = [ $foo, $bar ];

my $nv = $hash{$ref};
if ($v eq $nv) {
    ...
[download]

and you are told that this just works. Can it? does it? how so?

Comment on reference as key to hash Select or Download Code

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Re: reference as key to hash by ikegami (Patriarch) on Apr 24, 2009 at 01:54 UTC
To present the interface of a hash while not actually being a hash requires a feature called "magic". `tie` is a specific form of this magic, but not the only one. For example, `%ENV` is magical, but isn't tied. It's possible to overload hash dereference, but I don't see how that would help here even if you were using a hash reference and not a hash. So what's the real question?	[reply] [d/l] [select]
Re^2: reference as key to hash by shmem (Chancellor) on Apr 25, 2009 at 15:34 UTC
No real question here, only a mildly interesting puzzle. I recently realized that stringification of hash keys is true only for perl builtin hashes, not for tied hashes, and found the disguise as a SoPW appropriate for sharing the finding...	[reply]
Re: reference as key to hash by spx2 (Deacon) on Apr 24, 2009 at 15:09 UTC
why would you use a reference as a key to a hash ? references are ephemeral in many ways , what if you want to itertate over them later, when other items have taken the references that the old ones had when you made the hash ? I think this is not a very good idea.	[reply]
Re^2: reference as key to hash by ikegami (Patriarch) on Apr 26, 2009 at 03:07 UTC
why would you use a reference as a key to a hash ? You may have noticed the reference wasn't used as the key, just as a means of passing a more complex key.	[reply]
Re^2: reference as key to hash by shmem (Chancellor) on Apr 24, 2009 at 23:13 UTC
references are ephemeral in many ways So is anything you program or do. But while in scope you are present.	[reply]