Re: Looping through a hash reference is creating a key...?

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Re^2: Looping through a hash reference is creating a key...? by Roy Johnson (Monsignor) on Jun 10, 2004 at 19:13 UTC
I'm going to quibble with your advice regarding `exists`, which could be right or wrong depending on how one reads it. (I don't doubt that your understanding is correct, but I think others might read it differently than you intended.) Autovivification (the creation of intermediate references) is not prevented by using `exists`, so `exists $h{1}{2}` still creates `$h{1}`. To prevent autovivification, one would have to test `exists $h{1}` and only if it were true, examine `$h{1}{2}`. However, they could just test whether `$h{1}` were true and get pretty much the same result -- if it didn't exist before, it won't exist after. The only difference in using `exists` is if `$h{1}` has a non-reference value like zero, empty string, or undef. It seems a little odd to dereference it in those cases. Perhaps a better idea would be to test it with `ref`. The PerlMonk `tr///` Advocate	[reply] [d/l] [select]
Re^3: Looping through a hash reference is creating a key...? by perrin (Chancellor) on Jun 10, 2004 at 19:19 UTC
Good point. I remember finding that out the hard way when I wrote some code that did exists() checks on deeply nested nodes in a huge data structure. My exists checks made the structure grow about 10MB in memory! Adding uglier but more careful code (`if (exists $h{1} && exists $h{1}->{2} && exists ...`) made the problem go away.	[reply] [d/l]
Re^4: Looping through a hash reference is creating a key...? by thor (Priest) on Jun 10, 2004 at 20:20 UTC
I have to wonder if exists could be patched to not have this behavior. Short-circuiting comes to mind. If $h{1} doesn't exist, $h{1}{2} certainly won't. I'd look at it, but I can't code my way out of a paper bag in C. Plus, I have to believe that I'm not the first person to come up with such a notion. Just a thought, thor	[reply]
Re^5: Looping through a hash reference is creating a key...? by dave_the_m (Monsignor) on Jun 10, 2004 at 22:36 UTC
Exists and autovivification (was: Looping through a hash reference...) by Roy Johnson (Monsignor) on Jun 12, 2004 at 01:12 UTC
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Re^2: Looping through a hash reference is creating a key...? by drewbie (Chaplain) on Jun 11, 2004 at 03:41 UTC
FYI, the hash locking feature only started in 5.8.0. From the docs. Restricted hashes 5.8.0 introduces the ability to restrict a hash to a certain set of keys. No keys outside of this set can be added. It also introduces the ability to lock an individual key so it cannot be deleted and the value cannot be changed. This is intended to largely replace the deprecated pseudo-hashes. And hooray for removing pseudohashes! I have the notes from Damian Conway's Advanced OO Perl talk (Boston 2001) that talks a bit about pseudohashes. They were a cool idea, but the implementation was very flawed. The more you read about them, the less interested you became.	[reply]