Re: Re: Autovivification with hash of hashes

[pg]

I am quite confused with your second piece of code. What does it try to demo? What is your expected result (to print out the "popped into existence" or not, I tested it, and it printed)?

Also I don't know whether you realize that you actually used the concept of symblic reference in your second demo.

I changed your second demo a little bit, and added comments to explain:

use strict; #this basically disallows symblic ref
use warnings;

my %foo;

$foo{bar} = "key bar's value"; #now you made $foo{bar} EXISTS, there i
+s no need for auto-vivification to create it any more. MOST IMPORTANT
+LY, this can be a symblic reference depending on how you use it, cont
+inue...

if(exists($foo{qux})) {
    print "\$foo{qux} exists";
}

if(ref $foo{bar} eq 'HASH' && exists($foo{bar}{baz})) {#Here you are s
+trongly suggesting a symblic reference. As for the code, the part bef
+ore && obviously evaluate to false, so that exists after && will be e
+valuated, but no auto-vivfication here, as $foo{bar} exists any way
    print "\$foo{bar}{baz} exists\n";#will not print as there is no $f
+oo{bar}{baz}
}

if(exists($foo{bar})) {#yes
    print "\$foo{bar} popped into existence\n";#print out, but not pop
+ped into existence, it is actually created by you
}

#the following is added by me
$foo{bar}{baz} = 1; #violates strict refs, error, because you set $foo
+{bar} to a string earlier. There is no way to create $foo{bar}{baz} i
+n this context, unless you turn off use strict("refs")

update:

Yes, Aristotle is right, and I messed up with && and ||, Thanks for pointing out.

However this mistake does not reduce the value of this post, and the post as a whole is still correct, especially the discussion on symbolic reference.

I keep the mistake there (Remove the original mistake pointed out by other monks, would make other monk's reply funny, and sounds like irrelevant, and I don't do this ;-)

"And the latter is exactly what he's talking 
about protecting against by using ref."

Doubt.

[Aristotle]

Here you are strongly suggesting a symblic reference. As for the code, the part before && obviously evaluate to false, so that exists after && will be evaluated, but no auto-vivfication here, as $foo{bar} has been created by you earlier

Actually, a) the ref failing will immediately lead to the entire expression failing because he is using &&, not ||. b) If it got executed, strict would cause the exists to fail with a refs stricture violation.

And the latter is exactly what he's talking about protecting against by using ref.

Makeshifts last the longest.

[trs80]

Thank you, that is what I was trying to convey.

I think the most important thing to do when working with structures that may contain anonymous hash refereneces is to avoid using tests that access a possibly non existent hash reference key by name, which, as far as I know, is the only way to cause autovivification. I am not sure what, other then ref could even be used. This link provides some extended examples on the subject.
Autovivification Explained