does if(%{$ptr->{key}}) define?

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

A co-worker just sent me a snippet of code and asked why it behaved like it does, and I can't really figure out why it does, can someone explain this behavior to me?

perl -w -e 'use strict;my $t;
if (scalar (keys %{$t->{r}}))
{print "There are keys\n";}
print "t->{r} = $t->{r}\n"; 
'
t->{r} = HASH(0x80fbc50)
[download]

If I omit the if and only does:

 perl -w -e 'use strict;my $t = undef;
my %p = %{$t->{r}};
print "t->{r} = $t->{r}
";
'
[download]

I get this error:
Can't use an undefined value as a HASH reference at -e line 2.
Why is it behaving like this? I can't see why $t->{r} gets defined in the first snippet

Comment on does if(%{$ptr->{key}}) define? Select or Download Code

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Re: does if(%{$ptr->{key}}) define? by robartes (Priest) on Oct 03, 2002 at 09:08 UTC
Hi jmo, I bet you never would have predicted this when you got up this morning, but you are witnessing a process called autovivification. Basically, when you dereference an undefined value in a context that sssumes it exists, it gets created automatically. In your case: `keys %{$t->{r}}` is the place where the autovivification occurs. You are dereferencing $t as if it were a hashref, so it gets created as a hashref. The same thing happens to $t->{r}, because that gets dereferenced by the keys %{ } construct. Now, my understanding of autovivification is imperfect, so I might be wrong on the subtleties (or even the big picture) here. Somebody else will pipe in if I am. You can get more information on autovivification in perlref. CU Robartes-	[reply] [d/l]
Re: does if(%{$ptr->{key}}) define? by Juerd (Abbot) on Oct 03, 2002 at 08:23 UTC
I think this answers your question. `perl -w -e 'use strict;my $t = undef; my %p = keys %{$t->{r}}; print "t->{r} = $t->{r} "; '` [download] - Yes, I reinvent wheels. - Spam: Visit eurotraQ.	[reply] [d/l]