Re^7: Why does exists cause autovivication?

[demerphq]

Exists checks to see that the lookup of $var->{$key} is successful, meaning that $key has been used to store a value in the hash prior to the check. Thus we have:

# $user_by_uid{$uid}->{$host}
$var= $user_by_uid{$uid}
$key= $host

What this means is that if the result of $user_by_uid{$uid} is undef (either by having an explicit undef value or by not being in the %user_by_uid hash at all) then it is autovivified. If there is a hash reference stored in $user_by_uid{$uid} (either intentionally or via autoviv) then the it is checked to see if $host had ever been used as a key.

Tring to reduce this to a one liner: exists checks the the last hash lookup you feed it, not if the full "path" through the data structure exists. This is more obvious if you consider $user_by_id[$uid]->{$host}, obviously exists doesnt operate on arrays, and we get no error, so it means that only ->{$host} part is relevent to exists.

You could always write a simple routine:

sub my_exists {
  my ($hash_ref,$key)= @_;
  return $hash_ref and exists $hash_ref->{$key}
}

which would not autovivify. Compare it to how the real thing *might* be coded. Im pretty sure this actually wont work due to some subtleties of hash lookups and lvalue arguments but its pretty close to the real thing.

sub my_fake_exists {
  my ($hash_ref,$key)=@_;
  $hash_ref={} if not defined $hash_ref; #fake autoviv
  return exists $hash_ref->{$key}; # do the lookup
}

[Argel]

Okay, so is my original example:

next unless exists $user_by_uid{$uid}->{$host};

more like the following:

$href = defined $user_by_uid{$uid} ? $user_by_uid{$uid} : {}; #Fake au
+toviv
next unless exists $href->{$host};

Or would it be more like the following, using your example code?

next unless my_fake_exists $user_by_uid{$uid}, $host;

In other words, since there is no short-circuit evaluation to avoid the -> in my original example does that mean the autoviv occurs before anything is passed in to 'exists'?

Thanks very much for taking time to respond and hope you had a Happy New Year!!

[shmem]

No need to autovivify anything, only to test an empty hashref for a non-existent key.

next unless exists $user_by_uid{$uid};
next unless exists $user_by_uid{$uid}->{$host};

should do.