Chapter 2

exists() needs special support from the tied implementation in the form of 

EXISTS(). To complete the picture, DELETE() must be provided to remove a key from 

the hash, after which it is expected that EXISTS() will return false for that key. It is 

often difficult to decide what behavior to provide for these calls on a tied hash. In 

this case, you'd want to do the simple thing and just examine the underlying cache:

# method called when exists() is called on the hash

sub EXISTS {

   my ($self, $key) = @_;

   return exists $self >{cache}{$key};

}

# method called when delete() is called on the hash

sub DELETE {

   my ($self, $key) = @_;

   # delete both forward and reverse lookups if the key exists

   my $value;

   if (exists $self >{cache}{$key}) {

      $value = $self >{cache}{$key};

      delete $self >{cache}{$value};

      delete $self >{cache}{$key};

   }

   # return deleted value, just like the normal delete()

   return $value;

}

Perl provides a hook for a special case of delete() when the entire hash is 

being cleared. This is triggered by assigning an empty list to a hash:

%dns = ();

It's possible to implement this by looping over the keys and calling DELETE(), but 

there's usually a more efficient implementation. In this case you can just clear the 

cache:

sub CLEAR {

   my $self = shift;

   %{$self >{cache}} = ();

}

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