Perl Module Basics

Finally, you must provide an iterator by implementing FIRSTKEY() and NEXTKEY().

The iterator functions are used when several Perl operators are called keys(),

values(), and each(). The utility of allowing users to iterate over DNS lookups in 

the cache is questionable, but here's a possible implementation:

sub FIRSTKEY {

  my $self = shift;

  # reset iterator for the cache

  scalar keys %{$self >{cache}};

  # return the first key from the cache

  return scalar each %{$self >{cache}};

}

sub NEXTKEY {

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

   return scalar each %{$self >{cache}};

}

This implementation just proxies the call to each() on the underlying cache. As a 

result, it doesn't use the second parameter to NEXTKEY() the last key returned. This 

can be useful if the underlying data store isn't a hash but rather something that 

maintains an order.

Other Ties

In addition to scalars and hashes, you can also tie arrays and file handles. Once 

you've grokked implementing tied scalars and hashes, it's just a matter of learning 

the specific method names and interfaces. You can get this information from Perl's 

documentation with the command perlpod perltie.

Tying and Overloading

You might imagine that you could combine tying and overloading to form the 

ultimate magic Perl module. Unfortunately, due to a bug in the implementation of 

overloading in the version of Perl I'm using (5.6.1) and older versions, this isn't 

easily done. You can find the rather complicated details in the overload documen 

tation, but suffice it to say that for the time being you should choose tying or

overloading, not both.

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