Chapter 2

Tying Hashes

By far the most commonly used tied interface is the tied hash. Hashes are so inher 

ently flexible that they lend themselves well to representing an interface to a variety of 

data types. In fact, Perl's support for tied variables evolved from support for tying 

hashes to database files using dbmopen() into the general mechanism it is today.

One common use for tied hashes is to provide lazy computation and caching 

for some large dataset. For example, BOA::Network::DNS provides a tied hash 

interface for network name to IP mappings. Here's an example using the module:

use BOA::Network::DNS;

# tie hash to BOA::Network::DNS   provide nameserver as argument to constructor

tie %dns,  BOA::Network::DNS , nameserver =>  10.0.0.1 ;

# lookup IP address for www.perl.com

print "www.perl.com : ", $dns{ www.perl.com } || "not found!", "\n";

# do a reverse lookup for the DNS server

print "The name for the DNS server is: ", $dns{ 10.0.0.1 } || "not found!", "\n";

Obviously it would be impossible to prepopulate a hash with all the possible 

names and addresses on the Internet, but a tied hash allows you to pretend that 

you have. Also, as you'll see, the hash can very easily hold onto the results of past 

lookups to improve performance.

To implement a tied hash interface, you must provide eight methods 

TIEHASH(), FETCH(), STORE(), DELETE(), EXISTS(), CLEAR(), FIRSTKEY(), and NEXTKEY().

Here's TIEHASH() the constructor:

package BOA::Network::DNS;

sub TIEHASH {

   my $pkg = shift;

   my $self = { @_ }; # retrieve named options into $self hash ref

   # check for required  nameserver  option

   croak("Missing nameserver address!") unless exists $self >{nameserver};

   # initialize cache to an empty hash

   $self >{cache} = {};

   # bless $self and return

   return bless($self, $pkg);

}

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