NAME

Tie::HashDefaults - Let a hash have default values

SYNOPSIS

use Tie::HashDefaults;

tie %h, 'Tie::HashDefaults', \%defaults1, \%defaults0;

DESCRIPTION

     This creates a data structure which is essentially an array
     of hashes; this list contains all the hashes (passed by ref)
     in the argument list; but it also contains a new, internally
     created, anonymous hash.  This hash is used to store any
     insertions into the tied hash.

     Whenever a fetch (or an exists) is done on the tied hash,
     the requested key is searched for in each hash in the list,
     beginning with the internal "storage" hash; if it is not
     found in that hash, the key is looked for in the first
     default hash, then the next, and so on, until it is found in
     one of them, or there are none left to search.

     When an iteration (keys or each) is done on the tied hash,
     the set of keys returned is the union of keys from all of
     the default hashes, along with the storage hash.

     For operations that alter a hash -- store, delete, clear --
     the default hashes are never touched.  Only the storage hash
     is cleared.  One effect of this is that if the tied hash is
     cleared, e.g. via %h = ();, and immediately following that
     an iteration is started (via keys or each), it is likely
     that some keys will be returned.  (Unless, of course, there
     is no data in any of the given default hashes.)

     Manipulating the Defaults List

     The list of default hashes can be manipulated directly.  To
     do this, a special method on the tied object returns an
     array, by reference, containing the list of default hashes.
     Any changes to this array are reflected inside the
     Tie::HashDefaults object.  For example, to add another
     defaults source that takes precedence over the others
     already on the list:

       unshift @{ tied(%h)->get_defaults_list }, \%new_default_source;

     Or, to reverse the order in which the defaults are
     consulted:

       $ar = tied(%h)->get_defaults_list;
       @$ar = reverse @$ar;

     (Once you have the array-ref "handle" on the defaults list
     array, it's good for as long as the tied object stays tied.)

     NOTE: calling get_defaults_list also resets the iterator; so
     don't call it within an each loop on a hash tied to this
     class.

AUTHOR

jdporter@min.net (John Porter)

COPYRIGHT

     This is free software.  This software may be modified and
     distributed under the same terms as Perl itself.