Advanced Perl Programming

Tying Hashes

Accesses to tied hash tables are fully supported, unlike arrays. The tie-hash facility allows you to trap operations on the entire hash table (%h = ()), accesses to individual elements, and queries (exists, defined, each, keys, and values). Table 9.3 shows how these actions are mapped to method invocations on the tied object.

Table 9-3. tie and Hash Access

When you say:	Perl translates it to:
tie %h, 'Foo', 'a' => 1	$obj = Foo-->TIEHASH('a',1);
$h{a}	$obj-->FETCH ('a')
$h{a} = 1	$obj-->STORE ('a', 1)
delete $h{a}	$obj-->DELETE('a')
exists $h{a}	$obj-->EXISTS('a')
keys (%h),values(%h) each (%h)	$lk = $obj-->FIRSTKEY (); do { $val = $obj-->FETCH{$lk}; } while ($lk = $obj-->NEXTKEY($lk));
%h = ()	$obj-->CLEAR()
%h = (a=> 1)	$obj-->CLEAR() $obj-->STORE('a',1)
untie %h	$obj-->DESTROY()

FIRSTKEY and NEXTKEY are expected to return the next key in the sequence. This suffices if keys is invoked by the calling code; but if values or each is called, it calls FETCH for each key.

The most common (and natural-looking) use of tie is as a frontend for DBM files, which, as we mentioned earlier, are disk-based hash tables. Perl comes enabled with various flavors of DBM support. The following example uses the SDBM module, which comes with the standard Perl distribution:

use Fcntl; use SDBM_File; tie (%h, 'SDBM_File', 'foo.dbm', O_RDWR|O_CREAT, 0640) || die $!; # Open dbm file $h{a} = 10; # Write to file transparently while (($k, $v) = each %h) { # Iterate over ...