Problem

When you use keys on a hash whose keys are references, the references that keys returns no longer work. This situation often arises when you want to cross-reference two different hashes.

Solution

Use Tie::RefHash:

use Tie::RefHash;
tie %hash, "Tie::RefHash";
# you may now use references as the keys to %hash

Discussion

Hash keys are automatically "stringified," that is, treated as though they appeared between double quotes. In the case of numbers or strings, nothing is lost. This isn't the case with references, though.

Stringified references look like these:

Class::Somewhere=HASH(0x72048)
ARRAY(0x72048)

A stringified reference can't be dereferenced, because it is just a string and no longer a reference. This means you can't use references as the keys to a hash without losing their "magic."

Hand-rolled solutions to this problem involve maintaining a distinct hash whose keys are stringified references and whose values are the actual references. This is what Tie::RefHash does. We'll use IO objects for filehandles here to show you that even such strange references can be used to index a hash tied with Tie::RefHash.

Here's an example:

use Tie::RefHash;
use IO::File;

tie %name, "Tie::RefHash";
foreach $filename ("/etc/termcap", "/vmunix", "/bin/cat") {
    $fh = IO::File->new("< $filename") or next;
    $name{$fh} = $filename;
}
print "open files: ", join(", ", values %name), "\n";
foreach $file (keys %name) {
    seek($file, 0, 2);      # seek to the end
    printf("%s is %d bytes long.\n", $name{$file}, tell($file));
}

If you're storing objects as the keys to a hash, though, you almost always should be storing a unique attribute of the object (e.g., name or ID number) instead.

See Also

The documentation for the standard Tie::RefHash module; the "Hard References Don't Work as Hash Keys" section of Chapter 4 of Programming Perl, and the "Warning" section of perlref (1)