---
name: "Tie::RefHash::Weak"
section: "3pm"
platform: "linux"
package: "libtie-refhash-weak-perl"
version: "0.09-2.1"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/Tie::RefHash::Weak"
---

# Tie::RefHash::Weak(3pm)

A Tie::RefHash subclass with weakened references in the keys.

## NAME

Tie::RefHash::Weak - A Tie::RefHash subclass with weakened references in the keys.

## SYNOPSIS

```
        use Tie::RefHash::Weak;
        tie my %h, 'Tie::RefHash::Weak';
        # OR:
        use Tie::RefHash::Weak 'fieldhash';
        fieldhash my %h;
        { # new scope
                my $val = "foo";
                $h{\$val} = "bar"; # key is weak ref

                print join(", ", keys %h); # contains \$val, returns regular reference
        }
        # $val goes out of scope, refcount goes to zero
        # weak references to \$val are now undefined
        keys %h; # no longer contains \$val
        # see also Tie::RefHash
```

## DESCRIPTION

The Tie::RefHash module can be used to access hashes by reference. This is useful when you index by object, for example.

The problem with Tie::RefHash, and cross indexing, is that sometimes the index should not contain strong references to the objecs. Tie::RefHash's internal structures contain strong references to the key, and provide no convenient means to make those references weak.

This subclass of Tie::RefHash has weak keys, instead of strong ones. The values are left unaltered, and you'll have to make sure there are no strong references there yourself.

## FUNCTIONS

For compatibility with Hash::Util::FieldHash, this module will, upon request, export the following two functions. You may also write `"use Tie::RefHash::Weak ':all'"`.

- fieldhash %hash

  This ties the hash and returns a reference to it.

- fieldhashes \\%hash1, \\%hash2 ...

  This ties each hash that is passed to it as a reference. It returns the list of references in list context, or the number of hashes in scalar context.

## THREAD SAFETY

Tie::RefHash version 1.32 and above have correct handling of threads (with respect to changing reference addresses). If your module requires Tie::RefHash::Weak to be thread aware you need to depend on both Tie::RefHash::Weak and Tie::RefHash version 1.32 (or later).

Version 0.02 and later of Tie::RefHash::Weak depend on a thread-safe version of Tie::RefHash anyway, so if you are using the latest version this should already be taken care of for you.

## 5.10.0 COMPATIBILITY

Due to a minor change in Perl 5.10.0 a bug in the handling of magic freeing was uncovered causing segmentation faults.

This has been patched but not released yet, as of 0.08.

## CAVEAT

You can use an LVALUE reference (such as `"\substr ..."`) as a hash key, but due to a bug in perl (see \<http://rt.perl.org/rt3/Public/Bug/Display.html?id=46943\>) it might not be  possible to weaken a reference to it, in which case the hash element will  never be deleted automatically.

## AUTHORS

Yuval Kogman \<nothingmuch@woobling.org\>

some maintenance by Hans Dieter Pearcey \<hdp@pobox.com\>

## COPYRIGHT & LICENSE

```
        Copyright (c) 2004 Yuval Kogman. All rights reserved
        This program is free software; you can redistribute
        it and/or modify it under the same terms as Perl itself.
```

## SEE ALSO

Tie::RefHash, Class::DBI (the live object cache), "Perl\_magic\_killbackrefs" in mg.c