FAQ 4.70 How can I use a reference as a hash key?

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4.70: How can I use a reference as a hash key?

    (contributed by brian d foy and Ben Morrow)

    Hash keys are strings, so you can't really use a reference as the key.
    When you try to do that, perl turns the reference into its stringified
    form (for instance, "HASH(0xDEADBEEF)"). From there you can't get back
    the reference from the stringified form, at least without doing some
    extra work on your own.

    Remember that the entry in the hash will still be there even if the
    referenced variable goes out of scope, and that it is entirely possible
    for Perl to subsequently allocate a different variable at the same
    address. This will mean a new variable might accidentally be associated
    with the value for an old.

    If you have Perl 5.10 or later, and you just want to store a value
    against the reference for lookup later, you can use the core
    Hash::Util::Fieldhash module. This will also handle renaming the keys if
    you use multiple threads (which causes all variables to be reallocated
    at new addresses, changing their stringification), and
    garbage-collecting the entries when the referenced variable goes out of

    If you actually need to be able to get a real reference back from each
    hash entry, you can use the Tie::RefHash module, which does the required
    work for you.


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