


Hash::Util(3p)   Perl Programmers Reference Guide  Hash::Util(3p)


NAME
       Hash::Util - A selection of general-utility hash
       subroutines

SYNOPSIS
         # Restricted hashes

         use Hash::Util qw(
                            hash_seed all_keys
                            lock_keys unlock_keys
                            lock_value unlock_value
                            lock_hash unlock_hash
                            lock_keys_plus hash_locked
                            hidden_keys legal_keys
                          );

         %hash = (foo => 42, bar => 23);
         # Ways to restrict a hash
         lock_keys(%hash);
         lock_keys(%hash, @keyset);
         lock_keys_plus(%hash, @additional_keys);

         # Ways to inspect the properties of a restricted hash
         my @legal = legal_keys(%hash);
         my @hidden = hidden_keys(%hash);
         my $ref = all_keys(%hash,@keys,@hidden);
         my $is_locked = hash_locked(%hash);

         # Remove restrictions on the hash
         unlock_keys(%hash);

         # Lock individual values in a hash
         lock_value  (%hash, 'foo');
         unlock_value(%hash, 'foo');

         # Ways to change the restrictions on both keys and values
         lock_hash  (%hash);
         unlock_hash(%hash);

         my $hashes_are_randomised = hash_seed() != 0;

DESCRIPTION
       "Hash::Util" and "Hash::Util::FieldHash" contain special
       functions for manipulating hashes that don't really
       warrant a keyword.

       "Hash::Util" contains a set of functions that support
       restricted hashes. These are described in this document.
       "Hash::Util::FieldHash" contains an (unrelated) set of
       functions that support the use of hashes in inside-out
       classes, described in Hash::Util::FieldHash.

       By default "Hash::Util" does not export anything.




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Hash::Util(3p)   Perl Programmers Reference Guide  Hash::Util(3p)


   Restricted hashes
       5.8.0 introduces the ability to restrict a hash to a
       certain set of keys.  No keys outside of this set can be
       added.  It also introduces the ability to lock an
       individual key so it cannot be deleted and the ability to
       ensure that an individual value cannot be changed.

       This is intended to largely replace the deprecated pseudo-
       hashes.

       lock_keys
       unlock_keys
             lock_keys(%hash);
             lock_keys(%hash, @keys);

           Restricts the given %hash's set of keys to @keys.  If
           @keys is not given it restricts it to its current
           keyset.  No more keys can be added. delete() and
           exists() will still work, but will not alter the set
           of allowed keys. Note: the current implementation
           prevents the hash from being bless()ed while it is in
           a locked state. Any attempt to do so will raise an
           exception. Of course you can still bless() the hash
           before you call lock_keys() so this shouldn't be a
           problem.

             unlock_keys(%hash);

           Removes the restriction on the %hash's keyset.

           Note that if any of the values of the hash have been
           locked they will not be unlocked after this sub
           executes.

           Both routines return a reference to the hash operated
           on.

       lock_keys_plus
             lock_keys_plus(%hash,@additional_keys)

           Similar to "lock_keys()", with the difference being
           that the optional key list specifies keys that may or
           may not be already in the hash. Essentially this is an
           easier way to say

             lock_keys(%hash,@additional_keys,keys %hash);

           Returns a reference to %hash

       lock_value
       unlock_value
             lock_value  (%hash, $key);
             unlock_value(%hash, $key);




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Hash::Util(3p)   Perl Programmers Reference Guide  Hash::Util(3p)


           Locks and unlocks the value for an individual key of a
           hash.  The value of a locked key cannot be changed.

           Unless %hash has already been locked the key/value
           could be deleted regardless of this setting.

           Returns a reference to the %hash.

       lock_hash
       unlock_hash
               lock_hash(%hash);

           lock_hash() locks an entire hash, making all keys and
           values read-only.  No value can be changed, no keys
           can be added or deleted.

               unlock_hash(%hash);

           unlock_hash() does the opposite of lock_hash().  All
           keys and values are made writable.  All values can be
           changed and keys can be added and deleted.

           Returns a reference to the %hash.

       lock_hash_recurse
       unlock_hash_recurse
               lock_hash_recurse(%hash);

           lock_hash() locks an entire hash and any hashes it
           references recursively, making all keys and values
           read-only. No value can be changed, no keys can be
           added or deleted.

           Only recurses into hashes that are referenced by
           another hash. Thus a Hash of Hashes (HoH) will all be
           restricted, but a Hash of Arrays of Hashes (HoAoH)
           will only have the top hash restricted.

               unlock_hash_recurse(%hash);

           unlock_hash_recurse() does the opposite of
           lock_hash_recurse().  All keys and values are made
           writable.  All values can be changed and keys can be
           added and deleted. Identical recursion restrictions
           apply as to lock_hash_recurse().

           Returns a reference to the %hash.

       hash_unlocked
             hash_unlocked(%hash) and print "Hash is unlocked!\n";

           Returns true if the hash and its keys are unlocked.





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Hash::Util(3p)   Perl Programmers Reference Guide  Hash::Util(3p)


       legal_keys
             my @keys = legal_keys(%hash);

           Returns the list of the keys that are legal in a
           restricted hash.  In the case of an unrestricted hash
           this is identical to calling keys(%hash).

       hidden_keys
             my @keys = hidden_keys(%hash);

           Returns the list of the keys that are legal in a
           restricted hash but do not have a value associated to
           them. Thus if 'foo' is a "hidden" key of the %hash it
           will return false for both "defined" and "exists"
           tests.

           In the case of an unrestricted hash this will return
           an empty list.

           NOTE this is an experimental feature that is heavily
           dependent on the current implementation of restricted
           hashes. Should the implementation change, this routine
           may become meaningless, in which case it will return
           an empty list.

       all_keys
             all_keys(%hash,@keys,@hidden);

           Populates the arrays @keys with the all the keys that
           would pass an "exists" tests, and populates @hidden
           with the remaining legal keys that have not been
           utilized.

           Returns a reference to the hash.

           In the case of an unrestricted hash this will be
           equivalent to

             $ref = do {
                 @keys = keys %hash;
                 @hidden = ();
                 \%hash
             };

           NOTE this is an experimental feature that is heavily
           dependent on the current implementation of restricted
           hashes. Should the implementation change this routine
           may become meaningless in which case it will behave
           identically to how it would behave on an unrestricted
           hash.

       hash_seed
               my $hash_seed = hash_seed();




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Hash::Util(3p)   Perl Programmers Reference Guide  Hash::Util(3p)


           hash_seed() returns the seed number used to randomise
           hash ordering.  Zero means the "traditional" random
           hash ordering, non-zero means the new even more random
           hash ordering introduced in Perl 5.8.1.

           Note that the hash seed is sensitive information: by
           knowing it one can craft a denial-of-service attack
           against Perl code, even remotely, see "Algorithmic
           Complexity Attacks" in perlsec for more information.
           Do not disclose the hash seed to people who don't need
           to know it.  See also "PERL_HASH_SEED_DEBUG" in
           perlrun.

       hv_store
             my $sv = 0;
             hv_store(%hash,$key,$sv) or die "Failed to alias!";
             $hash{$key} = 1;
             print $sv; # prints 1

           Stores an alias to a variable in a hash instead of
           copying the value.

   Operating on references to hashes.
       Most subroutines documented in this module have equivalent
       versions that operate on references to hashes instead of
       native hashes.  The following is a list of these subs.
       They are identical except in name and in that instead of
       taking a %hash they take a $hashref, and additionally are
       not prototyped.

       lock_ref_keys
       unlock_ref_keys
       lock_ref_keys_plus
       lock_ref_value
       unlock_ref_value
       lock_hashref
       unlock_hashref
       lock_hashref_recurse
       unlock_hashref_recurse
       hash_ref_unlocked
       legal_ref_keys
       hidden_ref_keys

CAVEATS
       Note that the trapping of the restricted operations is not
       atomic: for example

           eval { %hash = (illegal_key => 1) }

       leaves the %hash empty rather than with its original
       contents.

BUGS
       The interface exposed by this module is very close to the



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Hash::Util(3p)   Perl Programmers Reference Guide  Hash::Util(3p)


       current implementation of restricted hashes. Over time it
       is expected that this behavior will be extended and the
       interface abstracted further.

AUTHOR
       Michael G Schwern <schwern@pobox.com> on top of code by
       Nick Ing-Simmons and Jeffrey Friedl.

       hv_store() is from Array::RefElem, Copyright 2000 Gisle
       Aas.

       Additional code by Yves Orton.

SEE ALSO
       Scalar::Util, List::Util and "Algorithmic Complexity
       Attacks" in perlsec.

       Hash::Util::FieldHash.







































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