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



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.

   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);

           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.

       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();

           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 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.



perl v5.16.2                                     2012-10-25                                  Hash::Util(3pm)