oracular (3) Hash::SafeKeys.3pm.gz

Provided by: libhash-safekeys-perl_0.04-1build3_amd64 bug

NAME
       Hash::SafeKeys - get hash contents without resetting each iterator


VERSION
       Version 0.04


SYNOPSIS
           use Hash::SafeKeys;
           while (my ($k,$v) = each %hash) {
              if (something_interesting_happens()) {
                 # get keys, values of %hash without resetting
                 # the 'each' iterator above
                 my @k = safekeys %hash;
                 my @v = safevalues %hash;
                 my %copy = safecopy %hash;
              }
           }


DESCRIPTION
       Every hash variable in Perl has its own internal iterator, accessed by the builtin "each", "keys", and
       "values" functions. The iterator is also implicitly used whenever the hash is evaluated in list context.
       The iterator is "reset" whenever "keys" or "values" is called on a hash, including the implicit calls
       when the hash is evaluated in list context. That makes it dangerous to do certain hash operations inside
       a "while ... each" loop:

           while (my($k,$v) = each %hash) {
              ...
              @k = sort keys %hash;               # Infinite loop!
              @v = grep { /foo/ }, values %hash;  # Ack!
              print join ' ', %hash;              # Run away!
           }

       "Hash::SafeKeys" provides alternate functions to access the keys, values, or entire contents of a hash in
       a way that does not reset the iterator, making them safe to use in such contexts:

           while (my($k,$v) = each %hash) {
              ...
              @k = sort safekeys %hash;               # Can do
              @v = grep { /foo/ }, safevalues %hash;  # No problem
              print join ' ', safecopy %hash;         # Right away, sir
           }


FUNCTIONS
   safekeys
   LIST = safekeys HASH
       Like the builtin keys function, returns a list consisting of all the keys of the named hash, in the same
       order that the builtin function would return them in. Unlike "keys", calling "safekeys" does not reset
       the HASH's internal iterator (see each).

   safevalues
   LIST = safevalues HASH
       Like the builtin values function, returns a list consisting of all the values of the named hash, in the
       same order that the builtin function would return them in. Unlike "values", calling "safevalues" does not
       reset the HASH's internal iterator (see each).

   safecopy
   LIST = safecopy HASH
       In list context, returns a shallow copy of the named HASH without resetting the HASH's internal iterator.
       Usually, evaluating a HASH in list context implicitly uses the internal iterator, resetting any existing
       state

   save_iterator_state
   restore_iterator_state
   HANDLE = save_iterator_state($hashref)
   restore_iterator_state($hashref, HANDLE)
       Low-level functions to manipulate the iterator of a hash reference.  The use cases for directly using
       these functions are

       1. Performance
           The absolute fastest way to safely access the keys of a hash is:

               $handle = Hash::Safekeys::save_iterator_state( \%hash );
               @keys = keys %hash;
               Hash::Safekeys::restore_iterator_state( \%hash, $handle );

           This is an improvement over "@keys = safekeys %hash" because it eliminates the O(n) list copy
           operation on return from the "safekeys" function.

       2. Access to aliased values
           The builtin "values" function returns aliases to the internal hash values, allowing you to modify the
           contents of the hash with constructions like

               s/foo/bar/g for values %hash

           As "safevalues %hash" returns a copy of the hash values, "s/foo/bar/g for safevalues %hash" will not
           modify the contents of the hash.

           To safely modify the values of the hash, a workaround with the low-level iterator functions is

               $handle = Hash::SafeKeys::save_iterator_state( \%hash );
               for (values %hash) { ... modify($_) ... }
               Hash::SafeKeys::restore_iterator_state( \%hash, $handle );

       3. Nested each calls on the same hash
           This construction will not work if $hash1 and $hash2 refer to the same hash:

               while (($key1,$val1) = each %$hash1) {
                   while (($key2,$val2) = each %$hash2) { ... }
               }

           but this construction is safe:

               while (($key1,$val1) = each %$hash1) {
                   $handle = Hash::SafeKeys::save_iterator_state($hash2);
                   while (($key2,$val2) = each %$hash2) { ... }
                   Hash::SafeKeys::restore_iterator_state($hash2, $handle);
               }

           The HANDLE that is returned by "save_iterator_state" and used as an input to "restore_iterator_state"
           is currently implemented as an integer that can be mapped internally to an original hash iterator.
           This implementation is subject to change in future releases and you should not rely on this value
           being an integer.

           It is a grave error to provide a different hash reference with the handle to the
           "restore_iterator_state" call than you provided to the "save_iterator_state" call that created the
           handle.

           Calling "save_iterator_state" without later calling "restore_iterator_state" will leak memory.


EXPORT
       "safekeys", "safevalues", and "safecopy" are all exported by default. Invoke Hash::SafeKeys with the
       empty arg list

           use Hash::SafeKeys ();

       if you don't want these functions to be imported into the calling package.

       The low-level iterator functions "save_iterator_state" and "restore_iterator_state" may also be exported
       by including them in the "use" call or by using the tag ":all"

           use Hash::SafeKeys ':all';   # also exports low-level iterator funcs


AUTHOR
       Marty O'Brien, "<mob at cpan.org>"


BUGS
       Please report any bugs or feature requests to "bug-hash-safekeys at rt.cpan.org", or through the web
       interface at <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Hash-SafeKeys>.  I will be notified, and
       then you'll automatically be notified of progress on your bug as I make changes.


SUPPORT
       You can find documentation for this module with the perldoc command.

           perldoc Hash::SafeKeys

       You can also look for information at:

       •   RT: CPAN's request tracker (report bugs here)

           <http://rt.cpan.org/NoAuth/Bugs.html?Dist=Hash-SafeKeys>

       •   AnnoCPAN: Annotated CPAN documentation

           <http://annocpan.org/dist/Hash-SafeKeys>

       •   CPAN Ratings

           <http://cpanratings.perl.org/d/Hash-SafeKeys>

       •   Search CPAN

           <http://search.cpan.org/dist/Hash-SafeKeys/>


ACKNOWLEDGEMENTS
       The "dclone" method in the Storable module demonstrated how to save and restore internal hash iterator
       state.  This module is indebted to the authors of this module and to user "gpojd" at stackoverflow.com
       <http://stackoverflow.com/a/10921567/168857> for directing me to it.

       A helpful comment by <Alexandr Evstigneev <http://search.cpan.org/~hurricup/>> let to further
       improvements.


       Copyright 2012-2016 Marty O'Brien.

       This program is free software; you can redistribute it and/or modify it under the terms of either: the
       GNU General Public License as published by the Free Software Foundation; or the Artistic License.

       See http://dev.perl.org/licenses/ for more information.