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Marty O'Brien


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


Version 0.04


    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;


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



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).


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).


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



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.


"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


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


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.


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

    perldoc Hash::SafeKeys

You can also look for information at:


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 for directing me to it.

A helpful comment by <Alexandr Evstigneev> 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.