SPUG: Reference to a hash

Matt Tucker tuck at whistlingfish.net
Fri Mar 22 02:24:46 CST 2002

-- Jim Flanagan <jimfl at tensegrity.net> spake thusly:

>   What you really want is an anonymous hashref, rather than a
> reference to   a specific named hash. One way to get that is the loop
> that you have   below. Another way is to just say
>       $HoH{$key}{$heading} = {%hash};
>   This creates a new, anonymous hash with the same keys/values as
> %hash.

Note that this essentially does a copy of the hash when you do that.
Sometimes a slightly more efficient solution is to use a locally scoped
hash that gets created anew each time. For instance, instead of:

    while (my $line = <STDIN>) {
        %hash = my_func($line);
        $things{$key} = {%hash};

It would be slightly more efficient (for large hashes at least) to do:

    while (my $line = <STDIN>) {
        my %hash = my_func($line);
        $things{$key} = \%hash;

Since %hash gets recreated each time through the loop, you don't have
the problem of it getting overwritten. This will only make a difference
for very large hashes, but I figured it was worth pointing out.
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