Local Copy of a member variable

[benrwebb]

Here is my situation: I'm inside a method. I access a local variable (a hash ref), munge the hash and then return my munged data. The problem comes in that I seem to be munging my member variable as well. I need to work with a local copy so that the member variable is uncorrupted. Here is my current (broken) code:

my $aref = $self->{atlas};
my %atlas = %$aref;
... munging ...

What do I need to do to make manipulating %atlas non-destructive to $self->{atlas} ?

Thanks!

[Roy Johnson]

You'll need a deep copy. See Data::COW. Update: Or, as holli suggests below, Scalar::Util::Clone. The former will do actual copies only of things you change; the latter will copy everything, but fast. So if it's a big structure that you're only modifying a little of, use Data::COW; if you're going to modify a substantial portion of it, use Scalar::Util::Clone.

[TedYoung]

If it is a simple hash then changes to %atlas won't be seen in $self->{atlas}. Now, if the values of $self->{atlas} contain references (i.e. other hashes, arrays, object, etc) then changes to those will be seen. If you need to make a deep copy (clone) of a complex datastructure, you could use dclone method is Storable.

use Storable;
my $aref = dclone($self->{atlas});
my %atlas = %$aref;

[holli]

From the docs of Storable:


There is a Clone module available on CPAN which implements deep cloning natively, i.e. without freezing to memory and thawing the result.

So I'd recomment Clone or Scalar::Util::Clone. There is also Clone::Any which is an interface for the existing clonecapable modules.

Of course, Storable has the advantage that you can save your structures to disk (including code references).

[jeffa]

Try this instead

my $atlas_copy = { %$atlas };

UPDATE: but a deep copy is the better choice here. I was assuming you were dealing with a simple hash structure and i failed to notice that you were copying to hash, not a hash ref. Nothing to see here ... move along. bad jeffa :)