Copying Data Structures
Problem
You need to copy a complex data structure.
Solution
Use the dclone
function from the Storable module
from CPAN:
use Storable; $r2 = dclone($r1);
Discussion
Two types of “copy” are sometimes confused. A surface copy (also known as shallow copy) simply copies references without creating copies of the data behind them:
@original = ( \@a, \@b, \@c ); @surface = @original;
A deep copy creates an entirely new structure with no overlapping references. This copies references to 1 layer deep:
@deep = map { [ @$_ ] } @original;
If @a
, @b
, and
@c
themselves contain references, the preceding
map
is no longer adequate. Writing your own code
to deep-copy structures is laborious and rapidly becomes tiresome.
The Storable module, found on CPAN, provides a function called
dclone
that recursively copies its argument:
use Storable qw(dclone); $r2 = dclone($r1);
This only works on references or blessed objects of type SCALAR,
ARRAY, or HASH; references of type CODE, GLOB, and IO and more
esoteric types are not supported. The safeFreeze
function from the FreezeThaw
module supports these
when used in the same address space by using a reference cache that
could interfere with garbage collection and object destructors under
some circumstances.
Because dclone
takes and returns references, you
must add extra punctuation if you have a hash of arrays that you want
to copy:
%newhash = %{ dclone(\%oldhash) };
See Also
The documentation for the CPAN modules Storable, Data::Dumper, and ...
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