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11.12. Copying Data Structures
11.12.1. Problem
You need to copy a complex data
structure.
11.12.2. Solution
Use the dclone function from the standard Storable
module:
use Storable;
$r2 = dclone($r1);
11.12.3. 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 one 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 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, HASH, or CODE;[20] references
of type GLOB, IO, and the more esoteric types are not supported. The
safeFreeze function from the FreezeThaw module
supports even these types when used in the same address space by
using a reference cache that could interfere with garbage collection
and object destructors under some circumstances.
[20]Believe it or not, it's true.
Storable can even serialize closures. See its manpage for how to
unthaw these using a Safe compartment.
Because dclone takes and returns references, you
must add extra punctuation if you have a hash or arrays to copy:
%newhash = %{ dclone(\%oldhash) };11.12.4. See Also
The documentation for the standard Storable and Data::Dumper modules,
and for the FreezeThaw CPAN module.
