Cloning in D

[Andrei Alexandrescu]

On 09/05/2010 10:15 PM, dsimcha wrote:
> I've started playing around with Orange a little to see whether it would meet
> D's cloning needs.  IMHO one must-have feature for proper cloning that truly
> "just works" is full aliasing preservation.

What do you mean by aliasing preservation? I'd think you want no 
aliasing between the clone and the original. (Perhaps you mean that the 
object graph of the clone and original look the same?)

First off, let's see what cloning methods we have:

1. We know how to clone built-in types except pointers. (Arrays are 
cloned by duplicating the array proper and cloning each element.)

2. Structs can be cloned with either this(this) or by cloning each of 
their fields.

3. For classes we can define a conventional method/interface, e.g. 
clone/Cloneable.

Now, given some arbitrary value v, we know how to obtain a clone of it 
by applying the rules above. The problem is, there are two opaque calls: 
this(this) for structs and obj.clone() for classes. If improperly 
implemented, the whole thing comes unglued. The problem is the standard 
library (and others) must rely on correct deep cloning without being 
able to check it.

Although it's difficult to implement deep cloning, it's relatively easy 
to _verify_ it. So I'm thinking, a generic deepdup() routine would apply 
cloning per the rules above and then would verify that opaque calls 
obj.clone() and this(this) did not produce any aliasing.

Bottom line, this clarifies we need dynamic field information for 
classes. Otherwise we wouldn't be able to "see" through polymorphism.


Andrei