How to realize copyable/postblit class

[John Colvin via Digitalmars-d-learn]

On Saturday, 13 June 2015 at 05:20:42 UTC, SimonN wrote:
> Hi,
>
> I have a few classes with need for deeper copying. I don't want 
> a bitwise copy necessarily. Ideally, I'd implement this(this).
>
> I've thought about changing them to struct. However, the type 
> feels much more like a D class than a D struct. It's often 
> passed by reference, and it's not plain old data. Changing it 
> to struct for the sole benefit of "this(this)" seems to be a 
> bad tradeoff.
>
> (Use case: Backing up game states in a for 
> savestating/networking.)
>
> I've resorted to a C++-style copy constructor:
>
> this(T rhs) {
>     myvalue = rhs.myvalue;
>     myarray = rhs.myarray.dup;
>     // ...
> }
>
> Downside: Boilerplate, each member appears once in the 
> declarations, a second time in the copy constructor.
>
> Alternative approaches seem to implement T T.dup() or T 
> T.clone(). Apparently, one has to pick one of these equally 
> good approaches, and stick with it throughout a codebase. It 
> seems good to implement it only for the classes that need it, 
> to minimize boilerplate.
>
> Is that the state of the art?
>
> -- Simon

perhaps:

class A
{
     struct S
     {
         //class contents
         this(this){ /*any extra post-copy fixeup you need*}
     }
     S s;
     //to avoid having to type A.s.myMember
     alias s this; //or could use std.typecons.Proxy
     this(A rhs)
     {
         s = rhs.s;
     }
}