DIP49 - Define qualified postblit

[Timon Gehr]

On 11/10/2013 12:07 PM, Kenji Hara wrote:
> Condider a case that copying "inout struct" inside inout function.
>
> struct S {
>      int[] arr;
>      this(this) ??? { }
> }
> int[] foo(inout S src)
> {
>      S dst = src; // copy inout S to S
>      return dst.arr;
> }
>
> If the struct S has postblit, what shold be done for "copying S from
> inout to mutable"?
>
> 1. You cannot modify elements of arr field, because originally it may be
> immutable.
> 2. You must re-initialize arr field by unique expression, otherwise it
> may break type system
> ...

2. is not necessary in the following case:

inout(int)[] foo(inout S src){
     inout(S) dst = src; // copy inout S to inout S
     return dst.arr;
}

But as far as I understand, there is no other postblit than inout to 
invoke under the current proposal, hence this could be wasteful.


> The requirements are exactly same as what necessary for unique postblit.
> Essentially "creating unique copy" is exactly same as "treating the copy
> source as inout".

I think it is a good design, but maybe still incomplete.

We could eg. keep

this(this)inout{ ... }

as the unique postblit and have

this(inout this)inout{ ... }

as a postblit for identically qualified source and target.