Is This a Solution For the Const/Rebindable Issue?

[Steven Schveighoffer]

On Mon, 13 Jun 2011 01:06:43 -0400, Mehrdad <wfunction at hotmail.com> wrote:

> == Quote from Steven Schveighoffer (schveiguy at yahoo.com)'s article
>> On Sun, 12 Jun 2011 23:58:01 -0400, Mehrdad <wfunction at hotmail.com>  
>> wrote:
>> I'll see if it's in my sent mail...
>> OK, I found it, it was actually the same logic but applied to shared
>> objects.  But Walter convinced me that the issues are the same (at least
>> for this problem).
>> Consider this type:
>> struct S
>> {
>>     Object o;
>> }
>> now, we have these two variables:
>> const(S) s;
>> const(Object) o;
>> finally, the issue:
>> void kryptonite(ref const(Object) o)
>> {
>>     o = new Object();
>> }
>> kryptonite(o); // fine
>> kryptonite(s.o);// oops!
>> The problem is, there isn't a way to distinguish a tail-const object
>> reference from a fully const object reference, yet both types can exist.
>> If you want to pass a reference to such a reference, then you run into
>> sticky issues like this one.
>> I understand that you want final to mean "head const", but final is a
>> storage class, not a type modifier -- it cannot be used as part of the
>> type info.
>> -Steve
>
> Hm... that's a reason, but it seems like we can get around it relatively  
> easily.
>
> It seems like it could be solved by making it so that const(S) also  
> makes the fields of S final. That way,
> you could no longer do `kryptonite(s.o)` because it's final.
>
> You could then say "final ref const Object o" to allow for passing that  
> field, because that would mean the
> callee cannot modify o.
>
>
> Wouldn't that work?

No.  final is not a type constructor, so it does not get carried around  
with the type.

final int i;

assert(typeof(i) == int); // after the declaration, the storage class is  
gone from the type!

int * ip = &i;

*ip = 4; // can't be forbidden.

final can only apply to the storage of the variable, and I'm not actually  
sure the above is valid in today's D2.  I think it's only a storage class  
in D1.  In D2, I think it's only use is to declare a function as  
not-virtual.

-Steve