From slices to perfect imitators: opByValue

[Sönke Ludwig via Digitalmars-d]

Am 08.05.2014 13:05, schrieb monarch_dodra:
> On Thursday, 8 May 2014 at 07:09:24 UTC, Sönke Ludwig wrote:
>> Just a general note: This is not only interesting for range/slice
>> types, but for any user defined reference type (e.g. RefCounted!T or
>> Isolated!T).
>
> Not necessarily: As soon as indirections come into play, you are
> basically screwed, since const is "turtles all the way down".
>
> So for example, the conversion from "const RefCounted!T" to
> "RefCounted!(const T)" is simply not possible, because it strips the
> const-ness of the ref count.
>
> What we would *really* need here is NOT:
> "const RefCounted!T" => "RefCounted!(const T)"
> But rather
> "RefCounted!T" => "RefCounted!(const T)"
>
> The idea is to cut out the "head const" directly. This also applies to
> most ranges too BTW.
>
> We'd be much better of if we never used `const MyRange!T` to begin with,
> but simply had a conversion from `MyRange!T` to `MyRange!(const T)`,
> which references the same data.
>
> In fact, I'm wondering if this might not be a more interesting direction
> to explore.

The reference count _must_ be handled separate from the payload's 
const-ness, or a const(RefCount!T) would be completely dysfunctional. 
Also, if you take the other example, Isolated!T, there is no reference 
count involved and const(Isolated!T) <-> Isolated!(const(T)) would be 
exactly what is needed.