Making RCSlice and DIP74 work with const and immutable

[via Digitalmars-d]

On Sunday, 1 March 2015 at 06:42:02 UTC, H. S. Teoh wrote:
> On Sun, Mar 01, 2015 at 12:53:24PM +1000, Manu via 
> Digitalmars-d wrote:
>> On 1 March 2015 at 12:48, Andrei Alexandrescu via Digitalmars-d
>> <digitalmars-d at puremagic.com> wrote:
>> > On 2/28/15 6:33 PM, Manu via Digitalmars-d wrote:
>> >>
>> >> one of the biggest recurring
>> >> complaints relating to the D type system
>> >
>> >
>> > That didn't get talked about in I don't remember. -- Andrei
>> 
>> So, what's the solution?
>> I've never complained about it, but I still have this problem
>> regularly. The solution in my experience is; 'everything is 
>> always
>> mutable'.
>
> I ran into the same problem, and as a result I hardly ever make 
> use of
> D's const system.
>
> One possible solution that occurred to me, though, is to 
> introduce a
> kind of "logical const" template that constructs a 
> partially-const type,
> with the "logical data" parts const/immutable/etc., but the 
> "metadata"
> parts mutable. User-defined attributes could be used for this 
> purpose:
>
> 	struct Metadata {}
>
> 	class MyClass {
> 		int field; // regular data field
> 		@Metadata int fieldCache; // metadata
> 		...
> 	}
>
> 	template Const(T) {
> 		class Const {
> 			const {
> 				// insert stuff not marked with
> 				// @Metadata here
> 			}
> 			// insert stuff marked with @Metadata here
> 		}
> 	}
>
> So then Const!MyClass is a modified version of MyClass where 
> the data
> fields are const (similarly, we can define Immutable for the 
> analogous
> purpose) but the fields marked as metadata will remain mutable.
>
> Of course, this is just a crude first stab at the problem; I'm 
> sure
> there's plenty of room for refinement to make it more usable, 
> and to
> address some obvious roadblocks, like how to make MyClass 
> implicitly
> convertible to Const!MyClass, etc.. But it seems likely that D's
> template machinery can actually express this in a way that does 
> not
> violate the guarantee of physical const.

You still cannot access it through a const reference, though.