Class views - a new concept

[Arafel]

On 27/6/22 1:33, forkit wrote:
> So I came up with this idea, based on discussion in another, rather 
> controverial, thread ;-)
> 
> In any case, I think this idea deserves it's own thread.
> 
> NOTE::it's just an idea, and certainly **not** a proposal.
> 
> I'd be interested to know, if anyone else out there knows of any 
> langauge, where a concept like this has been implemented (i.e. a way to 
> restrict specific class member functions to specific class variables).
> 
> So...here I introduce a new concept, which (for now), we'll call a 'view'.
> 
> It can solve multiple problems, at least for me.
> 
> First, it eliminates a need for that very controversial idea of 
> 'private(this)'
> 
> Second, it allows fine grained control of member access, to member 
> variables.
> 
> Anything declared private, in a view, is simply private to that view.
> 

There are probably some corner cases not covered, and I can't say how it 
will perform... but it should be already possible to get something 
really close and usable in most sensible cases:

```d
import std.typecons : Typedef;

class C {
    /* static */ struct View { // Can and should be static if it doesn't 
access any of C's internals
        // What everything not in this View must use
        public auto i() { return _i; }
        public auto i(int i) { _i = i; }

        // Internals not exposed anywhere else
        private int _i;
     };

     // The magic bit
     private Typedef!View _view;

     // Could be automated and probably improved with some Proxy-like 
meta-magic
     public auto ref i(Args...)(Args args) { return _view.i(args); }

     // Also something like this could perhaps be made to work:
     // alias i = view.i

     this() {
         i = 1; // Works
         _view.i = 2; // Works, but why would you use it?
         // view._i = 3 // Fails
     }
}

// Inheritance works as you'd expect
class D : C { }

void main() {
     C c = new C();
     c.i = 4; // Works
     c._view.i = 5; // Works, but why would you use it?
     // c.view._i = 6; // Fails

     D d = new D();
     d.i = 7 // Works, etc.;
}
```