New With Struct and Getting Class Object Pointers

[Alex]

On Sunday, 30 September 2018 at 09:30:38 UTC, Vijay Nayar wrote:
> Is there a way to either have a constant reference to a class 
> that can be set to a new value, or is there a way to convert 
> the class variable to a class pointer?
>
> For example:
>
> void main()
> {
> 	class Thing {}
> 	class ThingSaver {
>                 // A const(Thing) could not be changed in 
> setThing().
> 		const(Thing)* t;
>
> 		void setThing(in Thing thing) {
> 			t = thing;  // ERROR converting to pointer type!
> 		}
> 		const(Thing) getThing() const {
> 			return *t;
> 		}
> 	}
> 	
> 	Thing t1 = new Thing();
>
> 	ThingSaver saver = new ThingSaver();
> 	saver.setThing(t1);
> 	const(Thing) t2 = saver.getThing();
> }

I think, what you are facing here, is the different notion of 
const, as used from C++. The reasoning about it is described for 
example here:
http://jmdavisprog.com/articles/why-const-sucks.html
Jonathan is much better therein as I am.

However, there are approaches to solve what you want to do. For 
example, there is a Rebindable around:
https://dlang.org/library/std/typecons/rebindable.html

´´´
import std.stdio;
import std.typecons;

void main()
{
	class Thing {int dummy; }
	class ThingSaver {
                 /*
		A const(Thing) could not be changed in setThing(),
		but a Rebindable can be reassigned, keeping t const.
		*/
		Rebindable!(const Thing) t;

		void setThing(in Thing thing) {
			t = thing;  // No pointers in use :)
		}
		const(Thing) getThing() const {
			return t;
		}
	}
	
	Thing t1 = new Thing();

	ThingSaver saver = new ThingSaver();
	saver.setThing(t1);
	//saver.t.dummy = 5; fails as expected.
	const(Thing) t2 = saver.getThing();
}
´´´
I hope, I got your idea right...