Classes as enums in D?

[Marc Schütz via Digitalmars-d-learn]

On Monday, 30 November 2015 at 08:08:20 UTC, Meta wrote:
> class WhiteKey
> {
> 	private immutable int halfStepsToNext;
> 	private immutable int halfStepsToPrevious;
>
> 	enum
> 	{
> 		A = new WhiteKey(2, 2),
> 		B = new WhiteKey(2, 1),
> 		C = new WhiteKey(1, 2),
> 		D = new WhiteKey(2, 2),
> 		E = new WhiteKey(2, 1),
> 		F = new WhiteKey(1, 2),
> 		G = new WhiteKey(2, 2),
> 	}
> 	
> 	private this(int halfStepsToPrevious, int halfStepsToNext)
> 	{
> 		this.halfStepsToPrevious = halfStepsToPrevious;
> 		this.halfStepsToNext = halfStepsToNext;
> 	}
> }
>
> However, you do NOT want to do this, as everywhere you use 
> WhiteKey's members, a new object will be created. For example:
>
> auto f = WhiteKey.A;
> auto n = WhiteKey.A;
> 	
> import std.stdio;
> writeln(&f, " ", &n);
>

You're misinterpreting this:

     enum X {
         A = new Object,
         B = new Object,
     }

     void main() {
         import std.stdio;
         writeln(cast(void*) X.A);
         writeln(cast(void*) X.A);
     }

# output:
470910
470910

You're print the address of `f` and `n` on the stack, not the 
reference they're pointing to.

But it's true that enums of mutable _arrays_ do create a new 
instance every time they're used:

     enum X {
         A = [1,2,3],
         B = [4,5,6],
     }

     void main() {
         import std.stdio;
         writeln(X.A.ptr);
         writeln(X.A.ptr);
     }

# output:
7FD887F0E000
7FD887F0E010