C++ vs D: Default param values and struct to array casting

[Andrew Edwards]

On Friday, 6 September 2019 at 09:14:31 UTC, Andrew Edwards wrote:
> C++ allows the for following:
>
> struct Demo
> {
> 	float a, b, c, d;
> 	Demo() { a = b = c = d = 0.0f; }
> 	Demo(float _a, float _b, float _c, float _d) {
> 		a = _a;
> 		b = _b;
> 		c = _c;
> 		d = _d;
> 	}
> 	float  operator[] (size_t i) const { return (&a)[i]; } //[3]
> 	float& operator[] (size_t i) { return (&a)[i]; } //[4]
> }

This is my thought on how to accomplish op overloading:

struct Test
{
     float a, b, c, d;
     float opIndex(size_t i)
     in(i >= 0 && i <= 3)
     {
         final switch(i)
         {
             case 0: return a;
             case 1: return b;
             case 2: return c;
             case 3: return b;
         }
     }

     void opIndexAssign(float val, size_t i)
     in(i >= 0 && i <= 3)
     {
         final switch(i)
         {
             case 0: a = val; break;
             case 1: b = val; break;
             case 2: c = val; break;
             case 3: d = val; break;
         }
     }
}

Please advise if I've gone off the beaten path. It seems overkill 
for the two lines of code C++ requires so any suggestion is 
greatly appreciated.