Superfluous code in switch statement

[Timon Gehr via Digitalmars-d-learn]

On 09/04/2015 09:39 PM, Paul wrote:
> I discovered the other day (during a cut and paste malfunction!) that
> it's possible to have code before the first case in a switch. Google
> tells me that it's legal C code and something I read said it could be
> used for initialization but was rather vague.
>
> void main()
> {
>      import std.stdio;
>
>      int a=1;
>
>      switch(a)
>      {
>          a=2;
>          writeln("hello");
>
>          case 1:
>          break;
>          case 2:
>          break;
>          default:
>
>      }
>      writeln(a);
>
> }
>
> The code before the 'case' has to be legal D code to pass compilation
> but it seems to have no effect (which is probably a good thing!). I was
> a bit surprised that the compiler (dmd) didn't generate a warning when
> using the -w option.
>
> Can someone explain what's going on here please?
>

The switch statement is quite unstructured. You can have your case 
statements basically at arbitrary points where a statement is expected:

import std.stdio;
int main(){
     int x=2;
     switch(x){
         do{
             for({case 0:};){} x--;
         case 1: x--;
             if(false){
             default: writeln("!");
                 return 0;
             }
         }while(true);
     }
}

The only case where statements before the first case/default are 
potentially useful is when there is some way to jump back there. One 
could use goto or something like 
https://en.wikipedia.org/wiki/Duff%27s_device

It's not a very common thing to do though, and I don't think anyone 
would be sad if there was a dead code warning for the case where the 
code before the case statements cannot actually be reached. It's not 
done though. DMD never warns about dead code.