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?

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.

On Friday 04 September 2015 23:04, Timon Gehr wrote:

> DMD never warns about dead code.

It warns here:

----
import std.stdio;
void main()
{
    return;
    writeln("hi"); /* Warning: statement is not reachable */
}
----

On 09/04/2015 11:12 PM, anonymous wrote:
> On Friday 04 September 2015 23:04, Timon Gehr wrote:
>
>> DMD never warns about dead code.
>
> It warns here:
>
> ----
> import std.stdio;
> void main()
> {
>      return;
>      writeln("hi"); /* Warning: statement is not reachable */
> }
> ----
>

You are right, it does. Then I suppose there is no reason why it shouldn't warn in the switch case.

On Friday, 4 September 2015 at 21:20:11 UTC, Timon Gehr wrote:
> On 09/04/2015 11:12 PM, anonymous wrote:
>> On Friday 04 September 2015 23:04, Timon Gehr wrote:
>>
>>> DMD never warns about dead code.
>>
>> It warns here:
>>
>> ----
>> import std.stdio;
>> void main()
>> {
>>      return;
>>      writeln("hi"); /* Warning: statement is not reachable */
>> }
>> ----
>>
>
> You are right, it does. Then I suppose there is no reason why it shouldn't warn in the switch case.

I see, thanks.