• Paul (26/26) Sep 04 2015 I discovered the other day (during a cut and paste malfunction!)
• Timon Gehr (25/50) Sep 04 2015 The switch statement is quite unstructured. You can have your case
• anonymous (10/11) Sep 04 2015 It warns here:
• Timon Gehr (3/14) Sep 04 2015 You are right, it does. Then I suppose there is no reason why it
• Paul (2/20) Sep 05 2015 I see, thanks.

"Paul" <paul example.com> writes:

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?

Timon Gehr <timon.gehr gmx.ch> writes:

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.

anonymous <anonymous example.com> writes:

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 */
}
----

Timon Gehr <timon.gehr gmx.ch> writes:

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.

"Paul" <paul example.com> writes:

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.