Somehow I can't use ubyte variables behind 'case', but ulong works fine. Why is that?

void main() {
	alias TestType = ulong; // won't compile if = ubyte
	import std.stdio;
	TestType a,b,c;
	readf("%s %s %s ", &a, &b, &c);
	switch(c){
		case a: writeln("a");break;
		case b: writeln("b");break;
		default: assert(false);
	}
}

On 01/18/2017 05:22 PM, Yuxuan Shui wrote:
> Somehow I can't use ubyte variables behind 'case', but ulong works fine.
> Why is that?
>
> void main() {
>     alias TestType = ulong; // won't compile if = ubyte
>     import std.stdio;
>     TestType a,b,c;
>     readf("%s %s %s ", &a, &b, &c);
>     switch(c){
>         case a: writeln("a");break;
>         case b: writeln("b");break;
>         default: assert(false);
>     }
> }

case expressions must be constants:

  "The case expressions must all evaluate to a constant value or
   array, or a runtime initialized const or immutable variable of
   integral type."

  https://dlang.org/spec/statement.html#SwitchStatement

The fact that it compiles for ulong looks like a bug to me. It compiles probably because switch is most likely implemented in terms of a chained if-else-if statements by the compiler and it just works because there is no explicit check whether they are constant or not.

Ali

On Thursday, 19 January 2017 at 02:00:10 UTC, Ali Çehreli wrote:
> On 01/18/2017 05:22 PM, Yuxuan Shui wrote:
>> Somehow I can't use ubyte variables behind 'case', but ulong works fine.
>> Why is that?
>>
>
> case expressions must be constants:
>
>   "The case expressions must all evaluate to a constant value or
>    array, or a runtime initialized const or immutable variable of
>    integral type."
>
>   https://dlang.org/spec/statement.html#SwitchStatement
>
> The fact that it compiles for ulong looks like a bug to me. It compiles probably because switch is most likely implemented in terms of a chained if-else-if statements by the compiler and it just works because there is no explicit check whether they are constant or not.
>
> Ali

If you try:

void main() {
    alias TestType = ulong; // won't compile if = ubyte
    import std.stdio;
    TestType a,b,c;
    readf("%s %s %s ", &a, &b, &c);
    final switch(c){
        case a: writeln("a");break;
        case b: writeln("b");break;
        default: assert(false);
    }
}

Then the error message:

test.d(7): Error: case variables not allowed in final switch statements
test.d(8): Error: case variables not allowed in final switch statements

Makes it looks like that "case variable" is an intended feature.

On Thursday, 19 January 2017 at 01:22:56 UTC, Yuxuan Shui wrote:
> Somehow I can't use ubyte variables behind 'case', but ulong works fine. Why is that?
>
> void main() {
> 	alias TestType = ulong; // won't compile if = ubyte
> 	import std.stdio;
> 	TestType a,b,c;
> 	readf("%s %s %s ", &a, &b, &c);
> 	switch(c){
> 		case a: writeln("a");break;
> 		case b: writeln("b");break;
> 		default: assert(false);
> 	}
> }

It is a bug that this code compiled.
Case Variables can only be used on const values, to prevent mutation of them inside the switch itself.

try to make the type a const ubyte.