Re: switch statement with variable branches

[Stefan Koch via Digitalmars-d-learn]

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 ", , , );
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.

Re: switch statement with variable branches

[Yuxuan Shui via Digitalmars-d-learn]

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 ", , , );
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.

Re: switch statement with variable branches

[Ali Çehreli via Digitalmars-d-learn]

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 ", , , );
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

switch statement with variable branches

[Yuxuan Shui via Digitalmars-d-learn]

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 ", , , );
switch(c){
case a: writeln("a");break;
case b: writeln("b");break;
default: assert(false);
}
}