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const question re: switch statement

Joe101 (33)
Can anyone explain why 'case y' below, throws the error : 'y cannot appear in a constant expression' . 

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{
int x =3;
const int y = x;
int z = 5;

switch(x ){
//next line causes problem
    case y:
        ;
        break;

}

return 0;
}
Callum5042 (183)
Why is there a semi-colon under 'y'?

And 'x' is equal to 3 and can be changed in the program, while 'y' cannot be changed.

So if you changed 'x', 'y' would be trying to change to what 'x' is but can't.

If you understand what I mean, change 'x' into a const too and it will work
Peter87 (11234)
So if you changed 'x', 'y' would be trying to change to what 'x' is but can't.

No. y is a copy of x. Changing x will not affect y.

The problem is that you can only use compile time constants in the case labels. y is not known at compile time so it can't be used here.
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