nimforum mirror - When an enum overflows a bitsize'd field

dwhall256 (orginal) [2025-04-07T06:17:57+02:00] view original

Considering this code:


type
  FooKind = enum
    Alpha
    Bravo
    Charlie
    Delta
    Echo
  
  Bar {.packed.} = object
    typ {.bitsize: 2.}: FooKind
    val {.bitsize: 6.}: int
  
  Baz {.packed.} = object
    val {.bitsize: 6.}: int
    typ {.bitsize: 2.}: FooKind

var r = Bar(typ: Echo, val: 42)
var z = Baz(typ: Echo, val: 42)

echo "r: ", r
echo "z: ", z

The output is:


r: (typ: Alpha, val: -22)
z: (val: -22, typ: Alpha)

Of course one cannot encode 5 enum elements with only 2-bits. I understand that when I use pragmas, I'm overruling the compiler and am responsible for the results. But there are two things here that I didn't expect:

the enum value is bit-truncated and effectively wraps around

the integer field keeps its bit pattern, but changes value with no warning

Is this is the expected behavior?

Mirror of forum.nim-lang.org

12774 :: When an enum overflows a bitsize'd field