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6676 :: Object destroyed after editing enum property?

• ElAfalw (orginal) [2020-08-13T11:41:09+02:00] view original

  I have an object defined like this:

  
  Obj = ref object
      case kind: ObjKind:
          of typeA, typeB:	contents: seq[Obj]
          else: discard
  Run

  Then I create an object like that:

  
  example = Obj(kind: typeA, contents: @[some contents])
  Run

  at some point, let's say I decide to change the object's kind (but without changing its contents field). I do it like this:

  .kind = typeB
  Run

  Now, it seemingly works fine. The problem is the contents field's contents are gone (the only way to get them back is by keeping a copy of the previous contents, change the kind and re-set the contents).

  Any idea what is going on or what the best way to do it would be?

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• mratsim (orginal) [2020-08-13T11:52:53+02:00] view original

  Changing the kind effectively erase the object. This is due to the semantics of variant which are supposed to represent different runtime types and changing the kind would reinterpret the potentially garbage-filled fields.

  Note that things might have change in devel after this RFC was implemented: https://github.com/nim-lang/RFCs/issues/209

  If that RFC didn't change anything you can:

  • compile with -d:nimOldCaseObjects to use the previous behaviour
  • Use your own {.union.} object like so:

  type ObjKind = enum
    typeA, typeB
  
  type ObjField{.union.} = object
    fieldA: int
    fieldB: float
  
  type Obj = object
    kind: ObjKind
    f: ObjField
  Run

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• jibal (orginal) [2020-08-13T21:26:10+02:00] view original

  what the best way to do it would be

  Don't make kind a case variant:

  type
    ObjKind = enum typeA, typeB
    Obj = ref object
      kind: ObjKind
      contents: seq[Obj]
  Run

  if that doesn't work for your actual use case, then you need to provide more info. To me, just changing the kind of an object variant without changing the whole object doesn't make semantic sense.

  (BTW, if your object is really defined like that, you probably want to add an {.acyclic.} pragma.)

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• jibal (orginal) [2020-08-13T21:35:27+02:00] view original

  Use your own {.union.} object like so

  The manual says that GC isn't done on the content of unions (which matters for the OP's code, not yours) ... is that no longer the case?

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