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6763 :: For ``--gc:arc``, are ``GC_ref`` ``GC_unref`` still necessary?

• wt (orginal) [2020-09-03T03:54:41+02:00] view original

  In the following example --- use a container to store a ref pointer ref ref Obj:

  type
    Obj = object
      val: int
  
  var container: pointer = alloc0(sizeof(pointer))
  
  proc f() =
    var obj1: ref Obj = new(Obj)
    obj1.val = 100
    (cast[ref ref Obj](container))[] = obj1
    
    # GC_ref(obj) # still necessary? If not, can arc collect garbage correctly?
  
  proc main() =
    f()
    GC_fullCollect()
    
    var obj2 = new(Obj)
    obj2.val = 1000
    
    echo cast[ref ref Obj](container)[].val # Output: 100? 1000? unknown?
  
  main()
  Run

  When I use --gc:refc, the output is 1000, which indicates that GC_fullCollect() completely reclaims the memory of obj1. However, When I use --gc:arc, the output is 100. Does this mean that obj1 is automatically tracked by arc inside f()? Are GC_ref GC_unref necessary in this use case?

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• Araq (orginal) [2020-09-03T10:17:30+02:00] view original

  What you're doing is implementation defined and very dangerous. But your observation seems to be correct, in this case GC_ref is not required for --gc:arc. But it's a hard puzzle and I might change my mind about it. :-)

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• wt (orginal) [2020-09-04T03:59:52+02:00] view original

  Thanks, I got it. This is just a sample code. In fact, I am using a ptr UncheckedArray[T] to store some ref objects in a project. I was confused about the relationship between ptr UncheckedArray[T] and ref objects with --gc:arc, and now I understand.

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• Araq (orginal) [2020-09-04T08:52:12+02:00] view original

  Once you add both GC_ref and GC_unref calls into your code where they are required both ARC and the old GCs should be fine with it.

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