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5841 :: Is this a minor --gc:arg issue?

• Stefan_Salewski (orginal) [2020-01-20T19:19:10+01:00] view original

  This code compiles with default gc and with --gc:arc:

  type
    Language* = ref object
      impl*: pointer
  
  proc gBoxedFreePangoLanguage*(self: Language) =
    discard
  
  proc languageFromString*(): Language =
    var l: Language
    new(result)
    new(result, gBoxedFreePangoLanguage)
  Run

  
  $ nim c --gc:arc t6.nim
  Hint: 20040 LOC; 0.129 sec; 19.73MiB peakmem; Debug build; proj: /tmp/hhh/t6.nim; out: /tmp/hhh/t6 [SuccessX]
  Run

  But this code does only compile with default gc:

  type
    Language* = ref object
      impl*: pointer
  
  proc gBoxedFreePangoLanguage*(self: Language) =
    discard
  
  proc languageFromString*(): Language =
    new(result)
  
  proc getSampleLanguage*(): Language =
    new(result, gBoxedFreePangoLanguage)
  Run

  
  $ nim c t4.nim
  Hint: used config file '/home/stefan/Nim/config/nim.cfg' [Conf]
  Hint: used config file '/home/stefan/Nim/config/config.nims' [Conf]
  Hint: system [Processing]
  Hint: widestrs [Processing]
  Hint: io [Processing]
  Hint: t4 [Processing]
  Hint: gcc   -o /tmp/hhh/t4  /tmp/stefan/.cache/nim/t4_d/stdlib_system.nim.c.o /tmp/stefan/.cache/nim/t4_d/@mt4.nim.c.o    -ldl [Link]
  Hint: 22196 LOC; 0.153 sec; 19.762MiB peakmem; Debug build; proj: /tmp/hhh/t4.nim; out: /tmp/hhh/t4 [SuccessX]
  Run

  
  $ nim c --gc:arc t4.nim
  Hint: used config file '/home/stefan/Nim/config/nim.cfg' [Conf]
  Hint: used config file '/home/stefan/Nim/config/config.nims' [Conf]
  Hint: system [Processing]
  Hint: widestrs [Processing]
  Hint: io [Processing]
  Hint: t4 [Processing]
  /tmp/hhh/t4.nim(12, 6) Error: cannot bind another '=destroy' to: Language:ObjectType; previous declaration was constructed here implicitly: /tmp/hhh/t4.nim(9, 6)
  Run

  For me that was and still is confusing, I would prefer the last not compiling case, because in the other cases the finalizer can be called unintentionally.

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• Araq (orginal) [2020-01-20T20:11:34+01:00] view original

  Look, what is really going on here ais that you cannot use new(x) and new(x, finalize) at the same time. Once you have a call like new(x, finalize) in your program every instance of typeof(x) will call the finalizer.

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• Stefan_Salewski (orginal) [2020-01-21T08:20:32+01:00] view original

  I know that :-)

  The point is that only the bottom most code above compiling with --gc:arc give us the desired compile-time error. The topmost code compiles always without error, the bottom most code compiles with default gc.

  Well, it is indeed good enough, our actual code should most of the time look like the bottom most code, and we can compile always with --gc:arc to check for errors even when we intent to use another GC.

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• Araq (orginal) [2020-01-21T10:35:46+01:00] view original

  Either way the checking is incomplete, with --gc:arc you can compile new(x, final) and then afterwards new(x) won't be detected as an error.

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