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1787 :: How does one pass something by reference?

• ShinraTensei (orginal) [2015-11-10T15:50:17+01:00] view original

  Hi, everyone! I have this code: window.nim: http://pastebin.com/3YU2gz0S In main.nim i call the function with WinUpdate(w), also tried WinUpdate(addr(w)) but nothing seems to work. Is it even possible or am i doing it wrong? Thank you in advance for any help you can provide.

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• Araq (orginal) [2015-11-10T15:57:04+01:00] view original

  Use 'var' rather than 'ref'.

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• ShinraTensei (orginal) [2015-11-10T16:06:34+01:00] view original

  Thanks Araq. I thought that var is used only to declare a mutable variable. What's the purpose of ref then?

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• OderWat (orginal) [2015-11-10T21:17:18+01:00] view original

  ref is a traced pointer to "something" allocated on the heap. Usually this is an object but can be anything for real.

  type A = ref int
  
  let a = new A
  
  echo a[] # 0, because not inited but "zeroed"
  
  a[] = 2
  
  echo a[] # 2
  
  proc incA(x: A) = inc x[]
  
  incA(a)
  
  echo a[] # 3
  
  Run

  You see that you need to dereference [] the ref to modify the value and the value itself is "created" with new on the heap. Normally that would be an object or tuple and not "just" an int.

  See: http://nim-lang.org/docs/manual.html#types-reference-and-pointer-types

  Using var for a parameter creates an invisible pointer and the data does not need to lay on the heap but still needs to be mutable.

  See: http://nim-lang.org/docs/manual.html#procedures-var-parameters (and esp. the comment at the end of the section)

  I hope I got that all right :)

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• Arrrrrrrrr (orginal) [2015-11-10T21:28:32+01:00] view original

  vars with refs are a bit confusing, but with an example is easy to get:

  proc exA(i: int) =
    i = 0                 # Doesn't compile
  
  proc exB(i: var int) =
    i = 0                 # Compiles
  
  proc exC(i: ref MyRefObject) =
    i.myProp = 0          # Compiles
    i = new MyRefObject() # Doesn't compile
  
  proc exD(i: var ref MyRefObject) =
    i.myProp = 0          # Compiles
    i = new MyRefObject() # compiles
  Run

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