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10689 :: Using the var type to modify a separate object from another

• thedistantforest (orginal) [2023-11-27T03:28:54+01:00] view original

  Hi I'm trying to modify an object from the outside through another object.

  The reason for this is that I'd like a tween object to tween the variables of a target object. How would I achieve this?

  Example code:

  type Entity = ref object
    rotation: int
  
  type Tweener = ref object
    target: var int
  
  proc applyTween(t: Tweener, newValue: int) =
    t.target = newValue
  
  var e = new Entity
  var t = Tweener(target: e.rotation) # Error: invalid type: 'var int' in this context: 'Tweener' for var
  
  t.applyTween(20)
  echo "Entity(" & $e.rotation & ")" # Should output "Entity(20)"
  Run

  Playground link: https://play.nim-lang.org/#ix=4MzG

  I was thinking of using the var keyword, as my understanding of it is that it is similar to a pointer. However I am unable to get it working.

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• ElegantBeef (orginal) [2023-11-27T03:53:58+01:00] view original

  To do this safely one needs some form of borrow checker. Nim's experimental views does this https://nim-lang.org/docs/manual_experimental.html#view-types but it's likely to be easier to just use a ptr T or ref T which ever suffices for your problem until that feature becomes more usable.

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• Boston (orginal) [2023-11-27T03:55:55+01:00] view original

  This is the only way I know how to get the behavior you want, but I'm sure someone knows better.

  
  type Entity = ref object of RootObj
    rotation: int
  
  type Tweener = ref object of RootObj
    target: ptr int
  
  proc applyTween(t: Tweener, newValue: int) =
    t.target[] = newValue
  
  var e = new Entity
  var t = Tweener(target: e.rotation.addr)
  
  t.applyTween(20)
  echo "Entity(" & $e.rotation & ")" # Should output "Entity(20)"
  Run

  From what I understand var in Nim just communicates mutability, it does not necessarily mean a pointer everywhere in the code gen. Ref on the other hand is a traced pointer.

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