nimforum mirror - Changing chained attributes of object taken by var

PeterSR (orginal) [2017-08-06T11:25:18+02:00] view original

Consider the following code to create linked list-like structure:

type
  A = ref AObj
  AObj = object
    id: int
    parent: A
    child:  A

proc `$`(a: A): string = $a.id


proc takeAsVar(curr: var A, next: A) =
  if curr == nil:
    curr = next # var A is wanted here and nowhere else
  else:
    let const_curr = curr
    echo "takeAsVar: curr before: ", curr
    curr.parent.child = next # Try changing curr to const_curr for same result
    echo "takeAsVar: curr after:  ", curr
    # curr changes even though there is no "curr = ..." line.

proc takeAsConst(curr: A, next: A) =
  if curr == nil:
    #curr = next # can't do this
    discard
  else:
    echo "takeAsConst: curr before: ", curr
    curr.parent.child = next
    echo "takeAsConst: curr after:  ", curr


let a1 = A(id: 1, parent: nil, child: nil)
let a2 = A(id: 2, parent: nil, child: nil)
let a3 = A(id: 3, parent: nil, child: nil)

a1.child  = a2
a2.parent = a1

takeAsVar(a1.child, a3)


let a4 = A(id: 4, parent: nil, child: nil)
let a5 = A(id: 5, parent: nil, child: nil)
let a6 = A(id: 6, parent: nil, child: nil)

a4.child  = a5
a5.parent = a4

takeAsConst(a4.child, a6) # Works as expected

which outputs


takeAsVar: curr before: 2
takeAsVar: curr after:  3
takeAsConst: curr before: 5
takeAsConst: curr after:  5

Notice how the value of the variable curr changes in the first proc without an explicit curr = ....

We have a very simple object with a parent pointer and a child pointer. My initial goal was to create a linked list-like structure and inserting into a nil list could be handled by taking the original list by var since it allows for curr = next. Most of the logic for non-nil list has been removed to get a minimum working example, but the problem is this: When I want to change the child field of the parent of curr, then the variable curr itself is changed when taken by var.

In my mind taking a parameter by var would only change whether you can write curr = ... and change the original or not. Here it seems that it somehow infers that curr.parent.child is the same as curr and changes curr as well.

Have I misunderstood how var parameters work? Is this desired behaviour?

Using version 0.16.1 (git hash: 137b5f43022c3c5e22a7f28c8012f1d0ea6007c6)

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3093 :: Changing chained attributes of object taken by var - Is this desired behaviour?