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10654 :: overload operator `+=` for private member

• mimonot (orginal) [2023-11-19T17:13:30+01:00] view original

  I recently started learning the Nim language and ran into a problem. I can define operators to get and set a private member of an object, but I need the "+=" operator. How to properly define the "+=" operator for a private member of an object?

  type
    Obj* = object
      m: int
  
  proc m*(o: Obj): int = o.m
  proc `m=`*(o: var Obj, v: int) = o.m = v
  
  proc `+=` for m ?
  Run

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• aEverr (orginal) [2023-11-19T17:29:44+01:00] view original

  add

  proc m*(o: var Obj): var int = o.m
  Run

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• mimonot (orginal) [2023-11-19T18:17:15+01:00] view original

  Thank you. This works well if the "m" is the right type. But what if the private “m” is of a different type and needs conversion?

  type
    Obj = object
      m: cint
  
  proc `m`*(o: var Obj): var int = int(o.m)  # Error
  Run

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• exelotl (orginal) [2023-11-19T19:15:17+01:00] view original

  I don't think it can be done in that case. Well, there's an unsafe way of doing it if you know that the two types are binary compatible and of the same size.

  type
    Obj = object
      m: cint
  
  static:
    doAssert(sizeof(cint) == sizeof(int32))
  
  proc `m`*(o: var Obj): var int32 =
    cast[ptr int32](addr o.m)[]
  
  var obj: Obj
  obj.m += 5
  echo obj.m   # 5
  Run

  buuut this is extremely situational and I wouldn't recommend it at all. You're best off just making shorthand procedures that act on the object itself.

  type
    Obj = object
      m: cint
  
  proc incM*(o: var Obj, n: int) =
    o.m = cint(int(o.m) + n)
  
  proc decM*(o: var Obj, n: int) =
    o.m = cint(int(o.m) - n)
  
  var obj: Obj
  obj.incM(5)
  echo obj.m   # 5
  Run

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• mimonot (orginal) [2023-11-20T07:20:56+01:00] view original

  I'm writing a wrapper for a C library. I would like to preserve the '+=' and '-=' operators for structure members. Maybe you can use a template or a macro?

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• PMunch (orginal) [2023-11-20T07:28:19+01:00] view original

  This sounds like a job for dot operators!. They can be a bit tricky, but they can certainly do what you need

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• Araq (orginal) [2023-11-20T07:35:16+01:00] view original

  I would like to preserve the '+=' and '-=' operators for structure members.

  What does that mean.

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• mimonot (orginal) [2023-11-20T13:14:12+01:00] view original

  Thanks everyone. I solved my question by adding a special method.

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