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3363 :: Packed Strings, how to pack fixed length number of chars in an struct.

• treeform (orginal) [2017-11-23T16:34:12+01:00] view original

  I am trying to get some thing like this to work, but I am having issues with arrays and the type system:

  Working code, but I have to write array[16, char] which is awkward:

  proc pack[A](str: string): A =
    if str.len > result.len:
      raise newException(ValueError, "Can't pack " & $str.len & " string into " & $result.len)
    for i in 0..<result.len:
      if i >= str.len:
        result[i] = char(0)
      else:
        result[i] = str[i]
  
  
  # example struct
  type Test = object
    x: int32
    y: int32
    name: array[16, char]
    life: float32
  
  
  var test: Test
  test.name = pack[array[16, char]]("supernamething")
  
  echo sizeOf(Test)
  Run

  I am trying to wrap that as a type

  type PackedString[N] = array[N, char]
  
  
  proc pack[N](str: string): PackedString[N] =
    if str.len > result.len:
      raise newException(ValueError, "Can't pack " & $str.len & " string into " & $result.len)
    for i in 0..<result.len:
      if i >= str.len:
        result[i] = char(0)
      else:
        result[i] = str[i]
  
  
  # example struct
  type Test = object
    x: int32
    y: int32
    name: PackedString[16]
    life: float32
  
  
  var test: Test
  test.name = pack[16]("supernamething")
  
  echo sizeOf(Test)
  Run

  Can some one tell me what I am doing wrong?

  Thanks!

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• treeform (orginal) [2017-11-23T16:43:38+01:00] view original

  Araq helped me with this, turns out you need static[int] to get around "Error: cannot instantiate"

  This works:

  type  PackedString[N: static[int]] = array[N, char]
  
  
  proc pack[N](str: string): PackedString[N] =
    if str.len > result.len:
      raise newException(ValueError, "Can't pack " & $str.len & " string into " & $result.len)
    for i in 0..<result.len:
      if i >= str.len:
        result[i] = char(0)
      else:
        result[i] = str[i]
  
  
  # example struct
  type Test = object
    x: int32
    y: int32
    name: PackedString[16] #array[16, char]
    life: float32
  
  
  var test: Test
  test.name = pack[16]("supernamething")
  
  echo sizeOf(Test)
  
  Run

---