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7566 :: Nim can be so difficult to understand

• HJarausch (orginal) [2021-02-28T14:26:18+01:00] view original

  I'd like to create two objects hold a ref to the same Matrix, what am I missing?

  Many thanks for a hint!

  type
    Matrix[T] = ref object
      data : seq[T]
    
    MX[T] = object
      Theta : ref Matrix[T]
    
    MXU[T] = object
      Theta  : ref Matrix[T]
  
  var
    M  : MX[float]
    MU : MXU[float]
  
  M.Theta= new Matrix[float]
  
  #[ gives type mismatch: got 'Matrix[system.float]' for 'new Matrix[float]'
     but expected 'ref Matrix[system.float]'
  ]#
  
  M.Theta.data= new seq[float]
  
  M.Theta.data = @[1.0,2.0]
  
  MU.Theta = M.Theta   # do both point to the same matrix?
  
  MU.Theta.data[0]=7.0
  echo M    # I'd like to see  [7.0,2.0]
  Run

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• inventormatt (orginal) [2021-02-28T14:32:52+01:00] view original

  Your Matrix is already a reb object so for your theta attribute it wants a ref object of a ref object. if you change the ref matrix to a regular matrix it should then work the way you want it

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• inventormatt (orginal) [2021-02-28T14:39:22+01:00] view original

  and here is a nim playground of the corrected code https://play.nim-lang.org/#ix=2RbP

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• HJarausch (orginal) [2021-02-28T14:44:19+01:00] view original

  Thanks! But if I define Theta : Matrix[T] how can I have MU.Theta be the same object as M.Theta

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• Recruit_main707 (orginal) [2021-02-28T14:47:21+01:00] view original

  Matrix[T] is already a ref:

  type

  Matrix[T] = ref object # reference to an object

  data : seq[T]

  MX1[T] = object

  Theta : ref Matrix[T] # reference to a reference to an object

  MX2[T] = object

  Theta : Matrix[T] # (implicit) reference to an object

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• HJarausch (orginal) [2021-02-28T14:52:27+01:00] view original

  Thanks for the corrected code.

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• inventormatt (orginal) [2021-02-28T14:53:17+01:00] view original

  if you create a a matrix theta and set both m.Theta and MU.Theta to that same matrix then both of those variables will contain references to the same matrix like this https://play.nim-lang.org/#ix=2RbT

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• HJarausch (orginal) [2021-02-28T15:02:57+01:00] view original

  Thanks, as well.

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• alexeypetrushin (orginal) [2021-02-28T18:00:05+01:00] view original

  A side comment... I find using new Xxx extremely confusing, error prone and inconvenient, and try to avoid it as much as possible.

  The Xxx() constructor feels much better, and it would do implicit new call. And you don't need to change the code if later you would decide to change the object from ref to plain object.

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