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4446 :: Cast string to tuple?

• MarcL (orginal) [2018-12-08T19:02:08+01:00] view original

  Hi,

  I just discovered Nim and already doing little toy projects with it to get used to it. Love it.

  I am playing with networking. But reading from raw sockets, I need to cast the string to a tuple which represent the headers fields. That would enable me to select any field of the ethernet/ip/udp frame more easily.

  I have:

  type
      Raw_Frame = tuple
          mac_dest: array[6,char]
          mac_src: array[6,char]
  
  var t : ref Raw_Frame
  var data_coming_from_socket : string = "123456789123" #Imagine that the string comes from recv
  
  t = cast[ref Raw_Frame](addr c)
  
  echo("hi ", t.mac_dest,t.mac_src)
  Run

  nim

  I want t.mac_dest = ['1','2','3','4','5','6'] and t.mac_src = ['7','8','9','1','2','3'] as an array of char Then, different types will follow but I wanted my question to remain concise.

  Ultimately, I would cast the string to a tuple (I don't want objects, too much overhead) In C, you just cast your array of uint8 to a structure pointer and call it a day. Sure Nim does that too.

  I understood that string is actually a pointer but not too sure. If you have a better idea as to how to do it the Nim way, please help!

  Thanks,

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• MarcL (orginal) [2018-12-08T20:20:49+01:00] view original

  I understand my mistake.

  t = cast[ref Raw_Frame](addr c)
  Run

  should be

  t = cast[ref Raw_Frame](addr c[0])
  Run

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• Stefan_Salewski (orginal) [2018-12-08T20:45:34+01:00] view original

  Note that casting to a ref is dangerous, as ref's interfere with the Garbage-Collector. I guess when you really need a cast, you may want to cast to ptr instead of ref -- ptr is Nim's untraced pointer, similar to C's pointers.

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• MarcL (orginal) [2018-12-09T12:08:54+01:00] view original

  Thanks Stefan,

  I will copy the string on the heap to avoid it to be overwritten and cast it to a Raw_Frame ptr.

  1. only one memcpy is necessary
  2. it can be optimized better
  3. casting to a tuple eases coding as you don't have to remember where is what.
  4. no mem management gets in my way.

  Except if I miss something important and dangerous in Nim, it is the way to go for me. Thanks!

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