Mirror of forum.nim-lang.org

8073 :: select a type

• ingo (orginal) [2021-06-02T16:47:13+02:00] view original

  This does not work, but illustrates what I'm looking for:

  
  type FileBits = case BitDepth
    of  8: uint8
    of 16: uint16
    of 24: uint32
    of 32: uint32
    of 64: uint64
    else : discard
  Run

  How do I create a type depending on a variable?

  TIA

---

• sky_khan (orginal) [2021-06-02T16:56:27+02:00] view original

  You cant create a type at runtime. If BitDepth is not known at compile time, you'll need all these types. Then you can use generics, I guess

---

• ingo (orginal) [2021-06-02T17:04:18+02:00] view original

  BitDepth is known at compile time. Sorry, no idea what generics is.

---

• Zoom (orginal) [2021-06-02T17:09:40+02:00] view original

  Here you are:

  type
    BitDepth = enum
      BD8, BD16, BD24, BD32, BD64
    FileBits = object
      case kind: BitDepth:
      of  BD8:  smpl8: uint8
      of BD16: smpl16: uint16
      of BD24: smpl24: uint32
      of BD32: smpl32: uint32
      of BD64: smpl64: uint64
  Run

  The catch is, it's not any better and you can't really use it anyway without generics or lots of case branching each time you process FileBits.

---

• sky_khan (orginal) [2021-06-02T17:19:31+02:00] view original

  Ok, If you're sure you need just one FileBits definition,

  const BitDepth = 32
  
  when BitDepth == 8:
    type FileBits = uint8
  
  when BitDepth == 16:
    type FileBits = uint16
  
  when BitDepth == 24:
    type FileBits = uint32
  
  when BitDepth == 32:
    type FileBits = uint32
  
  when BitDepth == 64:
    type FileBits = uint64
  
  var
      bits : FileBits
  
  echo sizeof(bits)
  Run

---

• ingo (orginal) [2021-06-02T17:25:15+02:00] view original

  Thank you. I'll have a poke at both ways.

---

• shirleyquirk (orginal) [2021-06-02T18:13:16+02:00] view original

  this can be slightly less boilerplatey and error-checked if you fancy:

  from macros import error
  type FileBits = (
    when BitDepth == 8: uint8
    elif BitDepth == 16: uint16
    elif BitDepth in {24,32}: uint32
    elif BitDepth == 64: uint64
    else: static: error("bitdepth of " & $BitDepth & "unsupported"); nil
    )
  Run

---

• sky_khan (orginal) [2021-06-02T18:54:59+02:00] view original

  Yep, thats nicer but considering he/she had not heard of generics, I didnt want to talk about macros :)

---

• shirleyquirk (orginal) [2021-06-02T19:00:58+02:00] view original

  i know, i probably scared them away. i have no self-restraint. annoyingly i couldn't get it to work without them :[

---

• ingo (orginal) [2021-06-02T19:18:35+02:00] view original

  Haha, no I'm not that easily scared. More to study.

  Thanks

---

• Araq (orginal) [2021-06-03T15:14:47+02:00] view original

  Just use uint64 and let it contain unused bits.

---