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5650 :: Using setLen after newSeqOfCap is not safe?

• tauplus (orginal) [2019-12-07T16:48:53+01:00] view original

  • newSeqUninitialized doesn't initialize seq with zero. It's in the specifications.
  • When I use setLen after newSeqOfCap, expanded part of seq is not initialized with zero.
  • It's in the specifications, too? setLen after newSeqOfCap is not safe?

  const max_seq_size = 100
  const max_itr_size = 10_000
  
  var
    s: seq[int]
  
  for i in countup(1,max_itr_size):
    s = newSeq[int](max_seq_size)
    for j in countup(1, max_seq_size):
      s[j-1] = j
  
  s = newSeqUninitialized[int](max_seq_size)
  echo s[max_seq_size - 1]   # 100 (not initialized with zero, in the specifications)
  
  s = newSeqOfCap[int](max_seq_size)
  s.setLen(max_seq_size)
  echo s[max_seq_size - 1]   # 100 (not initialized with zero)
  Run

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• mratsim (orginal) [2019-12-09T10:27:09+01:00] view original

  setLen only changes the length, it does not initialize with zero except in one case, when Nim needs to reallocate the seq because the reserved buffer was too small but AFAIK that's the OS that gives zero-ed out memory pages by default.

  That can be seen with

  s = newSeqOfCap[int](max_seq_size)
  s.setLen(max_seq_size + 100000)
  echo s[max_seq_size - 1]
  Run

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