CGBitArray : A Packed Array of Flags

#define MAX 1000000

CGBitArray gba(MAX);
for (int i = 2; i < MAX / 2; i++) for (int j = 2 * i; j < MAX; j += i) gba.SetBit(j); for (i = 1; i < MAX; i++) if (!gba.GetBit(i)) printf("%d is primen", i);

And here’s the actual class:

class CGBitArray
{
public:
 void Init(int nStart, int nSize)
 {
  int nLen = nSize / 8;
  if (nSize & 0x07)
  nLen++;

  m_pData = new BYTE[nLen];
  memset(m_pData, 0, nLen);
  m_nStart = nStart;
  m_nSize = nSize;
  m_nSet = 0;
 }

 CGBitArray(int nSize) { Init(0, nSize); }
 CGBitArray(int nStart, int nSize) { Init(nStart, nSize); }
 ~CGBitArray() { delete [] m_pData; }

 void SetBit(int nIndex, BOOL bSet = TRUE)
 {
  nIndex -= m_nStart;
  int nOffset = nIndex / 8;
  int nMask = (1 << (nIndex & 0x07));
  BOOL bAlreadySet = m_pData[nOffset] & nMask;

  if (bSet)
   m_pData[nOffset] |= nMask;
  else
   m_pData[nOffset] &= ~nMask;

  if (bSet && !bAlreadySet)
   m_nSet++;
  else if (!bSet && bAlreadySet)
   m_nSet--;
 }

 BOOL GetBit(int nIndex)
 {
  nIndex -= m_nStart;
  return m_pData[nIndex / 8] & (1 << (nIndex & 0x07));
 }

 BOOL& operator[](ULONG nIndex)
 {
  static BOOL bRet;
  return bRet = GetBit(nIndex);
 }

 int GetSize() { return m_nSize; }
 int GetStart() { return m_nStart; }
 int GetSetCount() { return m_nSet; }
 int GetClearCount() { return m_nSize - m_nSet; }

protected:
 BYTE* m_pData;
 int m_nStart;
 int m_nSize;
 int m_nSet;
};