CArchive原理

FMD开发文集 -- CArchive原理
MFC 提供CArchive类实现数据的缓冲区读写，同时定义了类对象的存储与读取方案。 以下对CArchvie 的内部实现作分析。
一.概述
CArchive使用了缓冲区，即一段内存空间作为临时数据存储地，对CArchive的读写都先依次排列到此缓冲区，当缓冲区满或用户要求时，将此段整理后的数据读写到指定的存储煤质。
当建立CArchive对象时，应指定其模式是用于缓冲区读，还是用于缓冲区写。
可以这样理解，CArchive对象相当于铁路的货运练调度站，零散的货物被收集，当总量到达火车运量的时候，由火车装运走。
当接到火车的货物时，则货物由被分散到各自的货主。与货运不同的是，交货、取货是按时间循序执行的，而不是凭票据。因此必须保证送货的和取货的货主按同样的循序去存或取。
对于大型的货物，则是拆散成火车单位，运走，取货时，依次取各部分，组装成原物。
二.内部数据
缓冲区指针 BYTE* m_lpBufStart,指向缓冲区，这个缓冲区有可能是底层CFile(如派生类CMemFile)对象提供的，但一般是CArchive自己建立的。
缓冲区尾部指针 BYTE* m_lpBufMax;
缓冲区当前位置指针 BYTE* m_lpBufCur;
初始化时，如果是读模式,当前位置在尾部，如果是写模式，当前位置在头部:
m_lpBufCur = (IsLoading()) ? m_lpBufMax : m_lpBufStart;
三.基本数据读写
对于基本的数据类型，例如字节、双字等，可以直接使用">>"、"<<"符号进行读出、写入。
//操作符定义捕:

//插入操作
CArchive& operator<<(BYTE by);
CArchive& operator<<(WORD w);
CArchive& operator<<(LONG l);
CArchive& operator<<(DWORD dw);
CArchive& operator<<(float f);
CArchive& operator<<(double d);
CArchive& operator<<(int i);
CArchive& operator<<(short w);
CArchive& operator<<(char ch);
CArchive& operator<<(unsigned u);

//提取操作
CArchive& operator>>(BYTE& by);
CArchive& operator>>(WORD& w);
CArchive& operator>>(DWORD& dw);
CArchive& operator>>(LONG& l);
CArchive& operator>>(float& f);
CArchive& operator>>(double& d);

CArchive& operator>>(int& i);
CArchive& operator>>(short& w);
CArchive& operator>>(char& ch);
CArchive& operator>>(unsigned& u);
下面以双字为例，分析原码
双字的插入(写)
CArchive& CArchive::operator<<(DWORD dw)
{
if (m_lpBufCur + sizeof(DWORD) > m_lpBufMax) //缓冲区空间不够
  Flush();  //缓冲区内容提交到实际存储煤质。

if (!(m_nMode & bNoByteSwap))
  _AfxByteSwap(dw, m_lpBufCur);  //处理字节顺序
else
  *(DWORD*)m_lpBufCur = dw;      //添入缓冲区

m_lpBufCur += sizeof(DWORD);     //移动当前指针
return *this;
}
双字的提取(读)
CArchive& CArchive::operator>>(DWORD& dw)
{
if (m_lpBufCur + sizeof(DWORD) > m_lpBufMax) //缓冲区要读完了
  FillBuffer(sizeof(DWORD) - (UINT)(m_lpBufMax - m_lpBufCur));  //重新读入内容到缓冲区

dw = *(DWORD*)m_lpBufCur;  //读取双字
m_lpBufCur += sizeof(DWORD); //移动当前位置指针

if (!(m_nMode & bNoByteSwap))
  _AfxByteSwap(dw, (BYTE*)&dw);  //处理字节顺序
return *this;
}
四.缓冲区的更新

以上操作中，当缓冲区将插入满或缓冲区将提取空时，都将对缓冲区进行更新处理。

缓冲区将插入满时调用Flush();
void CArchive::Flush()
{
ASSERT_VALID(m_pFile);
ASSERT(m_bDirectBuffer || m_lpBufStart != NULL);
ASSERT(m_bDirectBuffer || m_lpBufCur != NULL);
ASSERT(m_lpBufStart == NULL ||
  AfxIsValidAddress(m_lpBufStart, m_lpBufMax - m_lpBufStart, IsStoring()));
ASSERT(m_lpBufCur == NULL ||
  AfxIsValidAddress(m_lpBufCur, m_lpBufMax - m_lpBufCur, IsStoring()));

if (IsLoading())
{
  // unget the characters in the buffer, seek back unused amount
  if (m_lpBufMax != m_lpBufCur)
   m_pFile-> Seek(-(m_lpBufMax - m_lpBufCur), CFile::current);
  m_lpBufCur = m_lpBufMax;    // 指向尾
}
else   //写模式
{
  if (!m_bDirectBuffer)
  {
   // 内容写入到文件
   if (m_lpBufCur != m_lpBufStart)
    m_pFile-> Write(m_lpBufStart, m_lpBufCur - m_lpBufStart);
  }
  else
  {
   //如果是直接针对内存区域的的(例如CMemFile中) (只需移动相关指针，指向新的一块内存)
   if (m_lpBufCur != m_lpBufStart)
    m_pFile-> GetBufferPtr(CFile::bufferCommit, m_lpBufCur - m_lpBufStart);
   // get next buffer
   VERIFY(m_pFile-> GetBufferPtr(CFile::bufferWrite, m_nBufSize,
    (void**)&m_lpBufStart, (void**)&m_lpBufMax) == (UINT)m_nBufSize);
   ASSERT((UINT)m_nBufSize == (UINT)(m_lpBufMax - m_lpBufStart));
  }
  m_lpBufCur = m_lpBufStart; //指向缓冲区首
}
}
缓冲区将提取空,会调用FillBuffer。 nBytesNeeded为当前剩余部分上尚有用的字节
void CArchive::FillBuffer(UINT nBytesNeeded)
{
ASSERT_VALID(m_pFile);
ASSERT(IsLoading());
ASSERT(m_bDirectBuffer || m_lpBufStart != NULL);
ASSERT(m_bDirectBuffer || m_lpBufCur != NULL);
ASSERT(nBytesNeeded > 0);
ASSERT(nBytesNeeded <= (UINT)m_nBufSize);
ASSERT(m_lpBufStart == NULL ||
  AfxIsValidAddress(m_lpBufStart, m_lpBufMax - m_lpBufStart, FALSE));
ASSERT(m_lpBufCur == NULL ||
  AfxIsValidAddress(m_lpBufCur, m_lpBufMax - m_lpBufCur, FALSE));

UINT nUnused = m_lpBufMax - m_lpBufCur;
ULONG nTotalNeeded = ((ULONG)nBytesNeeded) + nUnused;

// 从文件中读取
if (!m_bDirectBuffer)
{
  ASSERT(m_lpBufCur != NULL);
  ASSERT(m_lpBufStart != NULL);
  ASSERT(m_lpBufMax != NULL);

  if (m_lpBufCur > m_lpBufStart)
  {
   //保留剩余的尚未处理的部分，将它们移动到头
   if ((int)nUnused > 0)
   {
    memmove(m_lpBufStart, m_lpBufCur, nUnused);
    m_lpBufCur = m_lpBufStart;
    m_lpBufMax = m_lpBufStart + nUnused;
   }

   // read to satisfy nBytesNeeded or nLeft if possible
   UINT nRead = nUnused;
   UINT nLeft = m_nBufSize-nUnused;
   UINT nBytes;
   BYTE* lpTemp = m_lpBufStart + nUnused;
   do
   {
    nBytes = m_pFile-> Read(lpTemp, nLeft);
    lpTemp = lpTemp + nBytes;
    nRead += nBytes;
    nLeft -= nBytes;
   }
   while (nBytes > 0 && nLeft > 0 && nRead < nBytesNeeded);

   m_lpBufCur = m_lpBufStart;
   m_lpBufMax = m_lpBufStart + nRead;
  }
}
else
{
  // 如果是针对内存区域(CMemFile),移动相关指针，指向新的一块内存
  if (nUnused != 0)
   m_pFile-> Seek(-(LONG)nUnused, CFile::current);
  UINT nActual = m_pFile-> GetBufferPtr(CFile::bufferRead, m_nBufSize,
   (void**)&m_lpBufStart, (void**)&m_lpBufMax);
  ASSERT(nActual == (UINT)(m_lpBufMax - m_lpBufStart));
  m_lpBufCur = m_lpBufStart;
}

// not enough data to fill request?
if ((ULONG)(m_lpBufMax - m_lpBufCur) < nTotalNeeded)
  AfxThrowArchiveException(CArchiveException::endOfFile);
}
五.指定长度数据段落的读写

以下分析
UINT Read(void* lpBuf, UINT nMax); 读取长度为nMax的数据
void Write(const void* lpBuf, UINT nMax); 写入指定长度nMax的数据
对于大段数据的读写，先使用当前缓冲区中的内容或空间读取或写入，若这些空间够用了，则结束。
否则，从剩余的数据中找出最大的缓冲区整数倍大小的一块数据，直接读写到存储煤质(不反复使用缓冲区)。
剩余的余数部分，再使用缓冲区读写。
(说明:缓冲区读写的主要目的是将零散的数据以缓冲区大小为尺度来处理。对于大型数据，其中间的部分，不是零散的数据，使用缓冲区已经没有意思，故直接读写)
①读取
UINT CArchive::Read(void* lpBuf, UINT nMax)
{
ASSERT_VALID(m_pFile);
if (nMax == 0)
  return 0;

UINT nMaxTemp = nMax;  //还需要读入的长度，读入一部分，就减相应数值，直到此数值变为零

//处理当前缓冲区中剩余部分。
//如果要求读入字节小于缓冲区中剩余部分,则第一部分为要求读入的字节数，
//否则读入全部剩余部分
UINT nTemp = min(nMaxTemp, (UINT)(m_lpBufMax - m_lpBufCur));  
memcpy(lpBuf, m_lpBufCur, nTemp);
m_lpBufCur += nTemp;
lpBuf = (BYTE*)lpBuf + nTemp; //移动读出内容所在区域的指针
nMaxTemp -= nTemp;

//当前缓冲区中剩余部分不够要求读入的长度。
//还有字节需要读，则需要根据需要执行若干次填充缓冲区，读出，直到读出指定字节。
if (nMaxTemp != 0) 
{
  //计算出去除尾数部分的字节大小(整数个缓冲区大小)
  //对于这些部分，字节从文件对象中读出，放到输出缓冲区
  nTemp = nMaxTemp - (nMaxTemp % m_nBufSize); 
  UINT nRead = 0;

  UINT nLeft = nTemp;
  UINT nBytes;
  do
  {
   nBytes = m_pFile-> Read(lpBuf, nLeft); //要求读入此整数缓冲区部分大小
   lpBuf = (BYTE*)lpBuf + nBytes;
   nRead += nBytes;
   nLeft -= nBytes;
  }
  while ((nBytes > 0) && (nLeft > 0)); 知道读入了预定大小，或到达文件尾

  nMaxTemp -= nRead;

  if (nRead == nTemp) //读入的字节等于读入的整数倍部分  该读最后的余数部分了
  {
   // 建立装有此最后余数部分的内容的CArchive的工作缓冲区。
   if (!m_bDirectBuffer)
   {
    UINT nLeft = max(nMaxTemp, (UINT)m_nBufSize);
    UINT nBytes;
    BYTE* lpTemp = m_lpBufStart;
    nRead = 0;
    do
    {
     nBytes = m_pFile-> Read(lpTemp, nLeft);  //从文件中读入到CArchive缓冲区
     lpTemp = lpTemp + nBytes;
     nRead += nBytes;
     nLeft -= nBytes;
    }
    while ((nBytes > 0) && (nLeft > 0) && nRead < nMaxTemp);

    m_lpBufCur = m_lpBufStart;
    m_lpBufMax = m_lpBufStart + nRead;
   }
   else
   {
    nRead = m_pFile-> GetBufferPtr(CFile::bufferRead, m_nBufSize,
     (void**)&m_lpBufStart, (void**)&m_lpBufMax);
    ASSERT(nRead == (UINT)(m_lpBufMax - m_lpBufStart));
    m_lpBufCur = m_lpBufStart;
   }

   //读出此剩余部分到输出
   nTemp = min(nMaxTemp, (UINT)(m_lpBufMax - m_lpBufCur));
   memcpy(lpBuf, m_lpBufCur, nTemp);
   m_lpBufCur += nTemp;
   nMaxTemp -= nTemp;
  }
 
}
return nMax - nMaxTemp;
}
②保存,写入
void CArchive::Write(const void* lpBuf, UINT nMax)
{
if (nMax == 0)
  return;

//读入可能的部分到缓冲区当前的剩余部分
UINT nTemp = min(nMax, (UINT)(m_lpBufMax - m_lpBufCur));
memcpy(m_lpBufCur, lpBuf, nTemp);
m_lpBufCur += nTemp;
lpBuf = (BYTE*)lpBuf + nTemp;
nMax -= nTemp;

if (nMax > 0)  //还有未写入的部分
{
  Flush();    //将当前缓冲区写入到存储煤质

  //计算出整数倍缓冲区大小的字节数
  nTemp = nMax - (nMax % m_nBufSize);
  m_pFile-> Write(lpBuf, nTemp);  //直接写到文件
  lpBuf = (BYTE*)lpBuf + nTemp;
  nMax -= nTemp;

  //剩余部分添加到缓冲区
  if (m_bDirectBuffer)
  {
   // sync up direct mode buffer to new file position
   VERIFY(m_pFile-> GetBufferPtr(CFile::bufferWrite, m_nBufSize,
    (void**)&m_lpBufStart, (void**)&m_lpBufMax) == (UINT)m_nBufSize);
   ASSERT((UINT)m_nBufSize == (UINT)(m_lpBufMax - m_lpBufStart));
   m_lpBufCur = m_lpBufStart;
  }

  // copy remaining to active buffer
  ASSERT(nMax < (UINT)m_nBufSize);
  ASSERT(m_lpBufCur == m_lpBufStart);
  memcpy(m_lpBufCur, lpBuf, nMax);
  m_lpBufCur += nMax;
}
}
六.字符串的读写

①CArchive提供的WriteString和ReadString

字符串写
void CArchive::WriteString(LPCTSTR lpsz)
{
ASSERT(AfxIsValidString(lpsz));
Write(lpsz, lstrlen(lpsz) * sizeof(TCHAR));  //调用Write，将字符串对应的一段数据写入
}
字符串读(读取一行字符串)
LPTSTR CArchive::ReadString(LPTSTR lpsz, UINT nMax)
{
// if nMax is negative (such a large number doesn''t make sense given today''s
// 2gb address space), then assume it to mean "keep the newline".
int nStop = (int)nMax < 0 ? -(int)nMax : (int)nMax;
ASSERT(AfxIsValidAddress(lpsz, (nStop+1) * sizeof(TCHAR)));

_TUCHAR ch;
int nRead = 0;

TRY
{
  while (nRead < nStop)
  {
   *this >> ch;  //读出一个字节

   // stop and end-of-line (trailing ''\n'' is ignored)  遇换行—回车
   if (ch == ''\n'' || ch == ''\r'')
   {
    if (ch == ''\r'')
     *this >> ch;
    // store the newline when called with negative nMax
    if ((int)nMax != nStop)
     lpsz[nRead++] = ch;
    break;
   }
   lpsz[nRead++] = ch;
  }
}
CATCH(CArchiveException, e)
{
  if (e-> m_cause == CArchiveException::endOfFile)
  {
   DELETE_EXCEPTION(e);
   if (nRead == 0)
    return NULL;
  }
  else
  {
   THROW_LAST();
  }
}
END_CATCH

lpsz[nRead] = ''\0'';
return lpsz;
}
ReadString到CString对象,可以多行字符
BOOL CArchive::ReadString(CString& rString)
{
rString = &afxChNil;    // empty string without deallocating
const int nMaxSize = 128;
LPTSTR lpsz = rString.GetBuffer(nMaxSize);
LPTSTR lpszResult;
int nLen;
for (;;)
{
  lpszResult = ReadString(lpsz, (UINT)-nMaxSize); // store the newline
  rString.ReleaseBuffer();

  // if string is read completely or EOF
  if (lpszResult == NULL ||
   (nLen = lstrlen(lpsz)) < nMaxSize ||
   lpsz[nLen-1] == ''\n'')
  {
   break;
  }

  nLen = rString.GetLength();
  lpsz = rString.GetBuffer(nMaxSize + nLen) + nLen;
}

// remove ''\n'' from end of string if present
lpsz = rString.GetBuffer(0);
nLen = rString.GetLength();
if (nLen != 0 && lpsz[nLen-1] == ''\n'')
  rString.GetBufferSetLength(nLen-1);

return lpszResult != NULL;
}
②使用CString对象的"<<"与">>"符读写字符串
CString定义了输入输出符,可以象基本类型的数据一样使用CArchive 的操作符定义
friend CArchive& AFXAPI operator<<(CArchive& ar, const CString& string);
friend CArchive& AFXAPI operator>>(CArchive& ar, CString& string);
// CString serialization code
// String format:
//      UNICODE strings are always prefixed by 0xff, 0xfffe
//      if < 0xff chars: len:BYTE, TCHAR chars
//      if >= 0xff characters: 0xff, len:WORD, TCHAR chars
//      if >= 0xfffe characters: 0xff, 0xffff, len:DWORD, TCHARs

CArchive& AFXAPI operator<<(CArchive& ar, const CString& string)
{
// special signature to recognize unicode strings
#ifdef _UNICODE
ar << (BYTE)0xff;
ar << (WORD)0xfffe;
#endif

if (string.GetData()-> nDataLength < 255)
{
  ar << (BYTE)string.GetData()-> nDataLength;
}
else if (string.GetData()-> nDataLength < 0xfffe)
{
  ar << (BYTE)0xff;
  ar << (WORD)string.GetData()-> nDataLength;
}
else
{
  ar << (BYTE)0xff;
  ar << (WORD)0xffff;
  ar << (DWORD)string.GetData()-> nDataLength;
}
ar.Write(string.m_pchData, string.GetData()-> nDataLength*sizeof(TCHAR));
return ar;
}

// return string length or -1 if UNICODE string is found in the archive
AFX_STATIC UINT AFXAPI _AfxReadStringLength(CArchive& ar)
{
DWORD nNewLen;

// attempt BYTE length first
BYTE bLen;
ar >> bLen;

if (bLen < 0xff)
  return bLen;

// attempt WORD length
WORD wLen;
ar >> wLen;
if (wLen == 0xfffe)
{
  // UNICODE string prefix (length will follow)
  return (UINT)-1;
}
else if (wLen == 0xffff)
{
  // read DWORD of length
  ar >> nNewLen;
  return (UINT)nNewLen;
}
else
  return wLen;
}

CArchive& AFXAPI operator>>(CArchive& ar, CString& string)
{
#ifdef _UNICODE
int nConvert = 1;   // if we get ANSI, convert
#else
int nConvert = 0;   // if we get UNICODE, convert
#endif

UINT nNewLen = _AfxReadStringLength(ar);
if (nNewLen == (UINT)-1)
{
  nConvert = 1 - nConvert;
  nNewLen = _AfxReadStringLength(ar);
  ASSERT(nNewLen != -1);
}

// set length of string to new length
UINT nByteLen = nNewLen;
#ifdef _UNICODE
string.GetBufferSetLength((int)nNewLen);
nByteLen += nByteLen * (1 - nConvert);  // bytes to read
#else
nByteLen += nByteLen * nConvert;    // bytes to read
if (nNewLen == 0)
  string.GetBufferSetLength(0);
else
  string.GetBufferSetLength((int)nByteLen+nConvert);
#endif

// read in the characters
if (nNewLen != 0)
{
  ASSERT(nByteLen != 0);

  // read new data
  if (ar.Read(string.m_pchData, nByteLen) != nByteLen)
   AfxThrowArchiveException(CArchiveException::endOfFile);

  // convert the data if as necessary
  if (nConvert != 0)
  {
#ifdef _UNICODE
   CStringData* pOldData = string.GetData();
   LPSTR lpsz = (LPSTR)string.m_pchData;
#else
   CStringData* pOldData = string.GetData();
   LPWSTR lpsz = (LPWSTR)string.m_pchData;
#endif
   lpsz[nNewLen] = ''\0'';    // must be NUL terminated
   string.Init();   // don''t delete the old data
   string = lpsz;   // convert with operator=(LPWCSTR)
   CString::FreeData(pOldData);
  }
}
return ar;
}
七.CObject派生对象的读写
MFC中多数类都从CObject类派生，CObject类与CArchive类有着良好的合作关系，能实现将对象序列化储存到文件或其他媒介中去，或者读取预先储存的对象，动态建立对象等功能。
①CObject定义了针对CArvhive的输入输出操作符,可以向其他基本数据类型一样使用"<<"、"<<"符号
CArchive& AFXAPI operator<<(CArchive& ar, const CObject* pOb)
{ ar.WriteObject(pOb); return ar; }
CArchive& AFXAPI operator>>(CArchive& ar, CObject*& pOb)
{ pOb = ar.ReadObject(NULL); return ar; }
当使用这些符号时，实际上执行的是CArchive的WriteObject和ReadObject成员
②WriteObject与ReadObject
在WriteObject与ReadObject中先写入或读取运行时类信息(CRuntimeClas)，再调用Serialze(..),按其中的代码读写具体的对象数据。
因此，只要在CObject派生类中重载Serilize()函数，写入具体的读写过程，就可以使对象具有存储与创建能力。
//将对象写入到缓冲区
void CArchive::WriteObject(const CObject* pOb)
{
DWORD nObIndex;
// make sure m_pStoreMap is initialized
MapObject(NULL);

if (pOb == NULL)
{
  // save out null tag to represent NULL pointer
  *this << wNullTag;
}
else if ((nObIndex = (DWORD)(*m_pStoreMap)[(void*)pOb]) != 0)
  // assumes initialized to 0 map
{
  // save out index of already stored object
  if (nObIndex < wBigObjectTag)
   *this << (WORD)nObIndex;
  else
  {
   *this << wBigObjectTag;
   *this << nObIndex;
  }
}
else
{
  // write class of object first
  CRuntimeClass* pClassRef = pOb-> GetRuntimeClass();
  WriteClass(pClassRef);  //写入运行类信息

  // enter in stored object table, checking for overflow
  CheckCount();
  (*m_pStoreMap)[(void*)pOb] = (void*)m_nMapCount++;

  // 调用CObject的Serialize成员，按其中的代码写入类中数据。
  ((CObject*)pOb)-> Serialize(*this);
}
}
CObject* CArchive::ReadObject(const CRuntimeClass* pClassRefRequested)
{

// attempt to load next stream as CRuntimeClass
UINT nSchema;
DWORD obTag;
//先读入运行时类信息
CRuntimeClass* pClassRef = ReadClass(pClassRefRequested, &nSchema, &obTag);

// check to see if tag to already loaded object
CObject* pOb;
if (pClassRef == NULL)
{
  if (obTag > (DWORD)m_pLoadArray-> GetUpperBound())
  {
   // tag is too large for the number of objects read so far
   AfxThrowArchiveException(CArchiveException::badIndex,
    m_strFileName);
  }

  pOb = (CObject*)m_pLoadArray-> GetAt(obTag);
  if (pOb != NULL && pClassRefRequested != NULL &&
    !pOb-> IsKindOf(pClassRefRequested))
  {
   // loaded an object but of the wrong class
   AfxThrowArchiveException(CArchiveException::badClass,
    m_strFileName);
  }
}
else
{
  // 建立对象
  pOb = pClassRef-> CreateObject();
  if (pOb == NULL)
   AfxThrowMemoryException();

  // Add to mapping array BEFORE de-serializing
  CheckCount();
  m_pLoadArray-> InsertAt(m_nMapCount++, pOb);

  // Serialize the object with the schema number set in the archive
  UINT nSchemaSave = m_nObjectSchema;
  m_nObjectSchema = nSchema;
  pOb-> Serialize(*this); //调用CObject的Serialize,按其中代码读入对象数据。
  m_nObjectSchema = nSchemaSave;
  ASSERT_VALID(pOb);
}

return pOb;
}
③运行时类信息的读写
为了避免众多重复的同类对象写入重复的类信息，CArchive中使用CMap对象储存和检索类信息。
void CArchive::WriteClass(const CRuntimeClass* pClassRef)
{
ASSERT(pClassRef != NULL);
ASSERT(IsStoring());    // proper direction

if (pClassRef-> m_wSchema == 0xFFFF)
{
  TRACE1("Warning: Cannot call WriteClass/WriteObject for %hs.\n",
   pClassRef-> m_lpszClassName);
  AfxThrowNotSupportedException();
}

// make sure m_pStoreMap is initialized
MapObject(NULL);

// write out class id of pOb, with high bit set to indicate
// new object follows

// ASSUME: initialized to 0 map
DWORD nClassIndex;
if ((nClassIndex = (DWORD)(*m_pStoreMap)[(void*)pClassRef]) != 0)
{
  // previously seen class, write out the index tagged by high bit
  if (nClassIndex < wBigObjectTag)
   *this << (WORD)(wClassTag | nClassIndex);
  else
  {
   *this << wBigObjectTag;
   *this << (dwBigClassTag | nClassIndex);
  }
}
else
{
  // store new class
  *this << wNewClassTag;
  pClassRef-> Store(*this);

  // store new class reference in map, checking for overflow
  CheckCount();
  (*m_pStoreMap)[(void*)pClassRef] = (void*)m_nMapCount++;
}
}
CRuntimeClass* CArchive::ReadClass(const CRuntimeClass* pClassRefRequested,
UINT* pSchema, DWORD* pObTag)
{
ASSERT(pClassRefRequested == NULL ||
  AfxIsValidAddress(pClassRefRequested, sizeof(CRuntimeClass), FALSE));
ASSERT(IsLoading());    // proper direction

if (pClassRefRequested != NULL && pClassRefRequested-> m_wSchema == 0xFFFF)
{
  TRACE1("Warning: Cannot call ReadClass/ReadObject for %hs.\n",
   pClassRefRequested-> m_lpszClassName);
  AfxThrowNotSupportedException();
}

// make sure m_pLoadArray is initialized
MapObject(NULL);

// read object tag - if prefixed by wBigObjectTag then DWORD tag follows
DWORD obTag;
WORD wTag;
*this >> wTag;
if (wTag == wBigObjectTag)
  *this >> obTag;
else
  obTag = ((wTag & wClassTag) << 16) | (wTag & ~wClassTag);

// check for object tag (throw exception if expecting class tag)
if (!(obTag & dwBigClassTag))
{
  if (pObTag == NULL)
   AfxThrowArchiveException(CArchiveException::badIndex, m_strFileName);

  *pObTag = obTag;
  return NULL;
}

CRuntimeClass* pClassRef;
UINT nSchema;
if (wTag == wNewClassTag)
{
  // new object follows a new class id
  if ((pClassRef = CRuntimeClass::Load(*this, &nSchema)) == NULL)
   AfxThrowArchiveException(CArchiveException::badClass, m_strFileName);

  // check nSchema against the expected schema
  if ((pClassRef-> m_wSchema & ~VERSIONABLE_SCHEMA) != nSchema)
  {
   if (!(pClassRef-> m_wSchema & VERSIONABLE_SCHEMA))
   {
    // schema doesn''t match and not marked as VERSIONABLE_SCHEMA
    AfxThrowArchiveException(CArchiveException::badSchema,
     m_strFileName);
   }
   else
   {
    // they differ -- store the schema for later retrieval
    if (m_pSchemaMap == NULL)
     m_pSchemaMap = new CMapPtrToPtr;
    ASSERT_VALID(m_pSchemaMap);
    m_pSchemaMap-> SetAt(pClassRef, (void*)nSchema);
   }
  }
  CheckCount();
  m_pLoadArray-> InsertAt(m_nMapCount++, pClassRef);
}
else
{
  // existing class index in obTag followed by new object
  DWORD nClassIndex = (obTag & ~dwBigClassTag);
  if (nClassIndex == 0 || nClassIndex > (DWORD)m_pLoadArray-> GetUpperBound())
   AfxThrowArchiveException(CArchiveException::badIndex,
    m_strFileName);

  pClassRef = (CRuntimeClass*)m_pLoadArray-> GetAt(nClassIndex);
  ASSERT(pClassRef != NULL);

  // determine schema stored against objects of this type
  void* pTemp;
  BOOL bFound = FALSE;
  nSchema = 0;
  if (m_pSchemaMap != NULL)
  {
   bFound = m_pSchemaMap-> Lookup( pClassRef, pTemp );
   if (bFound)
    nSchema = (UINT)pTemp;
  }
  if (!bFound)
   nSchema = pClassRef-> m_wSchema & ~VERSIONABLE_SCHEMA;
   }

// check for correct derivation
if (pClassRefRequested != NULL &&
  !pClassRef-> IsDerivedFrom(pClassRefRequested))
{
  AfxThrowArchiveException(CArchiveException::badClass, m_strFileName);
}

// store nSchema for later examination
if (pSchema != NULL)
  *pSchema = nSchema;
else
  m_nObjectSchema = nSchema;

// store obTag for later examination
if (pObTag != NULL)
  *pObTag = obTag;

// return the resulting CRuntimeClass*
return pClassRef;
}