CArchive原理(三)

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;
}

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