CArchive原理(三)

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;
}