要将<< 和>> 两个运算子多载化，还要让Serialize 函数神不知鬼不觉地放入类别声明
之中，最好的作法仍然是使用宏。
类别之能够进行文件读写动作，前提是拥有动态生成的能力，所以，MFC 设计了两个宏
DECLARE_SERIAL 和IMPLEMENT_SERIAL：

#define DECLARE_SERIAL(class_name) \
DECLARE_DYNCREATE(class_name) \
friend CArchive& AFXAPI operator>>(CArchive& ar, class_name* &pOb);
#define IMPLEMENT_SERIAL(class_name, base_class_name, wSchema) \
CObject* PASCAL class_name::CreateObject() \
{ return new class_name; } \
_IMPLEMENT_RUNTIMECLASS(class_name, base_class_name, wSchema, \
class_name::CreateObject) \
CArchive& AFXAPI operator>>(CArchive& ar, class_name* &pOb) \
{ pOb = (class_name*) ar.ReadObject(RUNTIME_CLASS(class_name)); \
return ar; } \

为了在每一个对象被处理（读或写）之前，能够处理琐屑的工作，诸如判断是否第一次
出现、记录版本号码、记录文件名等工作，CRuntimeClass 需要两个函数Load 和Store

struct CRuntimeClass
{
// Attributes
LPCSTR m_lpszClassName;
int m_nObjectSize;
UINT m_wSchema; // schema number of the loaded class
CObject* (PASCAL* m_pfnCreateObject)(); // NULL => abstract class
CRuntimeClass* m_pBaseClass;
CObject* CreateObject();
void Store(CArchive& ar) const;
static CRuntimeClass* PASCAL Load(CArchive& ar, UINT* pwSchemaNum);
// CRuntimeClass objects linked together in simple list
static CRuntimeClass* pFirstClass; // start of class list
CRuntimeClass* m_pNextClass; // linked list of registered classes
};

你已经在上一节看过Load 函数，当时为了简化，我把它的参数拿掉，改为由屏幕上获
得类别名称，事实上它应该是从文件中读一个类别名称。至于Store 函数，是把类别名
称写入文件中：

// Runtime class serialization code
CRuntimeClass* PASCAL CRuntimeClass::Load(CArchive& ar, UINT* pwSchemaNum)
{
WORD nLen;
char szClassName[64];
CRuntimeClass* pClass;
ar >> (WORD&)(*pwSchemaNum) >> nLen;
if (nLen >= sizeof(szClassName) || ar.Read(szClassName, nLen) != nLen)
return NULL;
szClassName[nLen] = ~\0~;
for (pClass = pFirstClass; pClass != NULL; pClass = pClass->m_pNextClass)
{
if (lstrcmp(szClassName, pClass->m_lpszClassName) == 0)
return pClass;
}
return NULL; // not found
}
void CRuntimeClass::Store(CArchive& ar) const
// stores a runtime class description
{
WORD nLen = (WORD)lstrlenA(m_lpszClassName);
ar << (WORD)m_wSchema << nLen;
ar.Write(m_lpszClassName, nLen*sizeof(char));
}
class CScribDoc : public CDocument
{
DECLARE_DYNCREATE(CScribDoc)
…
};
class CStroke : public CObject
{
DECLARE_SERIAL(CStroke)
public:

void Serialize(CArchive&);
…
};
class CRectangle : public CObject
{
DECLARE_SERIAL(CRectangle)
public:
void Serialize(CArchive&);
…
};
class CCircle : public CObject
{
DECLARE_SERIAL(CCircle)
public:
void Serialize(CArchive&);
…
};

以及在.CPP 档中做这样的动作：

IMPLEMENT_DYNCREATE(CScribDoc, CDocument)
IMPLEMENT_SERIAL(CStroke, CObject, 2)
IMPLEMENT_SERIAL(CRectangle, CObject, 1)
IMPLEMENT_SERIAL(CCircle, CObject, 1)

然后呢？分头设计CStroke、CRectangle 和CCircle 的Serialize 函数吧。
当然，毫不令人意外地，MFC 源代码中的CObList 和CDWordArray 有这样的内容：

// in header files
class CDWordArray : public CObject
{
DECLARE_SERIAL(CDWordArray)
public:
void Serialize(CArchive&);
…
};
class CObList : public CObject
{
DECLARE_SERIAL(CObList)
public:
void Serialize(CArchive&);
…
};
// in implementation files

IMPLEMENT_SERIAL(CObList, CObject, 0)
IMPLEMENT_SERIAL(CDWordArray, CObject, 0)

而CObject 也多了一个虚拟函数Serialize：
class CObject
{
public:
virtual void Serialize(CArchive& ar);
…
}