很早看了MFC的一些宏的实现,什么RUNTIME_CLASS, DECLARE_DYNAMIC, DECLARE_DYNCREATE,IMPLEMENT_DYNCREATE, etc,看了就烦,现在整理下,免的忘了.

 代码实现

(注:以下宏及其实现取自MFC)

• DECLARE_DYNAMIC

Define:
#define DECLARE_DYNAMIC(class_name) "
public: "
    static const AFX_DATA CRuntimeClass class##class_name; "
    virtual CRuntimeClass* GetRuntimeClass() const; "

E.g.
DECLARE_DYNAMIC(RenderView)
(注:RenderView是继承于MFC中CFormView的一个类)

Equals:

public: 
    static const AFX_DATA CRuntimeClass classRenderView;
    virtual CRuntimeClass* GetRuntimeClass() const;

即declare了一个static的CRuntimeClass变量和一个虚拟函数GetRuntimeClass()

关于CRuntimeClass,其declaration:

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
#ifdef _AFXDLL
    CRuntimeClass* (PASCAL* m_pfnGetBaseClass)();
#else
    CRuntimeClass* m_pBaseClass;
#endif

// Operations
    CObject* CreateObject();
    BOOL IsDerivedFrom(const CRuntimeClass* pBaseClass) const;

// Implementation
    void Store(CArchive& ar) const;
    static CRuntimeClass* PASCAL Load(CArchive& ar, UINT* pwSchemaNum);

    // CRuntimeClass objects linked together in simple list
    CRuntimeClass* m_pNextClass;       // linked list of registered classes
};

结构体,6个成员:
m_lpszClassName 类名字
m_nObjectSize 对象大小
m_wSchema schema
m_pfnCreateObject 函数指针 (对象创建方法)
m_pBaseClass/m_pfnGetBaseClass 指向基类对象的指针/获取基类对象函数的指针 (Runtime的关键)
m_pNextClass 指向下一个此类对象

• DECLARE_DYNCREATE

Define:
// not serializable, but dynamically constructable
#define DECLARE_DYNCREATE(class_name) "
    DECLARE_DYNAMIC(class_name) "
    static CObject* PASCAL CreateObject();


E.g.
DECLARE_DYNCREATE(RenderView)

Equals:

public: 
    static const AFX_DATA CRuntimeClass classRenderView;
    virtual CRuntimeClass* GetRuntimeClass() const;
    static CObject* PASCAL CreateObject();

即declare了一个static的CRuntimeClass变量和一个虚拟函数GetRuntimeClass()和一个static的函数CreateObject()

关于CObject,其declaration:

#ifdef _AFXDLL
class CObject
#else
class AFX_NOVTABLE CObject
#endif
{
public:

// Object model (types, destruction, allocation)
    virtual CRuntimeClass* GetRuntimeClass() const;
    virtual ~CObject();  // virtual destructors are necessary

    // Diagnostic allocations
    void* PASCAL operator new(size_t nSize);
    void* PASCAL operator new(size_t, void* p);
    void PASCAL operator delete(void* p);
#if _MSC_VER >= 1200
    void PASCAL operator delete(void* p, void* pPlace);
#endif

#if defined(_DEBUG) && !defined(_AFX_NO_DEBUG_CRT)
    // for file name/line number tracking using DEBUG_NEW
    void* PASCAL operator new(size_t nSize, LPCSTR lpszFileName, int nLine);
#if _MSC_VER >= 1200
    void PASCAL operator delete(void *p, LPCSTR lpszFileName, int nLine);
#endif
#endif

    // Disable the copy constructor and assignment by default so you will get
    //   compiler errors instead of unexpected behaviour if you pass objects
    //   by value or assign objects.
protected:
    CObject();
private:
    CObject(const CObject& objectSrc);              // no implementation
    void operator=(const CObject& objectSrc);       // no implementation

// Attributes
public:
    BOOL IsSerializable() const;
    BOOL IsKindOf(const CRuntimeClass* pClass) const;

// Overridables
    virtual void Serialize(CArchive& ar);

#if defined(_DEBUG) || defined(_AFXDLL)
    // Diagnostic Support
    virtual void AssertValid() const;
    virtual void Dump(CDumpContext& dc) const;
#endif

// Implementation
public:
    static const AFX_DATA CRuntimeClass classCObject;
#ifdef _AFXDLL
    static CRuntimeClass* PASCAL _GetBaseClass();
#endif
};

包含:GetRuntimeClass()方法,static变量 CRuntimeClass classCObject,static方法 _GetBaseClass() (为NULL,因为没有Base),IsKindOf()方法等.

• RUNTIME_CLASS

#define RUNTIME_CLASS(class_name) ((CRuntimeClass*)(&class_name::class##class_name))

E.g.
RUNTIME_CLASS(RenderView)

Equals:
((CRuntimeClass*)(&RenderView::classRenderView))
即将classRenderView static变量转换成((CRuntimeClass*)指针

• IMPLEMENT_RUNTIMECLASS

Define:
#define IMPLEMENT_RUNTIMECLASS(class_name, base_class_name, wSchema, pfnNew) "
    AFX_COMDAT const AFX_DATADEF CRuntimeClass class_name::class##class_name = { "
        #class_name, sizeof(class class_name), wSchema, pfnNew, "
            RUNTIME_CLASS(base_class_name), NULL }; "
    CRuntimeClass* class_name::GetRuntimeClass() const "
        { return RUNTIME_CLASS(class_name); } "

E.g.
IMPLEMENT_RUNTIMECLASS(RenderView, CFormView, 0xFFFF, RenderView::CreateObject)

Equals:
AFX_COMDAT const AFX_DATADEF CRuntimeClass RenderView::classRenderView = { 
        #RenderView, sizeof(class RenderView), 0xFFFF, RenderView::CreateObject, 
            ((CRuntimeClass*)(&CFormView::classCFormView)), NULL }; 

    CRuntimeClass* RenderView::GetRuntimeClass() const 
        { return ((CRuntimeClass*)(&RenderView::classRenderView)); } 


(##为连接文本, #RenderView为取RenderView字符串)

即implement了static classRenderView变量和GetRuntimeClass()虚拟函数

• IMPLEMENT_DYNCREATE

Define:

#define IMPLEMENT_DYNCREATE(class_name, base_class_name) "
    CObject* PASCAL class_name::CreateObject() "
        { return new class_name; } "
    IMPLEMENT_RUNTIMECLASS(class_name, base_class_name, 0xFFFF, "
        class_name::CreateObject)

E.g.
IMPLEMENT_DYNCREATE(RenderView, CFormView)

Equals:

    CObject* PASCAL RenderView::CreateObject() 
        { return new RenderView; } 

    AFX_COMDAT const AFX_DATADEF CRuntimeClass RenderView::classRenderView = { 
        #RenderView, sizeof(class RenderView), 0xFFFF, RenderView::CreateObject, 
            ((CRuntimeClass*)(&CFormView::classCFormView)), NULL }; 

    CRuntimeClass* RenderView::GetRuntimeClass() const 
        { return ((CRuntimeClass*)(&RenderView::classRenderView)); } 

即implement了static classRenderView变量和GetRuntimeClass()虚拟函数和CreateObject()函数.

用途

综合来看,这套宏的目的是在目标对象(比如RenderView)里面嵌套了一个CRuntimeClass对象,用来支持类似Runtime类型的查询转换等(用以支持MFC的RTTI?).

支持这些,有什么用呢?一个用处是DYNAMIC_DOWNCAST,即MFC里实现的对象指针在类层次上的从上到下转换:
E.g.
...
pCFormView* pView = ...
pRenderView* pRenderView = DYNAMIC_DOWNCAST(RenderView, pView)
...

其实现如下:
CObject* AFX_CDECL AfxDynamicDownCast(CRuntimeClass* pClass, CObject* pObject)
{
    if (pObject != NULL && pObject->IsKindOf(pClass))
        return pObject;
    else
        return NULL;
}

BOOL CObject::IsKindOf(const CRuntimeClass* pClass) const
{
    ASSERT(this != NULL);
    // it better be in valid memory, at least for CObject size
    ASSERT(AfxIsValidAddress(this, sizeof(CObject)));

    // simple SI case
    CRuntimeClass* pClassThis = GetRuntimeClass();
    return pClassThis->IsDerivedFrom(pClass);
}

BOOL CRuntimeClass::IsDerivedFrom(const CRuntimeClass* pBaseClass) const
{
    ASSERT(this != NULL);
    ASSERT(AfxIsValidAddress(this, sizeof(CRuntimeClass), FALSE));
    ASSERT(pBaseClass != NULL);
    ASSERT(AfxIsValidAddress(pBaseClass, sizeof(CRuntimeClass), FALSE));

    // simple SI case
    const CRuntimeClass* pClassThis = this;
    while (pClassThis != NULL)
    {
        if (pClassThis == pBaseClass)
            return TRUE;
#ifdef _AFXDLL
        pClassThis = (*pClassThis->m_pfnGetBaseClass)();
#else
        pClassThis = pClassThis->m_pBaseClass;
#endif
    }
    return FALSE;       // walked to the top, no match
}

实现原理:RenderView继承自CFormView,后者又继承自CObject,它们本身又嵌套了static CRuntimeClass对象,那么查询一个指向CFormView对象的指针(pCFormView)是不是实际上就是一个指向RenderView对象的指针的功能是通过比较pCFormView指向的对象中的CRuntimeClass对象(或者其BaseRuntimeClass(或BaseRuntimeClass的BaseRuntimeClass...)对象)是不是就是(比较指针值)RenderView类所含的static CRuntimeClass对象(IsDerivedFrom方法)这么简单了?

如果对象一样(即指针值相等)的话则可以转换成功,否则失败.

(关键:嵌入的CRuntimeClass是静态的,可以通过类访问,又可以通过对象的非静态函数调用,这是实现的关键.因为继承于CObject层次上的每个类都有唯一的CRuntimeClass对象与之对应, 所以它可以成为类型的一个标识符,如果表示符一样了,那么肯定类型是一样的,而这个标识符既可以通过类访问又可以在运行时刻通过对象访问,所以取名CRuntimeClass.)