MFC RTTI 解析

在 C++ 标准 RTTI（dynamic_cast / typeid）普及之前，MFC就通过 DECLARE_DYNAMIC，IMPLEMENT_DYNAMIC 等宏实现了一套 RTTI。

CRuntimeClass 是核心部分，用于描述每个 class 信息。每个继承 CObject 的 class 和一个 CRuntimeClass 的结构体对应。

struct CRuntimeClass
{
// AttributesLPCSTR m_lpszClassName;int m_nObjectSize;UINT m_wSchema; // schema number of the loaded classCObject* (PASCAL* m_pfnCreateObject)(); // NULL => abstract classCRuntimeClass* m_pBaseClass;// OperationsCObject* CreateObject();BOOL IsDerivedFrom(const CRuntimeClass* pBaseClass) const;...// CRuntimeClass objects linked together in simple listCRuntimeClass* m_pNextClass;       // linked list of registered classes
};

DECLARE_DYNAMIC 在 class 里面注入一个静态成员和虚函数

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

RUNTIME_CLASS 返回某个类所对应的 CRuntimeClass

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

IMPLEMENT_RUNTIMECLASS 初始化静态成员并实现虚函数

#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); } \

MFC 提供了一个更简单的宏 IMPLEMENT_DYNAMIC

#define IMPLEMENT_DYNAMIC(class_name, base_class_name) \IMPLEMENT_RUNTIMECLASS(class_name, base_class_name, 0xFFFF, NULL)

举例

// 展开前
class Foo : CObject
{DECLARE_DYNAMIC(Foo);
};
IMPLEMENT_DYNAMIC(Foo, CObject);// 展开后
class Foo : CObject
{
public:static const CRuntimeClass classFoo;virtual CRuntimeClass* GetRuntimeClass() const;
};CRuntimeClass class_name::classFoo =
{"Foo",sizeof(class Foo),0xFFFF,NULL,(CRuntimeClass*)&CObject::classCObject,NULL
};
CRuntimeClass* class_name::GetRuntimeClass() const
{return (CRuntimeClass*)&Foo::classFoo;
}

IsKindOf 是最常用的类型判断方法。每个 class 的 IsKindOf 方法都调用了 CRuntimeClass::IsDerivedFrom

BOOL CObject::IsKindOf(const CRuntimeClass* pClass) const
{...// simple SI caseCRuntimeClass* pClassThis = GetRuntimeClass();return pClassThis->IsDerivedFrom(pClass);
}

IsDerivedFrom 通过成员 m_pBaseClass 不断向基类遍历，判断继承关系，这样就实现了运行时类型识别

BOOL CRuntimeClass::IsDerivedFrom(const CRuntimeClass* pBaseClass) const
{...// simple SI caseconst CRuntimeClass* pClassThis = this;while (pClassThis != NULL){if (pClassThis == pBaseClass)return TRUE;pClassThis = pClassThis->m_pBaseClass;}return FALSE;       // walked to the top, no match
}