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The MFC CRuntimeClass Structure and the RUNTIME_CLASS Macro
In a real MFC program, an instance of the CRuntimeClass structure replaces the static s_lpszClassName data member shown above. This structure has data members for the class name and the object size; it also contains a pointer to a special static function, CreateObject, that's supposed to be implemented in the target class. Here's a simplified version of CRuntimeClass:
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;
// Dynamic name lookup and creation
static CRuntimeClass* PASCAL FromName(LPCSTR lpszClassName);
static CRuntimeClass* PASCAL FromName(LPCWSTR lpszClassName);
static CObject* PASCAL CreateObject(LPCSTR lpszClassName);
static CObject* PASCAL CreateObject(LPCWSTR lpszClassName);
// 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
const AFX_CLASSINIT* m_pClassInit;
};
| Note | The real MFC CRuntimeClass structure has additional data members and functions that navigate through the class's hierarchy. This navigation feature is not supported by the official C++ RTTI implementation. |
This structure supports not only class name retrieval but also dynamic creation. Each class you derive from CObject has a static CRuntimeClass data member, provided you use the MFC DECLARE_DYNAMIC, DECLARE_DYNCREATE, or DECLARE_SERIAL macro in the declaration and the corresponding IMPLEMENT macro in the implementation file. The name of the static data member is, by convention, class<class_name>. If your class were named CMyClass, the CRuntimeClass data member would be named classCMyClass.
If you want a pointer to a class's static CRuntimeClass object, you use the MFC RUNTIME_CLASS macro, defined as follows:
#define _RUNTIME_CLASS(class_name)((CRuntimeClass*)(&class_name::class##class_name))
#ifdef _AFXDLL
#define RUNTIME_CLASS(class_name) (class_name::GetThisClass())
#else
#define RUNTIME_CLASS(class_name) _RUNTIME_CLASS(class_name)
#endif
Here's how you use the macro to get the name string from a class name:
ASSERT(RUNTIME_CLASS(CMyClass)->m_lpszClassName == "CMyClass");
If you want the class name string from an object, you call the virtual CObject::GetRuntimeClass function. The function simply returns a pointer to the class's static CRuntimeClass object, just as earlier the GetClassName function returned the name string. Here's the function for CMyClass:
virtual CRuntimeClass* GetRuntimeClass()
const { return &classCMyClass; }
And here's how you call it:
ASSERT(pMyObject->GetRuntimeClass()->m_lpszClassName == "CMyClass");
