DYNAMIC_DOWNCAST
DYNAMIC_DOWNCAST 是MFC中的,字面意思是,“动态向下转型”,主要用于 父类指针 转换为 子类指针,安全,可以用返回是否为 NULL 判断;而用强制转换,不安全。
必须是由MFC中的CObject派生,且支持了RTTI(运行时类型识别)即 DECLARE_DYNAMIC / IMPLEMENT_DYNAMIC 看:MFC中动态创建DECLARE_DYNCREATE和运行时类型识别DECLARE_DYNAMIC
例如:
// 这个object参数,就是pointer(指针)
#define DYNAMIC_DOWNCAST(class_name, object)
(class_name)AfxDynamicDownCast(RUNTIME_CLASS(class_name), object)
CObject AFX_CDECL AfxDynamicDownCast(CRuntimeClass pClass, CObject pObject)
{
if(pObject != NULL && pObject->IsKindOf(pClass))
return pObject;
else
return NULL;
}
//使用方法
class_name p = DYNAMIC_DOWNCAST(class_name, pointer);
//将指针pointer安全地转换为 class_name ,若pointer确是指向class_name类型的对象,则返回适当的指针;否则,返回NULL
//强制类型转换(不安全),因为,若pointer不指向class_name类型的对象,没有提示,后续的操作会异常。
class_name p = (class_name)pointer;
//自己写了个MFC对话框类 CMFCDialogDlg,继承CDialog -> CWnd
CWnd pWnd = new CMFCDialogDlg;
CMFCDialogDlg pDlg = DYNAMIC_DOWNCAST(CMFCDialogDlg, pWnd);//ok
ASSERT(pDlg);
CDialog pDlg2 = DYNAMIC_DOWNCAST(CDialog, pWnd);//ok
ASSERT(pDlg2);
CView pV = DYNAMIC_DOWNCAST(CView, pWnd);//NULL
ASSERT(pV);
class CObject
{
public:
// Object model (types, destruction, allocation)
virtual CRuntimeClass* GetRuntimeClass() const;
virtual ~CObject() = 0; // virtual destructors are necessary
// 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 CRuntimeClass classCObject;
#ifdef _AFXDLL
static CRuntimeClass* PASCAL _GetBaseClass();
static CRuntimeClass* PASCAL GetThisClass();
#endif
};
//...cpp
BOOL IsKindOf(const CRuntimeClass* pClass) const //常量指针
{
CRuntimeClass* pClassThis = GetRuntimeClass();//GetRuntimeClass()是虚函数,调用的是实际子类中的函数,返回的是实际的类型
while(pClassThis!=NULL)
{
if(pClassThis==pClass)
return TRUE;
pClassThis=pClass->m_pBaseClass;//基类的指针是在IMPLEMENT_DYNAMIC过程中赋值的
}
return FALSE;
}
//所以,pObject->IsKindOf(RUNTIME_CLASS(类名));当“类名”是 pObject实际指向的对象的类型,或其父类,返回TRUE;否则返回FALSE
// Helper macros
#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
#define ASSERT_KINDOF(class_name, object)
ASSERT((object)->IsKindOf(RUNTIME_CLASS(class_name)))
View Code
dynamic_cast
dynamic_cast 是C++标准里的,一般都可以使用,包括向下转型(父->子)、向上转型(子->父)。
可以看看这篇博客:C++强制类型转换:static_cast、dynamic_cast、const_cast、reinterpret_cast
例如:
class Base{};
class Derived :public Base
{
//...
};
Base pB = new Derived;
Derived pD = dynamic_cast
原文链接: https://www.cnblogs.com/htj10/p/13053802.html
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