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TN016: Using C++ Multiple Inheritance with MFC

This note describes how to use multiple inheritance (MI) with the Microsoft Foundation Classes. The use of MI is not required with MFC. MI is not used in any MFC classes and is not required to write a class library.

The following subtopics describe how MI affects the use of common MFC idioms as well as covering some of the restrictions of MI. Some of these restrictions are general C++ restrictions. Others are imposed by the MFC architecture.

At the end of this technical note you will find a complete MFC application that uses MI.

The persistence and dynamic object creation mechanisms of MFC use the CRuntimeClass data structure to uniquely identify classes. MFC associates one of these structures with each dynamic and/or serializable class in your application. These structures are initialized when the application starts by using a special static object of type AFX_CLASSINIT.

The current implementation of CRuntimeClass does not support MI runtime type information. This does not mean you cannot use MI in your MFC application. However, you will have certain responsibilities when you work with objects that have more than one base class.

The CObject::IsKindOf method will not correctly determine the type of an object if it has multiple base classes. Therefore, you cannot use CObject as a virtual base class, and all calls to CObject member functions such as CObject::Serialize and CObject::operator new must have scope qualifiers so that C++ can disambiguate the appropriate function call. When a program uses MI within MFC, the class that contains the CObject base class needs to be the left-most class in the list of base classes.

An alternative is to use the dynamic_cast operator. Casting an object with MI to one of its base classes will force the compiler to use the functions in the supplied base class. For more information, see dynamic_cast Operator.

All significant classes derive directly or indirectly from class CObject. CObject does not have any member data, but it does have some default functionality. When you use MI, you will typically inherit from two or more CObject-derived classes. The following example illustrates how a class can inherit from a CFrameWnd and a CObList:

class CListWnd : public CFrameWnd, public CObList
{
 ...
};
CListWnd myListWnd;

In this case CObject is included two times. This means that you need a way to disambiguate any reference to CObject methods or operators. The operator new and operator delete are two operators that must be disambiguated. As another example, the following code causes an error at compile time:

myListWnd.Dump(afxDump);
    // compile time error, CFrameWnd::Dump or CObList::Dump ?

When you create a new class that has two or more CObject derived base classes, you should reimplement the CObject methods that you want other people to use. Operators new and delete are mandatory and Dump is recommended. The following example reimplements the new and delete operators and the Dump method:

class CListWnd : public CFrameWnd, public CObList
{
public:
    void* operator new(size_t nSize)
        { return CFrameWnd::operator new(nSize); }
    void operator delete(void* p)
        { CFrameWnd::operator delete(p); }

    void Dump(CDumpContent& dc)
        { CFrameWnd::Dump(dc);
          CObList::Dump(dc); }
     ...
};

It might seem that virtually inheriting CObject would solve the problem of function ambiguity, but that is not the case. Because there is no member data in CObject, you do not need virtual inheritance to prevent multiple copies of a base class member data. In the first example that was shown earlier, the Dump virtual method is still ambiguous because it is implemented differently in CFrameWnd and CObList. The best way to remove ambiguity is to follow the recommendations presented in the previous section.

The run-time typing mechanism supported by MFC in CObject uses the macros DECLARE_DYNAMIC, IMPLEMENT_DYNAMIC, DECLARE_DYNCREATE, IMPLEMENT_DYNCREATE, DECLARE_SERIAL and IMPLEMENT_SERIAL. These macros can perform a run-time type check to guarantee safe downcasts.

These macros support only a single base class and will work in a limited way for multiply inherited classes. The base class you specify in IMPLEMENT_DYNAMIC or IMPLEMENT_SERIAL should be the first (or left-most) base class. This placement will enable you to do type checking for the left-most base class only. The run-time type system will know nothing about additional base classes. In the following example, the run-time systems will do type checking against CFrameWnd, but will know nothing about CObList.

class CListWnd : public CFrameWnd, public CObList
{
    DECLARE_DYNAMIC(CListWnd)
    ...
};
IMPLEMENT_DYNAMIC(CListWnd, CFrameWnd)

For the MFC message map system to work correctly, there are two additional requirements:

  • There must be only one CWnd-derived base class.

  • The CWnd-derived base class must be the first (or left-most) base class.

Here are some examples that will not work:

class CTwoWindows : public CFrameWnd, public CEdit
    { ... };
        // error : two copies of CWnd

class CListEdit : public CObList, public CEdit
    { ... };
        // error : CEdit (derived from CWnd) must be first

The following sample is a stand-alone application that consists of one class derived from CFrameWnd and CWinApp. We do not recommend that you structure an application in this manner, but this is an example of the smallest MFC application that has one class.

#include <afxwin.h>

class CHelloAppAndFrame : public CFrameWnd, public CWinApp
{ 
public:
    CHelloAppAndFrame()
        { }

    // Necessary because of MI disambiguity
    void* operator new(size_t nSize)
        { return CFrameWnd::operator new(nSize); }
    void operator delete(void* p)
        { CFrameWnd::operator delete(p); }

    // Implementation
    // CWinApp overrides
    virtual BOOL InitInstance();
    // CFrameWnd overrides
    virtual void PostNcDestroy();
    afx_msg void OnPaint();

    DECLARE_MESSAGE_MAP()

};

BEGIN_MESSAGE_MAP(CHelloAppAndFrame, CFrameWnd)
    ON_WM_PAINT()
END_MESSAGE_MAP()

// because the frame window is not allocated on the heap, we must
// override PostNCDestroy not to delete the frame object
void CHelloAppAndFrame::PostNcDestroy()
{
    // do nothing (do not call base class)
}

void CHelloAppAndFrame::OnPaint()
{
    CPaintDC dc(this);
    CRect rect;
    GetClientRect(rect);

    CString s = "Hello, Windows!";
    dc.SetTextAlign(TA_BASELINE | TA_CENTER);
    dc.SetTextColor(::GetSysColor(COLOR_WINDOWTEXT));
    dc.SetBkMode(TRANSPARENT);
    dc.TextOut(rect.right / 2, rect.bottom / 2, s);
}

// Application initialization
BOOL CHelloAppAndFrame::InitInstance()
{
    // first create the main frame
    if (!CFrameWnd::Create(NULL, "Multiple Inheritance Sample",
        WS_OVERLAPPEDWINDOW, rectDefault))
        return FALSE;

    // the application object is also a frame window
    m_pMainWnd = this;          
    ShowWindow(m_nCmdShow);
    return TRUE;
}

CHelloAppAndFrame theHelloAppAndFrame;

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