HOWTO: Override an Interface in an MFC Application

ID: Q141277

The information in this article applies to:

The Microsoft Foundation Classes (MFC), included with:
- Microsoft Visual C++, 32-bit Editions, versions 2.0, 2.1, 2.2, 4.0, 5.0, 6.0

SUMMARY

In an MFC application, you can override existing interfaces in a class as well as provide additional interfaces. Overriding an interface in this case is synonymous with replacing an interface. The example in this article illustrates how to override an interface in a class while preserving the original interface implementation so that it can be delegated to by the new interface implementation.

This article doesn't deal with overriding the IDispatch implementation as this is a special case. The following article demonstrates how to override IDispatch in MFC:

Q140616 MFCDISP: Replacing MFC's IDispatch implementation

MORE INFORMATION

The following steps will override the IOleObject implementation for a default OLE Control generated by the Control Wizard.

To add the declaration of the IOleObject implementation to the control, add the following code to the header file for the COleControl-derived class:


      // Interface Maps
      protected:
           // IOleObject
           BEGIN_INTERFACE_PART(MyOleObject, IOleObject)
               INIT_INTERFACE_PART(CIOleOverCtrl, MyOleObject)
               STDMETHOD(SetClientSite)(LPOLECLIENTSITE);
               STDMETHOD(GetClientSite)(LPOLECLIENTSITE*);
               STDMETHOD(SetHostNames)(LPCOLESTR, LPCOLESTR);
               STDMETHOD(Close)(DWORD);
               STDMETHOD(SetMoniker)(DWORD, LPMONIKER);
               STDMETHOD(GetMoniker)(DWORD, DWORD, LPMONIKER*);
               STDMETHOD(InitFromData)(LPDATAOBJECT, BOOL, DWORD);
               STDMETHOD(GetClipboardData)(DWORD, LPDATAOBJECT*);
               STDMETHOD(DoVerb)(LONG, LPMSG, LPOLECLIENTSITE, LONG, HWND,
                       LPCRECT);
               STDMETHOD(EnumVerbs)(IEnumOLEVERB**);
               STDMETHOD(Update)();
               STDMETHOD(IsUpToDate)();
               STDMETHOD(GetUserClassID)(CLSID*);
               STDMETHOD(GetUserType)(DWORD, LPOLESTR*);
               STDMETHOD(SetExtent)(DWORD, LPSIZEL);
               STDMETHOD(GetExtent)(DWORD, LPSIZEL);
               STDMETHOD(Advise)(LPADVISESINK, LPDWORD);
               STDMETHOD(Unadvise)(DWORD);
               STDMETHOD(EnumAdvise)(LPENUMSTATDATA*);
               STDMETHOD(GetMiscStatus)(DWORD, LPDWORD);
               STDMETHOD(SetColorScheme)(LPLOGPALETTE);
           END_INTERFACE_PART(MyOleObject)

      DECLARE_INTERFACE_MAP();

This adds a nested class XMyOleObject to your control class. Note that these macros declare interface methods including the IUnknown interface methods, so you must implement the IUnknown methods as well.

Add the IOleObject interface to the interface map for the control by adding an INTERFACE_PART macro to the implementation file for the control:
```
      BEGIN_INTERFACE_MAP(CIOleOverCtrl, COleControl)
         INTERFACE_PART(CIOleOverCtrl, IID_IOleObject, MyOleObject)
      END_INTERFACE_MAP() 
```
Replace CIOleOverCtrl with the name of your control and MyOleObject with the name you chose for the nested class that supports IOleObject.

Implement the interface methods you declared. Add the following code to the implementation file for the control:


      STDMETHODIMP_(ULONG) CIOleOverCtrl::XMyOleObject::AddRef()
      {
          METHOD_MANAGE_STATE(CIOleOverCtrl, MyOleObject)
          ASSERT_VALID(pThis);

          return pThis->m_xOleObject.AddRef();
      }

      STDMETHODIMP_(ULONG) CIOleOverCtrl::XMyOleObject::Release()
      {
          METHOD_MANAGE_STATE(CIOleOverCtrl, MyOleObject)
          ASSERT_VALID(pThis);

          return pThis->m_xOleObject.Release ();
      }

      STDMETHODIMP CIOleOverCtrl::XMyOleObject::QueryInterface(
          REFIID iid, LPVOID far* ppvObj)
      {
          METHOD_MANAGE_STATE(CIOleOverCtrl, MyOleObject)
          ASSERT_VALID(pThis);

          return pThis->m_xOleObject.QueryInterface ( iid,  ppvObj);
      }

      STDMETHODIMP
      CIOleOverCtrl::XMyOleObject::SetClientSite(LPOLECLIENTSITE
      pClientSite)
      {
          METHOD_MANAGE_STATE(CIOleOverCtrl, MyOleObject)
             ASSERT_VALID(pThis);

          return pThis->m_xOleObject.SetClientSite ( pClientSite );
      }
      ...

The rest of the methods follow the same pattern where CIOleOverCtrl is the name of the control, XMyOleObject is the name of the nested class that supports IOleObject, and m_xMyOleObject is calculated by removing the I from the interface being supported and adding m_x.

Note that these methods simply pass the call on to the original IOleObject implementation. However, this is not a requirement; you could add functionality and delegate to the original implementation or not delegate at all.

REFERENCES

Technical Notes #38 and #39.

Keywords : kbole kbMFC kbVC200 kbVC210 kbVC220 kbVC400 kbVC500 kbVC600
Version : Winnet:2.0,2.1,2.2,4.0,5.0,6.0
Platform : NT WINDOWS
Issue type : kbhowto