Constructs a CExcelWnd object and attaches it to Excel's application window
[C++]
CExcelWnd();
This constructor is all that is required to initialise the object. 
            You should ensure that MFC is properly in scope before instantiating the class, 
            by declaring the XLL_FIX_STATE macro, as in the following example.
            XLL_FIX_STATE;
CExcelWnd wndParent;
CMyDialog dlg(&wndParent);
dlg.DoModal();
Header: xllplus.h