Doc的窗口就创建一个，如果已经存在就激活那个窗口，不存在就建立Doc（转自CSDN）

CDocument* CMyApp::CreateOrShow(CMultiDocTemplate*   pDocTemplate) 
{ 
POSITION   posDoc=pDocTemplate-> GetFirstDocPosition(); 
CDocument*   pDocument   =NULL; 
if(posDoc!=NULL){ 
pDocument   =pDocTemplate-> GetNextDoc(posDoc); 
//To   get   the   first   view   in   the   list   of   views: 

POSITION   posView   =   pDocument   -> GetFirstViewPosition(); 
CView*   pFirstView   =   pDocument   -> GetNextView(posView); 
ASSERT(pFirstView   !=NULL); 
pFirstView   -> GetParentFrame()-> GetLastActivePopup()-> BringWindowToTop(); 
return pDocument; 
} 
else{ 
pDocument   =pDocTemplate-> CreateNewDocument(); 
if   (pDocument   ==   NULL) 
{ 
TRACE0( "CDocTemplate::CreateNewDocument   returned   NULL.\n "); 
AfxMessageBox(AFX_IDP_FAILED_TO_CREATE_DOC); 
return   NULL; 
} 
ASSERT_VALID(pDocument); 
CString strTitle; 
pDocTemplate-> GetDocString(strTitle,CDocTemplate::windowTitle); 
pDocument-> SetTitle(strTitle); 
BOOL   bAutoDelete   =   pDocument-> m_bAutoDelete; 
pDocument-> m_bAutoDelete   =   FALSE;       //   don 't   destroy   if   something   goes   wrong 
CFrameWnd*   pFrame   =   pDocTemplate-> CreateNewFrame(pDocument,   NULL); 
pDocument-> m_bAutoDelete   =   bAutoDelete; 
if   (pFrame   ==   NULL) 
{ 
AfxMessageBox(AFX_IDP_FAILED_TO_CREATE_DOC); 
delete   pDocument;               //   explicit   delete   on   error 
return   NULL; 
} 
ASSERT_VALID(pFrame); 
pDocTemplate-> InitialUpdateFrame(pFrame   ,pDocument,TRUE); 
pFrame-> ShowWindow(SW_SHOWMAXIMIZED); 
} 
return pDocument   ; 
} 

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BOOL   CMainFrame::FntCreateOrShow(CMultiDocTemplate*   pDocTemplate) 
{ 
POSITION   posDoc=pDocTemplate-> GetFirstDocPosition(); 
CDocument*   pDocument   =NULL; 
if   (posDoc!=NULL) 
{ 
pDocument   =pDocTemplate-> GetNextDoc(posDoc); 
POSITION   posView   =   pDocument   -> GetFirstViewPosition(); 
CView*   pFirstView   =   pDocument   -> GetNextView(posView); 
ASSERT(pFirstView   !=NULL); 
pFirstView-> GetParentFrame()-> GetLastActivePopup()-> BringWindowToTop();//激活视图 
return   TRUE; 
} 
else 
{ 
pDocTemplate-> OpenDocumentFile(NULL); 
} 
return   FALSE; 
}