[MFC] How to get the pointers of Doc,View,MainFrmae,App in SDI, MDI

When developing MFC program on Document/View architecture, the first problem we usually have is how to get the pointers of Doucment, View, MainFrame and App.
Below are some common methods to get the pointers:

1. Get the pointer of Application Class CWinApp:Use MFC Global Function AfxGetApp() to get it.
    
   
2. Get the pointer of MainFrame
   • User member variable: m_pMainWnd
     (m_pMainWnd is the member variable of CWinThread, and CWinApp derived from CWinThread, so we can use the member of CWinThread to get the pointer of Main frame )
     CMainFrame* pMainWnd = (CMainFrame*) AfxGetApp()->m_pMainWnd;
   • Use MFC Global Function AfxGetMainWnd() to get it.
     CMainFrame* pMainWnd =(CMainFrame*) AfxGetMainWnd();
3. Get the pointer of Document in View
   Use the member function GetDocument() to get it.
   CMyAppDoc* pDoc = GetDocument();
4. Get the pointer of Active View
   First, get the pointer of MainFrame, and then use GetActiveView() to get the pointer of View by MainFrame.
   CMainFrame* pMainWnd = (CMainFrame*) AfxGetApp()->m_pMainWnd; 
   CMyAppView* pView = ( CMyAppView*) pMainWnd->GetActiveView();
5. Get the pointer of Active Document
   First, get the pointer of FrameWnd, and then use GetActiveDocument() to get the pointer of Document by FrameWnd.
   CFrameWnd* pMainFrm = (CFrameWnd*) AfxGetApp()->m_pMainWnd;
   CDocument* pCurDoc = (CFrameWnd*) pMainFrm ->GetActiveDocument();