MFC Doc/View: How to get the active document anywhere in my application?

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Q: How to get the active document anywhere in my application?

A: There are several methods, one is to get first the active frame then call CFrameWnd::GetActiveDocument.
In an SDI application the main frame is the main window itself so a function which gets the active document may look like:

CDocument* GetSDIActiveDocument()
{
    CDocument* pDoc = NULL;

    CWnd* pWndMain = AfxGetMainWnd();
    ASSERT(pWndMain);
    ASSERT(pWndMain->IsKindOf(RUNTIME_CLASS(CFrameWnd)) && 
          !pWndMain->IsKindOf(RUNTIME_CLASS(CMDIFrameWnd))); // Not an SDI app.
    
    pDoc = ((CFrameWnd*)pWndMain)->GetActiveDocument();

    return pDoc;
}

In MDI applications, we have to additionally get the active MDI child frame.

CDocument* GetMDIActiveDocument()
{
    CDocument* pDoc = NULL;

    CWnd* pWndMain = AfxGetMainWnd();
    ASSERT(pWndMain);
    ASSERT(pWndMain->IsKindOf(RUNTIME_CLASS(CMDIFrameWnd))); // Not an MDI app.

    CFrameWnd* pFrame = ((CMDIFrameWnd*)pWndMain)->MDIGetActive();
    if(NULL != pFrame)
    {
        pDoc = pFrame->GetActiveDocument(); // get the active document
    }
    return pDoc;
}

Putting them together we can write the following generic function to be used either in SDI or MDI applications.

CDocument* GenericGetActiveDocument()
{
    CDocument* pDoc = NULL;
    CWnd* pWndMain = AfxGetMainWnd();
    if(NULL != pWndMain)
    {
        if(pWndMain->IsKindOf(RUNTIME_CLASS(CMDIFrameWnd)))
        {
            // MDI application, so first we have to get the active MDI child frame.
            CFrameWnd* pFrame = ((CMDIFrameWnd*)pWndMain)->MDIGetActive();
            if(NULL != pFrame)
            {
                pDoc = pFrame->GetActiveDocument();
            }
        }
        else if(pWndMain->IsKindOf(RUNTIME_CLASS(CFrameWnd)))
        {
            // SDI appllication so main window is the active frame. 
            pDoc = ((CFrameWnd*)pWndMain)->GetActiveDocument();
        }
        else
        {
            ASSERT(FALSE); // Neither MDI nor SDI application.
        }
    }
    return pDoc;
}

Before casting to a given document type we have to check if indeed the returned pointer is of that type.

    CDocument* pDoc = GenericGetActiveDocument();
    if((NULL != pDoc) && pDoc->IsKindOf(RUNTIME_CLASS(CMyDocument)))
    {
        CMyDocument* pMyDoc = (CMyDocument*)pDoc;
        // enjoy of ponter to CMyDocument object.
    }

To avoid such type of tests in many places, the GenericGetActiveDocument function can be improved, giving to it info about the required document type.

CDocument* GenericGetActiveDocument(CRuntimeClass* pClass)
{
    ASSERT(NULL != pClass); // must be not NULL and derived from CDocument.
    ASSERT(pClass->IsDerivedFrom(RUNTIME_CLASS(CDocument))); 

    CDocument* pDoc = NULL;
    CWnd* pWndMain = AfxGetMainWnd();
    if(NULL != pWndMain)
    {
        if(pWndMain->IsKindOf(RUNTIME_CLASS(CMDIFrameWnd)))
        {
            // MDI application, so first we have to get the active MDI child frame.
            CFrameWnd* pFrame = ((CMDIFrameWnd*)pWndMain)->MDIGetActive();
            if(NULL != pFrame)
            {
                CDocument* pActiveDoc = pFrame->GetActiveDocument();
                if((NULL != pActiveDoc) && pActiveDoc->IsKindOf(pClass))
                {
                    // The found document is of required type
                    pDoc = pActiveDoc;
                }
            }
        }
        else if(pWndMain->IsKindOf(RUNTIME_CLASS(CFrameWnd)))
        {
            // SDI appllication so main window is the active frame. 
            pDoc = ((CFrameWnd*)pWndMain)->GetActiveDocument();
        }
        else
        {
            ASSERT(FALSE); // Neither MDI nor SDI application.
        }
    }
    return pDoc;
}

With the help of a little macro...

#define GET_ACTIVE_DOC(x) (x*)GenericGetActiveDocument(RUNTIME_CLASS(x))

...all remained to write anywhere in the SDI/MDI MFC application is

    CMyDocument* pDoc = GET_ACTIVE_DOC(CMyDocument);

See also:

• How to obtain a pointer to various objects?
• How To Get Current CDocument or CView from Anywhere