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accidentprone

asked on

CString Access Violation mystery

Help me figure out how to fix this!
I'm trying to pass a CString object from a message handler to another function that sets up a file name for opening.

My app has a runtime error at the point where the assignment is made.
When I debug, I go here:

in ...\mfc\include\afx.inl:
______________________________________
// CString
_AFX_INLINE CStringData* CString::GetData() const
      { ASSERT(m_pchData != NULL); return ((CStringData*)m_pchData)-1; }
_AFX_INLINE void CString::Init()
_________________________________________

the call before that was from ...\mfc\src\strcore.cpp:
__________________________________________________
void CString::AllocBeforeWrite(int nLen)
{
if (GetData()->nRefs > 1 || nLen > GetData()->nAllocLength)
   {
      Release();
      AllocBuffer(nLen);
   }
   ASSERT(GetData()->nRefs <= 1);
}
____________________________________________
My code looks like this:
in the header:
_______________________________________
class CFe517App : public CWinApp
{
public:
      CFe517App();
      void SetTutName (LPCTSTR);
private:
      CString m_TutorName;
_____________________________________
in the cpp file:
__________________________________________________
void CFe517App::SetTutName ( LPCTSTR TName)
{      
      m_TutorName = TName;    // this breaks!!
}
___________________________________________________
I've tried this with m_TutorName as both public and private.
Please help me see what I'm doing wrong!
Avatar of galkin
galkin

Where is your message handler?
I'd like to know what TName is when it enters the function..can you put a TRACE in and detail the result?
Your TName may be NULL or corrupted pointer. Check it is value in debuger.
Avatar of accidentprone

ASKER

Running the debugger, I get that TName = 0x0042276c "stat.mwd", when I begin SetTutName, so that is correct. I got kiched out to
...\mfc\src\STRCORE.cpp, like this:
_____________________________
void CString::AllocBeforeWrite(int nLen)
{
      if (GetData()->nRefs > 1 || nLen >  GetData()->nAllocLength) // breaking here!
      {
            Release();
            AllocBuffer(nLen);
      }
      ASSERT(GetData()->nRefs <= 1);
}
__________________________________
and the debugger tells me this:
-      this      0x0012fb58 {""}
-        m_pchData        0x00000000 ""
            CXX0030: Error: expression cannot be evaluated
        nLen      8
 

If I try to F10 or F11 the function, I get a message box saying:
"Unhandled exception in fe517.exe (MFC42D.DLL): 0xC0000005: Access Violation"

Also, galkin, my message handler is in another file: it looks like this:
headerinfo:
___________________________________________
class CTutorDlg : public CDialog
{
// Construction
public:
      CTutorDlg(CWnd* pParent = NULL);   // standard constructor

// Dialog Data
      //{{AFX_DATA(CTutorDlg)
      enum { IDD = IDD_TUTORIALDLG };
      CString      m_TutName;
      //}}AFX_DATA


// Overrides
      // ClassWizard generated virtual function overrides
      //{{AFX_VIRTUAL(CTutorDlg)
      protected:
      virtual void DoDataExchange(CDataExchange* pDX);    // DDX/DDV support
      //}}AFX_VIRTUAL

// Implementation
protected:

      // Generated message map functions
      //{{AFX_MSG(CTutorDlg)
      virtual void OnOK();
      //}}AFX_MSG
      DECLARE_MESSAGE_MAP()
};

and the implementation looks like this:
___________________________________________
void CTutorDlg::OnOK()
{
      CDialog::OnOK();

      AfxMessageBox (m_TutName);

      CFe517App* pApp;                        
      pApp -> SetTutName (m_TutName);      
}


Adjusted points to 78
I also want to mention that if there is a better or easier way to be doing this, that will definitely suffice for an answer.

The idea here is to get a string (CString) out of a CComboBox and into a function somewhere else that can use it.

I'd be grateful if you told me how YOU'd do such a task...see, I even ADDED 48 POINTS!
I also want to mention that if there is a better or easier way to be doing this, that will definitely suffice for an answer.

The idea here is to get a string (CString) out of a CComboBox and into a function somewhere else that can use it.

I'd be grateful if you told me how YOU'd do such a task...see, I even ADDED 48 POINTS!
Well, I think I need to change my question, because I have figured out what I did wrong, but I don't know yet how to do it right.

I think I was  getting a runtime error because, in the message handler, I was creating a pointer to a new copy of the calling object, and not the calling object itself.

I have overridden OnOK() to return a string, but I don't know how to grab hold of it before the dialog box exits.

Thanks for reading this far!
Well, I think I need to change my question, because I have figured out what I did wrong, but I don't know yet how to do it right.

I think I was  getting a runtime error because, in the message handler, I was creating a pointer to a new copy of the calling object, and not the calling object itself.

I have overridden OnOK() to return a string, but I don't know how to grab hold of it before the dialog box exits.

Thanks for reading this far!
Well, I think I need to change my question, because I have figured out what I did wrong, but I don't know yet how to do it right.

I think I was  getting a runtime error because, in the message handler, I was creating a pointer to a new copy of the calling object, and not the calling object itself.

I have overridden OnOK() to return a string, but I don't know how to grab hold of it before the dialog box exits.

Thanks for reading this far!
Well, I think I need to change my question, because I have figured out what I did wrong, but I don't know yet how to do it right.

I think I was  getting a runtime error because, in the message handler, I was creating a pointer to a new copy of the calling object, and not the calling object itself.

I have overridden OnOK() to return a string, but I don't know how to grab hold of it before the dialog box exits.

Thanks for reading this far!
Well, I think I need to change my question, because I have figured out what I did wrong, but I don't know yet how to do it right.

I think I was  getting a runtime error because, in the message handler, I was creating a pointer to a new copy of the calling object, and not the calling object itself.

I have overridden OnOK() to return a string, but I don't know how to grab hold of it before the dialog box exits.

Thanks for reading this far!
Well, I think I need to change my question, because I have figured out what I did wrong, but I don't know yet how to do it right.

I think I was  getting a runtime error because, in the message handler, I was creating a pointer to a new copy of the calling object, and not the calling object itself.

I have overridden OnOK() to return a string, but I don't know how to grab hold of it before the dialog box exits.

Thanks for reading this far!
ASKER CERTIFIED SOLUTION
Avatar of javan
javan

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This was very good, but because I wanted to get the string out of
the dialog box, the code I ended up using looked like this:


BOOL CFe517App::OnFileViewTutorials()
{
        CTutorDlg      tutordlg;
      if (tutordlg.DoModal () == IDOK)
      {
            m_TutorName = tutordlg.m_TutName;
            CWinApp::OpenDocumentFile (m_TutorName);
            return true;
      }
      return false;
}
thank you very much!!!