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CString Access Violation mystery

Posted on 1998-05-06
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Last Modified: 2013-11-20
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!
0
Comment
Question by:accidentprone
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15 Comments
 
LVL 7

Expert Comment

by:galkin
ID: 1308095
Where is your message handler?
0
 
LVL 1

Expert Comment

by:Blondie050798
ID: 1308096
I'd like to know what TName is when it enters the function..can you put a TRACE in and detail the result?
0
 
LVL 8

Expert Comment

by:gelbert
ID: 1308097
Your TName may be NULL or corrupted pointer. Check it is value in debuger.
0
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Author Comment

by:accidentprone
ID: 1308098
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);      
}


0
 

Author Comment

by:accidentprone
ID: 1308099
Adjusted points to 78
0
 

Author Comment

by:accidentprone
ID: 1308100
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!
0
 

Author Comment

by:accidentprone
ID: 1308101
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!
0
 

Author Comment

by:accidentprone
ID: 1308102
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!
0
 

Author Comment

by:accidentprone
ID: 1308103
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!
0
 

Author Comment

by:accidentprone
ID: 1308104
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!
0
 

Author Comment

by:accidentprone
ID: 1308105
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!
0
 

Author Comment

by:accidentprone
ID: 1308106
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!
0
 

Author Comment

by:accidentprone
ID: 1308107
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!
0
 

Accepted Solution

by:
javan earned 210 total points
ID: 1308108
I don't know if this answers your first question, but I think this will answer the question you posed in your last comment.  To get a CString from a combo box in another function?

In the function where you want something to happen, do the following

CString sSelection;

//get the currently selected item.
CComboBox *pCombo = (CComboBox*)GetDlgItem(IDD_COMBOID);

int nCurSel = pCombo->GetCurSel();
if(nCurSel == -1)
     return false;

pCombo->GetLBText(nCurSel, sSelection);


0
 

Author Comment

by:accidentprone
ID: 1308109
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!!!
0

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