Solved

CString Access Violation mystery

Posted on 1998-05-06
15
954 Views
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
15 Comments
 
LVL 7

Expert Comment

by:galkin
Comment Utility
Where is your message handler?
0
 
LVL 1

Expert Comment

by:Blondie050798
Comment Utility
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
Comment Utility
Your TName may be NULL or corrupted pointer. Check it is value in debuger.
0
 

Author Comment

by:accidentprone
Comment Utility
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
Comment Utility
Adjusted points to 78
0
 

Author Comment

by:accidentprone
Comment Utility
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
Comment Utility
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
What Should I Do With This Threat Intelligence?

Are you wondering if you actually need threat intelligence? The answer is yes. We explain the basics for creating useful threat intelligence.

 

Author Comment

by:accidentprone
Comment Utility
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
Comment Utility
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
Comment Utility
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
Comment Utility
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
Comment Utility
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
Comment Utility
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 70 total points
Comment Utility
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
Comment Utility
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

Featured Post

How your wiki can always stay up-to-date

Quip doubles as a “living” wiki and a project management tool that evolves with your organization. As you finish projects in Quip, the work remains, easily accessible to all team members, new and old.
- Increase transparency
- Onboard new hires faster
- Access from mobile/offline

Join & Write a Comment

Introduction: Dialogs (1) modal - maintaining the database. Continuing from the ninth article about sudoku.   You might have heard of modal and modeless dialogs.  Here with this Sudoku application will we use one of each type: a modal dialog …
If you use Adobe Reader X it is possible you can't open OLE PDF documents in the standard. The reason is the 'save box mode' in adobe reader X. Many people think the protected Mode of adobe reader x is only to stop the write access. But this fe…
This video will show you how to get GIT to work in Eclipse.   It will walk you through how to install the EGit plugin in eclipse and how to checkout an existing repository.
This video explains how to create simple products associated to Magento configurable product and offers fast way of their generation with Store Manager for Magento tool.

743 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question

Need Help in Real-Time?

Connect with top rated Experts

17 Experts available now in Live!

Get 1:1 Help Now