C++/MFC Error accessing control's variable - c++

Question

I created a control's variable for CEdit:
class CGateDlg : public CDialog
{
...
public:
// here is my control's variable
CEdit m_edit_a;
// here I map variable to control
virtual void DoDataExchange(CDataExchange* pDX);
}
And this is how I map my variable to the control:
void CGateDlg::DoDataExchange(CDataExchange* pDX)
{
CDialog::DoDataExchange(pDX);
DDX_Control(pDX, IDC_EDIT_A, m_edit_a);
}
This is how it works: user types some text into the edit box. Then he presses the "Reset" button which clears the edit box. This is a piece of code responsible for clearing edit box after clicking Reset button:
void CGateDlg::OnBnClickedReset()
{
// clear edit box
m_edit_a.SetWindowTextW(L"");
}
Application starts without any errors. I type some text into EditBox and hit "Reset" button. Then I get an error which leads me to winocc.cpp, line 245 (ENSURE(this)):
void CWnd::SetWindowText(LPCTSTR lpszString)
{
ENSURE(this);
ENSURE(::IsWindow(m_hWnd) || (m_pCtrlSite != NULL));
if (m_pCtrlSite == NULL)
::SetWindowText(m_hWnd, lpszString);
else
m_pCtrlSite->SetWindowText(lpszString);
}
I think the problem is with the hWnd:
this 0x0030fa54 {CEdit hWnd=0x00000000} CWnd * const
but how to fix it ?
Everything works fine when I access my control's value using this:
CEdit *m_edit_a;
m_edit_a = reinterpret_cast<CEdit *>(GetDlgItem(IDC_EDIT_A));
m_edit_a->SetWindowTextW(L"");
What am I doing wrong ?

Answer 1

I can see two possibilities:
The control does not exist when the dialog starts. The first thing that CDialog::OnInitDialog will do is call DoDataExchange, so if you're creating the control later in the initialization process it's too late.
Your own OnInitDialog is not calling CDialog::OnInitDialog so DoDataExchange is not being called.

Answer 2

I think you should no use directly the meber of your control (in this case m_edit_a). Instead you should use a memeber variable, let's say CStrimg m_edit_data, and you should link it to the control:
DDX_Text(pDX, IDC_EDIT_A, m_edit_data); // as you did it in DDC_Cotrol
Now you can use directy the variable, but in order the control to be updated you should use the following code before using it:
UpdateData(true); // unlocks the control in a sense
m_edit_data = "this is my test";
UpdateData(false); // locks the control again (in a sense)
This is normal procedure in MFC :), hope I helped...
ohh... you should also add the control to String Table ... (let me know if you do not know)

Answer 3

I can not find something wrong with you. I Create a new project using VC6.0,and associate a variable to the Edit,just link you do. the exe operates normally.
class CEditTestDlg : public CDialog
{
// Construction
public:
CEditTestDlg(CWnd* pParent = NULL); // standard constructor
// Dialog Data
//{{AFX_DATA(CEditTestDlg)
enum { IDD = IDD_EDITTEST_DIALOG };
CEdit m_Edit;
//}}AFX_DATA
// ClassWizard generated virtual function overrides
//{{AFX_VIRTUAL(CEditTestDlg)
protected:
virtual void DoDataExchange(CDataExchange* pDX); // DDX/DDV support
//}}AFX_VIRTUAL
......
.cpp
void CEditTestDlg::DoDataExchange(CDataExchange* pDX)
{
CDialog::DoDataExchange(pDX);
//{{AFX_DATA_MAP(CEditTestDlg)
DDX_Control(pDX, IDC_EDIT1, m_Edit);
//}}AFX_DATA_MAP
}
void CEditTestDlg::OnBnClickedReset()
{
// TODO: Add your control notification handler code here
m_Edit.SetWindowText("tttt");
}
so,I think it is not a code problem.You had better try again.

Answer 4

If your dialog starts off calling CDialog::OnInitDialog() and your DoDataExchange starts off calling CDialog::DoDataExchange but still you have null hWnd pointers and get CNotSupportedException, make sure your resource (rc) file's dialog template includes all the controls (IDC_) and such you have in DoDataExchange.
Check for overriding definitions if using a DLL that also provides resources.