why am I getting this error when compiling an mfc application? - c++

Question

I have this code:
void CALLBACK CTestTimeUpDlg::MyTimerProc(
HWND hWnd, // handle of CWnd that called SetTimer
UINT nMsg, // WM_TIMER
UINT_PTR nIDEvent, // timer identification
DWORD dwTime // system time
)
{
const int m_TimerValue=0;
double timeValueSec=m_TimerValue/1000.0;
CString valueString;
valueString.Format(L"%3.3f",timeValueSec);
m_TimerDisplayValue.SetWindowTextW(valueString);
}
void CTestTimeUpDlg::OnBnClickedButtonStart()
{
m_TimerValue=0;
m_Timer = SetTimer(1, 1, &CTestTimeUpDlg::MyTimerProc);
}
but when I compiled it, I am getting this error:
'CWnd::SetTimer' : cannot convert parameter 3 from 'void (__stdcall CTestTimeUpDlg::* )(HWND,UINT,UINT_PTR,DWORD)' to 'void (__stdcall *)(HWND,UINT,UINT_PTR,DWORD)'
the code is similar to the code from Microsoft documentation:
http://msdn.microsoft.com/en-us/library/49313fdf.aspx

Answer 1

You should make CTestTimeUpDlg::MyTimerProc static. However, by doing this, you can't access instance members such as m_TimerDisplayValue.
You shouldn't use callback in this case. Set lpfnTimer NULL, as the first timer in the sample of the link. That way, the timer posts the message WM_TIMER, and you can handle it by your non-static member function.
ADD:
Seems the document (plus my words above) is lacking in explanation.
Do as followings to implement a handler of WM_TIMER.
Declare handler in your class declaration:
afx_msg void OnTimer(UINT_PTR nIDEvent);
In your cpp file, add message mapping:
BEGIN_MESSAGE_MAP(CTestTimeUpDlg, ...)
ON_WM_TIMER()
END_MESSAGE_MAP()
and implementation:
void CTestTimeUpDlg::OnTimer(UINT_PTR nIDEvent)
{
// your code here...
}