HOWTO: Filter Keystrokes in Controls Derived from CEdit Class

Last reviewed: May 28, 1997
Article ID: Q92394

The information in this article applies to:

The Microsoft Foundation Classes (MFC) included with:

    - Microsoft C/C++ version 7.0
    - Microsoft Visual C++ for Windows, versions 1.0, 1.5, 1.51, 1.52
    - Microsoft Visual C++, 32-bit Edition, versions 1.0, 2.0, 2.1, 2.2,
      4.0

SUMMARY

Using the Microsoft Foundation Classes (MFC), if an application derives an edit control of the CEdit class, it can capture messages for that control, process the messages, and then pass them to the Default member function for default processing, if desired.

MORE INFORMATION

One of the messages a control receives is the WM_CHAR message, which is processed by the OnChar member function. By default, OnChar calls the Default member function.

Attempting to change the character passed to an edit control using the following technique can create problems:

   afx_msg void CMyEdit::OnChar(UINT nChar, UINT nRepCnt, UINT nFlags)
   {
      if (nChar == 'a' || nChar == 'A')
         nChar = 'X';

      CEdit::OnChar(nChar, nRepCnt, nFlags);
   }

Note that this code attempts to change every "A" or "a" character into an "X" character by changing the wChar value before calling the base class CEdit::OnChar function. However, CEdit::OnChar calls the Default member function inherited from CWnd. Default uses the original wParam sent to the edit control with the WM_CHAR message and not the wChar value passed to the CEdit::OnChar member function.

To perform this type of processing, the application must directly call the DefWindowProc member function. The following code demonstrates this technique:

   /*
      Assume that CMyEdit is derived from the CEdit class.
   */

   afx_msg void CMyEdit::OnChar(UINT nChar, UINT nRepCnt, UINT nFlags)
   {
      if (nChar == 'a' || nChar == 'A')
         nChar = 'X';
      DefWindowProc(WM_CHAR, nChar, MAKELONG(nRepCnt, nFlags));
   }

Keywords : MfcUI kbfasttip kbhowto
Technology : kbmfc
Version : 1.0 1.5 1.51 1.52 2.0 2.1 2.2 4.
Platform : MS-DOS NT WINDOWS

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Last reviewed: May 28, 1997
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