A Validating Edit Control

Input Limitation

To reduce the chances of error even further, I lock out an characters that are not actually valid. So for my floating-point control, I disallow all characters except the digits, plus and minus signs, decimal point, and the letters 'e' and 'E'. And backspace. Don't forget backspace! 

A common piece of advice that appears is to "put this in the PreTranslateMessage handler of your dialog". This doesn't make any sense to me; it violates any number of issues of abstraction and object orientation. It makes a lot more sense to me to put this in the control that wants to filter the characters. To do this, I write a handler like the one shown below, which appears in my subclassed dialog. This gets created when I add a WM_CHAR handler using ClassWizard, and all I do is fill in the code shown.

void CFloatingEdit::OnChar(UINT nChar, UINT nRepCnt, UINT nFlags) 
   {
    switch(nChar)
       { /* validate */
	case _T('+'):
	case _T('-'):
	case _T('.'):
	case _T('E'):
	case _T('e'):
	case _T('0'):
	case _T('1'):
	case _T('2'):
	case _T('3'):
	case _T('4'):
	case _T('5'):
	case _T('6'):
	case _T('7'):
	case _T('8'):
	case _T('9'):
	case _T('\b'):
	   break;
	default:
	   MessageBeep(0);
	   return;
       } /* validate */
    CEdit::OnChar(nChar, nRepCnt, nFlags);
}

All this does is accept the characters shown, and call the superclass handler, or simply issue a beep and return, thus discarding the character.

Summary

This article summarizes a set of techniques I use extensively in the applications I build. I've not seen this set of ideas documented elsewhere, so this seemed a good topic for an essay.

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