If yes how can we use an array without first declaring it?

The only way I know how to test for them would be to make the conditionals that “return false” more stringent through the use of and’s. For example, taking out

case '#': // match a digit
    if (!isdigit(strUserInput[nIndex]))
        return false;

and replacing it with an added and, like so:

case '#': // match a digit
    if (!isdigit(strUserInput[nIndex]) && strUserInput[nIndex] != '?') // note added and
        return false;

means the test conditional will not return “false” if a question mark is entered where a digit is supposed to be. That allows the user to substiture a question mark for a digit, but only for a digit.

I had a problem with two loops because I used clear and ignore at wrong places. So if 123abc was entered, somehow my app just kept silent until I pressed enter. Using your sample code for
“2) If you don’t want this to be valid input, it is not rejected (and you have garbage in your stream).”

fixed my problems and now my integer validation works reliably no matter when and how often I use it.

Thanks.

while(i = _getch())				                           //validating input
{
	if(i > '0' && i < '7')
	break;
	cout << "Wrong number!"; //input isn't valid
}