The standard functions isalpha(), isdigit(), isprint() etc., defined in the standard header or often fail to produce the right results because they assume that their arguments are of type ‘unsigned char’. On certain implementations, char is signed by default and this causes ctype functions to fail. For efficiency reasons, most implementations use bitwise operators rather than relational operators to implement these functions. The problem is that bitwise operations work well only when applied to unsigned values. What can you do to fix this problem? Explicitly cast the arguments of these functions to unsigned char. This will ensure that the ctype functions produce the correct results, whether their arguments are unsigned or not:

 #include char c;cout << "enter a character";cin >> c;bool alpha = isalpha ( (unsigned char) c); // explicit castif (!alpha)  cout << "invalid character entered";