Why would we call cin.clear() and cin.ignore() after reading input?

Question

Google Code University's C++ tutorial used to have this code:

// Description: Illustrate the use of cin to get input // and how to recover from errors.  #include <iostream> using namespace std;  int main() {   int input_var = 0;   // Enter the do while loop and stay there until either   // a non-numeric is entered, or -1 is entered.  Note that   // cin will accept any integer, 4, 40, 400, etc.   do {     cout << "Enter a number (-1 = quit): ";     // The following line accepts input from the keyboard into     // variable input_var.     // cin returns false if an input operation fails, that is, if     // something other than an int (the type of input_var) is entered.     if (!(cin >> input_var)) {       cout << "Please enter numbers only." << endl;       cin.clear();       cin.ignore(10000,'\n');     }     if (input_var != -1) {       cout << "You entered " << input_var << endl;     }   }   while (input_var != -1);   cout << "All done." << endl;    return 0; } 

What is the significance of cin.clear() and cin.ignore()? Why are the 10000 and \n parameters necessary?

Answer 1

The cin.clear() clears the error flag on cin (so that future I/O operations will work correctly), and then cin.ignore(10000, '\n') skips to the next newline (to ignore anything else on the same line as the non-number so that it does not cause another parse failure). It will only skip up to 10000 characters, so the code is assuming the user will not put in a very long, invalid line.