Infinite loop with cin when typing string while a number is expected

Question

In the following loop, if we type characters as the cin input instead of numbers which are expected, then it goes into infinite loop. Could anyone please explain to me why this occurs?

When we use cin, if the input is not a number, then are there ways to detect this to avoid abovementioned problems?

unsigned long ul_x1, ul_x2;

while (1)
{
  cin >> ul_x1 >> ul_x2;
  cout << "ux_x1 is " << ul_x1 << endl << "ul_x2 is " << ul_x2 << endl;
}

Answer 1

Well you always will have an infinite loop, but I know what you really want to know is why cin doesn't keep prompting for input on each loop iteration causing your infinite loop to freerun.

The reason is because cin fails in the situation you describe and won't read any more input into those variables. By giving cin bad input, cin gets in the fail state and stops prompting the command line for input causing the loop to free run.

For simple validation, you can try to use cin to validate your inputs by checking whether cin is in the fail state. When fail occurs clear the fail state and force the stream to throw away the bad input. This returns cin to normal operation so you can prompt for more input.

  if (cin.fail())
  {
     cout << "ERROR -- You did not enter an integer";

     // get rid of failure state
     cin.clear(); 

     // From Eric's answer (thanks Eric)
     // discard 'bad' character(s) 
     cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');

  }

For more sophisticated validation, you may wish to read into a string first and do more sophisticated checks on the string to make sure it matches what you expect.