Why does my program go into an infinite loop when someone enters an invalid input character?, C++ FAQ

C++ FAQ / Section 15 / FAQ 15.2

Section 15:

15.1	Advantages of `<iostream>` over `<cstdio>`?
15.2	Understanding infinite loop when the input is invalid?
15.3	How can I get `std::cin` to skip invalid input characters?
15.4	How does that funky `while (std::cin >> foo)` syntax work?
15.5	Why does my input seem to process past the end of file?
15.6	Why is my program ignoring my input request after the first iteration?
15.7	Should I end my output lines with `std::endl` or `'\n'`?
15.8	How can I provide printing for my `class` `Fred`?
15.9	Using a `printOn()` method vs. a `friend` function?
15.10	How can I provide input for my `class` `Fred`?
15.11	Providing printing for an entire hierarchy of classes?
15.12	How can I open a stream in binary mode?
15.13	"Reopening" `std::cin` and `std::cout` in binary mode?
15.14	How can I write/read objects of my class to/from a data file?
15.15	Sending objects over a network?
15.16	Backslashes in filenames?
15.17	How to capture key presses before the ENTER key?
15.18	How to turn off screen echo on keyboard input?
15.19	How can I move the cursor around on the screen?
15.20	How can I clear the screen?
15.21	How can I change the colors on the screen?
15.22	How can I print the numeric value of a `char` or `char*`? Updated!

[15.2] Why does my program go into an infinite loop when someone enters an invalid input character?

For example, suppose you have the following code that reads integers from std::cin:

#include <iostream>

int main()
{
  std::cout << "Enter numbers separated by whitespace (use -1 to quit): ";
  int i = 0;
  while (i != -1) {
    std::cin >> i;        // BAD FORM — See comments below
    std::cout << "You entered " << i << '\n';
  }
  ...
}

The problem with this code is that it lacks any checking to see if someone entered an invalid input character. In particular, if someone enters something that doesn't look like an integer (such as an 'x'), the stream std::cin goes into a "failed state," and all subsequent input attempts return immediately without doing anything. In other words, the program enters an infinite loop; if 42 was the last number that was successfully read, the program will print the message You entered 42 over and over.

An easy way to check for invalid input is to move the input request from the body of the while loop into the control-expression of the while loop. E.g.,

#include <iostream>

int main()
{
  std::cout << "Enter a number, or -1 to quit: ";
  int i = 0;
  while (std::cin >> i) {    // GOOD FORM
    if (i == -1) break;
    std::cout << "You entered " << i << '\n';
  }
  ...
}

This will cause the while loop to exit either when you hit end-of-file, or when you enter a bad integer, or when you enter -1.

(Naturally you can eliminate the break by changing the while loop expression from while (std::cin >> i) to while ((std::cin >> i) && (i != -1)), but that's not really the point of this FAQ since this FAQ has to do with iostreams rather than generic structured programming guidelines.)