Why does this code give me an infinite loop?

Question

When I enter in a correct value (an integer) it is good. But when I enter in a character, I get an infinite loop. I've looked at every side of this code and could not find a problem with it. Why is this happening? I'm using g++ 4.7 on Windows.

#include <iostream>
#include <limits>

int main()
{
    int n;
    while (!(std::cin >> n))
    {
        std::cout << "Please try again.\n";
        std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
        std::cin.clear();
    }
}

Input: x
Output:

enter image description here

is there an unprintable character like CR coming along with the input when you type or does cin grab one character? — bright-star
– bright-star, Commented Nov 27, 2013 at 22:39
@qwrrty Well, scanf is marginally faster than cin >> (not sure why, it has to parse the format string), so if the question is "performance!1!!11elevenone", it may be the answer. — Joker_vD
– Joker_vD, Commented Nov 28, 2013 at 6:26

john · Accepted Answer · 2013-11-27 22:46:56Z

6

It's because your recover operations are in the wrong order. First clear the error then clear the buffer.

    std::cin.clear();
    std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');

answered Nov 27, 2013 at 22:46

john

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2 Comments

template boy Over a year ago

Thanks. I couldn't even tell! :)

john Over a year ago

The point is that because the stream is in an error state ignore will also fail unless you clear the error first.

Kerrek SB · Accepted Answer · 2013-11-27 22:46:56Z

4

You have to clear the error state first, and then ignore the unparsable buffer content. Otherwise, ignore will do nothing on a stream that's not in a good state.

You will separately need to deal with reaching the end of the stream.

answered Nov 27, 2013 at 22:46

Kerrek SB

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