std::cin infinite loop from invalid input

Question

Please read the following code:

#include <iostream>
#include <cstdlib>
int main() {
    std::cout << "Please input one integer." << std::endl;
    int i;
    while (true) {
        std::cin >> i;
        if (std::cin) {
            std::cout << "i = " << i << std::endl;
            break;
        } else {
            std::cout << "Error. Please try again."<< std::endl;
            std::cin.ignore();
            std::cin.clear();
        }
    }
    std::cout << "Thank you very much." << std::endl;
    std::system("pause");
    return 0;
}

When I give std::cin an invalid input, such as w, then Error. Please try again. is outputed infinitely. I thought std::cin.ignore would blank the input stream, and std::cin.clear would resume it to normal state. So why does the infinite loop happen?

`clear` first. `ignore` second. – T.C. Aug 11 '16 at 07:55 — T.C., Aug 11 '16 at 07:55

score 4 · Accepted Answer · answered Aug 11 '16 at 08:01

By default, std::basic_istream::ignore() ignores only 1 character:

basic_istream& ignore( std::streamsize count = 1, int_type delim = Traits::eof() );

(From http://en.cppreference.com/w/cpp/io/basic_istream/ignore)

More idiomatic use for your case seems to be the one from the example there:

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

Also, note that you want to call clear() before the ignore() call, so the input extraction of ignore() will succeed instead of returning immediately.

That solved my problem. Thank you very much! – Dannyu NDos Aug 11 '16 at 08:06 — Dannyu NDos, Aug 11 '16 at 08:06

std::cin infinite loop from invalid input

1 Answers1