Reading till new line in C++ goes in an infinite loop

Question

Given the following program

#include <iostream>
#include <string>
using namespace std;

int main() {
    int integer;
    cin >> integer;
    if (!cin) {
        string str;
        char ch;
        while ((ch = cin.get()) != '\n') {
            cout << "scanning" << endl;
            cout << "got " << static_cast<int>(ch) << endl;
        }
    }
    return 0;
}

When given this input file (redirected input)

x123

With a newline at the end, why does the program go into an infinite loop? Shouldn't it stop after encountering the newline at the end of the file? I keep getting the value of ch fetched as -1..

Thanks!

Note cin.ignore() doesn't seem to help resolve the issue here

It does, and then when I scan till the end to try and find a newline, I get an this infinite loop — Curious, Sep 13 '16 at 23:34
There's no code to clear the error condition, so `cin` just keeps erroring. — David Schwartz, Sep 13 '16 at 23:39
There are so many duplicates and variants of this it's hard to choose one. Not even hard to find. — Baum mit Augen, Sep 13 '16 at 23:49

Fantastic Mr Fox · Accepted Answer · 2016-09-13T23:47:50.060

3

If you get an error on std::cin (which is of type std::istream), then you need to clear it:

int integer;
cin >> integer;
if (!cin) {
    cin.clear(); // If an error occurred, we need to clear it!
    ...

Then it will work.

edited Sep 13 '16 at 23:47

answered Sep 13 '16 at 23:40

Fantastic Mr Fox

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no! it only stops the infinite loop but nothing correct! – Raindrop7 Sep 14 '16 at 00:05
try { cin >> integer; if (cin.fail()) throw;} catch(...) { cout << "cin threw this exception!" << endl;} – Raindrop7 Sep 14 '16 at 00:15

score 1 · Answer 2 · answered Sep 13 '16 at 23:41

1

The solution is to recover from the failed state with cin.clear() and then retry to scan till a newline..

answered Sep 13 '16 at 23:41

Curious

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Reading till new line in C++ goes in an infinite loop

2 Answers2