C++ cout then cin hanging

Question

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

void doOneSet();

int main() {
    srand(time(0));
    doOneSet();

    return 0;
}

void doOneSet() {
    int answer;
    for (int i = 0; i < 5; i++) {
        cout << (rand() % 100) << " + " << (rand() % 101) << ": ";
        cin >> answer;
    }
}

The following code hangs, and never outputs the cout statement, and also doesn't get the get the cin statement. When I end the cin statement as follows, it works fine, but I'm not sure why. I want to have the input on the same line, not on the next line.

cout << (rand() % 100) << " + " << (rand() % 101) << ": " << endl;

I've been able to do this easily before, but for some reason I'm having an issue now, not sure why. Thanks in advance!

Answer 1

I know this is old, but for future searchers, I just hit this issue. I was setting up a new PC and installed VS2019 then remembered I needed VS2013 installed too to support legacy builds. A simple Hello World app would hang on the cout.

I reran the VS2019 install as Repair and, following the reboot, cout behaves as expected.

Answer 2

I could not reproduce your behavior in g++ 4.9.4, but I have an idea. You could try using the fflush function which flushes the buffer of a stream. Here is the code:

#include <stdio.h>
#include <iostream>
#include <string>
#include <cstdlib>
using namespace std;

void doOneSet();

int main() {
    srand(time(0));
    doOneSet();

    return 0;
}

void doOneSet() {
    int answer;
    for (int i = 0; i < 5; i++) {
        cout << (rand() % 100) << " + " << (rand() % 101) << ": ";
        fflush(stdout);
        cin >> answer;
    }
}