AlternateFooBar.cpp

#include <iostream>
#include <thread>
#include <string>
#include <vector>
#include <mutex>
#include <functional>
#include <condition_variable>
#include <atomic>

using namespace std;

bool printedFoo = false;
condition_variable cv;
mutex mtx;

void printFoo(int n)
{
    for (int i = 0; i < n; i++)
    {
        unique_lock<mutex> ulock(mtx);
        // Sleep until printedFoo becomes false.
        // or wakeup when printedFoo = false
        cv.wait(ulock, [&]
                { return printedFoo == false; });
        cout
            << "foo";
        printedFoo = true;
        ulock.unlock(); // without this program works correctly
        // but unlocking early helps other thread to wake up immediately
        // so there is less lock contention
        cv.notify_one();
    }
}

void printBar(int n)
{
    for (int i = 0; i < n; i++)
    {
        unique_lock<mutex> ulock(mtx);
        // wakeup when printedFoo becomes true
        cv.wait(ulock, [&]
                { return printedFoo == true; });
        cout
            << "bar";
        printedFoo = false;
        ulock.unlock();
        cv.notify_one();
    }
}

int main()
{
    int n = 10;
    thread t1(printFoo, n);
    thread t2(printBar, n);

    t1.join();
    t2.join();

    return 0;
}