condition_variable.cpp

#include <condition_variable>
#include <iostream>
#include <mutex>
#include <string>
#include <thread>

std::mutex              m;
std::condition_variable cv;
std::string             data;
bool                    ready     = false;
bool                    processed = false;

void worker_thread()
{
        // wait until main() sends data
        std::unique_lock<std::mutex> lk(m);
        cv.wait(lk, [] { return ready; });

        // after the wait, we own the lock
        std::cout << "Worker thread is processing data\n";
        data += " after processing";

        // send data back to main()
        processed = true;
        std::cout << "Worker thread signals data processing completed\n";

        // manual unlocking is done before notifying, to avoid waking up
        // the waiting thread only to block again (see notify_one for details)
        lk.unlock();
        cv.notify_one();
}

int main()
{
        std::thread worker(worker_thread);

        data = "Example data";
        // send data to the worker thread
        {
                std::lock_guard<std::mutex> lk(m);
                ready = true;
                std::cout << "main() signals data ready for processing\n";
        }
        cv.notify_one();

        // wait for the worker
        {
                std::unique_lock<std::mutex> lk(m);
                cv.wait(lk, [] { return processed; });
        }
        std::cout << "Back in main(), data = " << data << '\n';

        worker.join();
}