Item 8 (2/2)- Enhanced support for multithreading(English)

Inter-thread Communication

std::condition_variable is used for communication between threads, allowing one thread to wait for a notification from another thread.

Multithreading

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

std::mutex mtx;
std::condition_variable cv;
bool ready = false;

void print_id(int id) {
     std::cout << "Thread " << id << " is waiting " << std::endl;
     std::unique_lock<std::mutex> lock(mtx);

     std::cout << "Thread " << id << " is waiting cv " << std::endl;
     cv.wait(lock, [] { return ready; }); // Wait for ready to be true

     std::cout << "Thread " << id <<" is running " << std::endl;
}

 void go() {
     std::unique_lock<std::mutex> lock(mtx);
     ready = true;
     cv.notify_all(); // Notify all waiting threads
}

 int main() {
     std::thread threads[5];
     for (int i = 0; i < 5; ++i) {
         threads[i] = std::thread(print_id, i);
     }


     std::cout << "Main Thread is sleeping " << std::endl;
     std::this_thread::sleep_for(std::chrono::seconds(3));
     std::cout << "Main Thread is waking up " << std::endl;

     go();

     for (auto& t : threads) {
         t.join();
     }

     return 0;
}

Execution result:

Explanation of execution order:

1. main thread goes to sleep
2. t3 is waiting
3. t2 is waiting
4. t1 is waiting
5. t4 is waiting
6. t0 is waiting and t3 is waiting for cv (condition_variable)
7. t2 is waiting for cv
8. t1 is waiting for cv
9. t4 is waiting for cv
10. t0 is waiting for cv
11. main thread wakes up and calls notifyAll
12. t0 executes
13. t1 executes
14. t4 executes
15. t2 executes
16. t3 executes