cond_var_models_semaphore.cpp

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

using namespace std;

condition_variable CondVar;
mutex Mutex;
int SemaphorCount{0};
bool RunFlag{false}, StopFlag{false};

void worker() {
  for (auto stop_flag{false}; !stop_flag;) {
    {
      unique_lock<mutex> lock{Mutex};
      CondVar.wait(lock, [] { return SemaphorCount; });
      --SemaphorCount;
      cout << ">" << this_thread::get_id() << endl;
    }

    unique_lock<mutex> lock{Mutex};
    ++SemaphorCount;
    stop_flag = StopFlag;
    cout << "<" << this_thread::get_id() << endl;
  }
}

int main() {
  vector<future<void>> workers;
  workers.push_back(async(launch::async, []() { worker(); }));
  workers.push_back(async(launch::async, []() { worker(); }));
  workers.push_back(async(launch::async, []() { worker(); }));
  workers.push_back(async(launch::async, []() { worker(); }));

  cout << "Press any key to run..." << endl;
  string line;
  getline(cin, line);
  {
    unique_lock<mutex> lock{Mutex};
    SemaphorCount = workers.size() / 2;
    CondVar.notify_all();
  }

  cout << "Press any key to stop..." << endl;
  getline(cin, line);
  {
    unique_lock<mutex> lock{Mutex};
    StopFlag = true;
    CondVar.notify_all();
  }

  vector<future<void>>{}.swap(workers);
  cout << SemaphorCount << endl;
}