#include <iostream>
#include <thread>
#include <mutex>
#include <shared_mutex>
#include <chrono>

std::mutex print_mtx;
void print_value(int x)
{
  std::lock_guard<std::mutex> lock(print_mtx);
  std::cout << x << std::endl;
}

class X {
  std::shared_timed_mutex mtx_;
  int count_ = 0;
public:
  // 書き込み側：カウンタを加算する
  void writer()
  {
    std::lock_guard<std::shared_timed_mutex> lock(mtx_);
    ++count_;
  }

  // 読み込み側：カウンタの値を読む
  void reader()
  {
    int local_count;
    {
      std::shared_lock<std::shared_timed_mutex> lock(mtx_);
      local_count = count_;
    } // 共有ロックをここで手放す
    print_value(local_count);
  }
};

X obj;
void writer_thread()
{
  for (int i = 0; i < 3; ++i) {
    std::this_thread::sleep_for(std::chrono::milliseconds(1));
    obj.writer();
  }
}

void reader_thread()
{
  for (int i = 0; i < 10; ++i) {
    std::this_thread::sleep_for(std::chrono::milliseconds(1));
    obj.reader();
  }
}

int main()
{
  // 書き込みユーザー1人
  // 読み込みユーザー3人
  std::thread writer1(writer_thread);
  std::thread reader1(reader_thread);
  std::thread reader2(reader_thread);
  std::thread reader3(reader_thread);

  writer1.join();
  reader1.join();
  reader2.join();
  reader3.join();
}