// ReadLock -- ReadUnlock
// WriteLock -- WriteUnlock
// read priority

private:
    std::condition_variable can_read_, can_write_;
    std::atomic<int> readers_;
    std::atomic<bool> locked_;

public:
    WriteLock() {
        std::unique_lock<std::mutex> lock;
        while(locked_) {
            can_write_.wait(lock);
        }
        locked_.store(true);
        while (readers_.load() != 0)
            can_write_.wait(lock);
    }

    ReadLock() {
        std::unique_lock <std::mutex> lock;
        while(locked_) {
            can_read_.wait(lock);
        }
        readers_.fetch_add(1);
    }

    WriteUnlock() {
        if (readers_.load() != 0) {
            can_read_.notify_one();
        } else {
            can_write_.notify_one();
        }
        locked_.store(false);
    }

    ReadUnlock() {
        if (readers_.fetch_add(-1) != 0) {
            can_read_.notify_one();
        } else {
            can_write_.notify_one();
        }
    }