static std::mutex shared by all the instances of a class

Question

I have a non-blocking Timer class that:

• counts down from whatever int the class is instantiated with
• each new instance of the class waits for any timer that is currently running to first finish before they start counting down

Current Design of class:

class Timer{
private: 
  std::thread *thread;

public:
  Timer(const int &timer, std::mutex *mutex){
    this->thread = new std::thread(
        [&timer, mutex](){
          mutex->lock();
          std::cout<<"Counting to: "<<timer<<std::endl;
          for (int i=0; i < timer; i++) {
            std::cout<<i<<std::endl;
            sleep(1);
          }
          std::cout<<"Time is up"<<std::endl;
          mutex->unlock();
    });
  }

  ~Timer(){
    this->thread->join();
    delete this->thread;
  }
};

I am calling it from main with this:

int main() {
  std::mutex mutex;
  Timer time(3, &mutex);
  Timer newTime(9, &mutex);
  //Do something else in the meantime ...
  return 0;
}

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Quesion:

I would like to simplify the main by internalising the mutex as a static variable of the class. Something like this:

• a simpler main

int main() {
  Timer time(3);
  Timer newTime(9);
  //Do something else in the meantime ...
  return 0;
}

• a more representative Timer class:

class Timer{
private: 
  std::thread *thread;
  static std::mutex timerMutex;

public:
  Timer(const int &timer){
    this->thread = new std::thread(
        [&timer, this](){
          timerMutex.lock();
          std::cout<<"Counting to: "<<timer<<std::endl;
          for (int i=0; i < timer; i++) {
            std::cout<<i<<std::endl;
            sleep(1);
          }
          std::cout<<"Time is up"<<std::endl;
          timerMutex.unlock();
    });
  }

  ~Timer(){
    this->thread->join();
    delete this->thread;
  }
};

• How can I do this ?

---

Current Error:

 error: linker command failed with exit code 1