Passing mutex reference from main to a class - c++

Question

I need to work with the same mutex and unique_lock across the main function and class instances. However, I am having trouble assigning the mutex/unique_lock address to a class member variable (that is a mutex&).
This is what I have:
Worker.h
class Worker
{
private:
std::mutex &m_mu;
std::unique_lock<std::mutex> &locker;
public:
void start(std::mutex &mu, std::unique_lock<std::mutex> &locker);
};
Worker.cpp
void Worker::start(std::mutex &mu, std::unique_lock<std::mutex> &locker)
{
this->mu = mu; // error
this->locker = locker; // error
}
I tried doing this->mu(mu); but that doesn't work either. Is there anything I can do to make this work?
Thanks.

Answer 1

You need to pass the mutex reference when you construct your class.
Worker::Worker(std::mutex &mu, std::unique_lock<std::mutex> &locker)
:m_mu(mu), locker(locker)
{}
That's the only place you can initialize a reference. Once it's constructed, you cannot change what it references.
Why do you need the locker? The mutex makes the synchronization, the lock is just a RAII object to ease acquiring the mutex.

Answer 2

You don't need to pass the lock object to the function. As long as the class is referring to the correct mutex you can lock the mutex inside the function like this:
class Worker
{
private:
std::mutex& m_mu;
public:
Worker(std::mutex& mu): m_mu(mu) {} // bind reference during initialization
void start();
};
// Worker.cpp
void Worker::start()
{
std::unique_lock<std::mutex> locker(m_mu); // lock the shared resource
// Do something with it here
}
int main()
{
std::mutex mu;
std::vector<Worker> workers(4, Worker(std::ref(mu)));
// etc...
}