ticket-based fair mutex implementation

ticket_fair_mutex.cpp

#include <condition_variable>
#include <mutex>
#include <thread>

class fair_mutex {
  std::mutex mtx_;
  std::condition_variable cv_;
  unsigned int next_, curr_;

public:
  fair_mutex() : next_(0), curr_(0) {}
  ~fair_mutex() = default;

  fair_mutex(const fair_mutex&) = delete;
  fair_mutex& operator=(const fair_mutex&) = delete;

  void lock()
  {
    std::unique_lock<decltype(mtx_)> lk(mtx_);
    const unsigned int self = next_++;
    cv_.wait(lk, [&]{ return (self == curr_); });
  }
  bool try_lock()
  {
    std::lock_guard<decltype(mtx_)> lk(mtx_);
    if (next_ != curr_)
      return false;
    ++next_;
    return true;
  }
  void unlock()
  {
    std::lock_guard<decltype(mtx_)> lk(mtx_);
    ++curr_;
    cv_.notify_all();
  }
};

integrate into https://github.com/yohhoy/yamc

Can you please provide explanation?

This will not work as fair mutex. Reason: self = next_++ assignment is done under regular (non-fair) mutex. This defeats the purpose of the whole implementation.