naive fair mutex implementation (with recursive semantics)

naive_fair_mutex.cpp

#include <cassert>
#include <queue>
#include <condition_variable>
#include <mutex>
#include <thread>

class fair_recursive_mutex {
  std::mutex mtx_;
  std::condition_variable cv_;
  std::queue<std::thread::id> q_;
  unsigned int count_;

public:
  fair_recursive_mutex() : count_(0) {}
  ~fair_recursive_mutex() = default;

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

  void lock()
  {
    const std::thread::id tid = std::this_thread::get_id();
    std::unique_lock<decltype(mtx_)> lk(mtx_);
    if (!q_.empty() && q_.front() != tid)
      cv_.wait(lk, [&]{ return q_.empty(); });
    if (q_.empty())
      q_.push(tid);
    ++count_;
    assert(q_.front() == tid);
  }
  bool try_lock()
  {
    const std::thread::id tid = std::this_thread::get_id();
    std::lock_guard<decltype(mtx_)> lk(mtx_);
    if (!q_.empty() && q_.front() != tid)
      return false;
    if (q_.empty())
      q_.push(tid);
    ++count_;
    assert(q_.front() == tid);
    return true;
  }
  void unlock()
  {
    std::lock_guard<decltype(mtx_)> lk(mtx_);
    assert(q_.front() == std::this_thread::get_id());
    if (--count_ == 0) {
      q_.pop();
      cv_.notify_all();
    }
  }
};