nothrow_mutex.h

#pragma once
#include <atomic>
#include <condition_variable>

/// A mutex guaranted noexcept that can be used in destructors.
/// See:
/// http://stackoverflow.com/questions/16568990/locking-a-mutex-in-a-destructor-in-c11
template <class Tp_ = std::mutex>
class nothrow_mutex {
 public:
  nothrow_mutex() noexcept : mutex_(), flag_(), both_(false) {
    flag_.clear(std::memory_order_release);
  }

  ~nothrow_mutex() = default;

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

  auto lock() noexcept {
    try {
      mutex_.lock();
      while (flag_.test_and_set(std::memory_order_acquire)) both_ = true;
    } catch (...) {
      while (flag_.test_and_set(std::memory_order_acquire)) both_ = false;
    }
  }

  auto unlock() noexcept {
    if (both_) {
      mutex_.unlock();
    }
    flag_.clear(std::memory_order_release);
  }

 private:
  Tp_ mutex_;
  std::atomic_flag flag_;
  bool both_;
};