Unbuffered Channel in C++

channel.hpp

  /// Unbuffered channel.
  /// Optimized for a single value (busy-waits on a second one).
  template <typename V>
  struct Channel {
    V _v;
    std::mutex _mutex;  // Locked when there is no value.
    std::atomic_int_fast8_t _state; enum State {EMPTY = 0, WRITING = 1, FULL = 2};
    Channel(): _state (EMPTY) {_mutex.lock();}
    // Waits until the Channel is empty then stores the value.
    template <typename VA> void send (VA&& v) {
      for (;;) {
        int_fast8_t expectEmpty = EMPTY; if (_state.compare_exchange_weak (expectEmpty, WRITING)) break;
        std::this_thread::sleep_for (std::chrono::milliseconds (20));
      }
      try {_v = std::forward<V> (v);} catch (...) {_state = EMPTY; throw;}
      _state = FULL;
      _mutex.unlock();  // Allows the reader to proceed.
    }
    // Waits untill there is a value to receive.
    V receive() {
      _mutex.lock();  // Wait.
      V tmp = std::move (_v);
      assert (_state == FULL);
      _state = EMPTY;
      return tmp;
    }
  };