namespace std {
  class shared_mutex;
  class shared_timed_mutex;
  template<class Mutex> class shared_lock;
  template<class Mutex>
    void swap(shared_lock<Mutex>& x, shared_lock<Mutex>& y) noexcept;
}

namespace std {
  class shared_mutex {
  public:
    shared_mutex();
    ~shared_mutex();
 
    shared_mutex(const shared_mutex&) = delete;
    shared_mutex& operator=(const shared_mutex&) = delete;
 
    // exclusive ownership
    void lock();                // blocking
    bool try_lock();
    void unlock();
 
    // shared ownership
    void lock_shared();         // blocking
    bool try_lock_shared();
    void unlock_shared();
 
    using native_handle_type = /* implementation-defined */;
    native_handle_type native_handle();
  };
}

namespace std {
  class shared_timed_mutex {
  public:
    shared_timed_mutex();
    ~shared_timed_mutex();
 
    shared_timed_mutex(const shared_timed_mutex&) = delete;
    shared_timed_mutex& operator=(const shared_timed_mutex&) = delete;
 
    // exclusive ownership
    void lock();                // blocking
    bool try_lock();
    template<class Rep, class Period>
      bool try_lock_for(const chrono::duration<Rep, Period>& rel_time);
    template<class Clock, class Duration>
      bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time);
    void unlock();
 
    // shared ownership
    void lock_shared();         // blocking
    bool try_lock_shared();
    template<class Rep, class Period>
      bool try_lock_shared_for(const chrono::duration<Rep, Period>& rel_time);
    template<class Clock, class Duration>
      bool try_lock_shared_until(const chrono::time_point<Clock, Duration>& abs_time);
    void unlock_shared();
  };
}

namespace std {
  template<class Mutex>
  class shared_lock {
  public:
    using mutex_type = Mutex;
 
    // construct/copy/destroy
    shared_lock() noexcept;
    explicit shared_lock(mutex_type& m);        // blocking
    shared_lock(mutex_type& m, defer_lock_t) noexcept;
    shared_lock(mutex_type& m, try_to_lock_t);
    shared_lock(mutex_type& m, adopt_lock_t);
    template<class Clock, class Duration>
      shared_lock(mutex_type& m, const chrono::time_point<Clock, Duration>& abs_time);
    template<class Rep, class Period>
      shared_lock(mutex_type& m, const chrono::duration<Rep, Period>& rel_time);
    ~shared_lock();
 
    shared_lock(const shared_lock&) = delete;
    shared_lock& operator=(const shared_lock&) = delete;
 
    shared_lock(shared_lock&& u) noexcept;
    shared_lock& operator=(shared_lock&& u) noexcept;
 
    // locking
    void lock();                                // blocking
    bool try_lock();
    template<class Rep, class Period>
      bool try_lock_for(const chrono::duration<Rep, Period>& rel_time);
    template<class Clock, class Duration>
      bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time);
    void unlock();
 
    // modifiers
    void swap(shared_lock& u) noexcept;
    mutex_type* release() noexcept;
 
    // observers
    bool owns_lock() const noexcept;
    explicit operator bool () const noexcept;
    mutex_type* mutex() const noexcept;
 
  private:
    mutex_type* pm;                             // exposition only
    bool owns;                                  // exposition only
  };
 
  template<class Mutex>
    void swap(shared_lock<Mutex>& x, shared_lock<Mutex>& y) noexcept;
}