namespace std { class mutex; class recursive_mutex; class timed_mutex; class recursive_timed_mutex; struct defer_lock_t { explicit defer_lock_t() = default; }; struct try_to_lock_t { explicit try_to_lock_t() = default; }; struct adopt_lock_t { explicit adopt_lock_t() = default; }; inline constexpr defer_lock_t defer_lock { }; inline constexpr try_to_lock_t try_to_lock { }; inline constexpr adopt_lock_t adopt_lock { }; template <class Mutex> class lock_guard; template <class... MutexTypes> class scoped_lock; template <class Mutex> class unique_lock; template <class Mutex> void swap(unique_lock<Mutex>& x, unique_lock<Mutex>& y) noexcept; template <class L1, class L2, class... L3> int try_lock(L1&, L2&, L3&...); template <class L1, class L2, class... L3> void lock(L1&, L2&, L3&...); struct once_flag; template<class Callable, class... Args> void call_once(once_flag& flag, Callable&& func, Args&&... args); }