void swap(shared_lock& sl) noexcept;
Effects: Swaps the data members of *this and sl.
mutex_type* release() noexcept;
Returns: The previous value of pm.
Postconditions: pm == nullptr and owns == false.
template <class Mutex>
void swap(shared_lock<Mutex>& x, shared_lock<Mutex>& y) noexcept;