std::unique_lock::unique_lock(3)

NAME
std::unique_lock::unique_lock - std::unique_lock::unique_lock

Synopsis
   unique_lock() noexcept; (1) (since C++11)
   unique_lock( unique_lock&& other ) noexcept; (2) (since C++11)
   explicit unique_lock( mutex_type& m ); (3) (since C++11)
   unique_lock( mutex_type& m, std::defer_lock_t t ) noexcept; (4) (since C++11)
   unique_lock( mutex_type& m, std::try_to_lock_t t ); (5) (since C++11)
   unique_lock( mutex_type& m, std::adopt_lock_t t ); (6) (since C++11)
   template< class Rep, class Period >
   unique_lock( mutex_type& m, (7) (since C++11)
                const std::chrono::duration<Rep, Period>&
   timeout_duration );
   template< class Clock, class Duration >
   unique_lock( mutex_type& m, (8) (since C++11)
                const std::chrono::time_point<Clock, Duration>&
   timeout_time );
   Constructs a unique_lock, optionally locking the supplied mutex.
   1) Constructs a unique_lock with no associated mutex.
   2) Move constructor. Initializes the unique_lock with the contents of other. Leaves
   other with no associated mutex.
   3-8) Constructs a unique_lock with m as the associated mutex. Additionally:
   3) Locks the associated mutex by calling m.lock().
   4) Does not lock the associated mutex.
   5) Tries to lock the associated mutex without blocking by calling m.try_lock(). The
   behavior is undefined if Mutex does not satisfy Lockable.
   6) Assumes the calling thread already holds a non-shared lock (i.e., a lock acquired
   by lock, try_lock, try_lock_for, or try_lock_until) on m. The behavior is undefined
   if not so.
   7) Tries to lock the associated mutex by calling m.try_lock_for(timeout_duration).
   Blocks until specified timeout_duration has elapsed or the lock is acquired,
   whichever comes first. May block for longer than timeout_duration. The behavior is
   undefined if Mutex does not satisfy TimedLockable.
   8) Tries to lock the associated mutex by calling m.try_lock_until(timeout_time).
   Blocks until specified timeout_time has been reached or the lock is acquired,
   whichever comes first. May block for longer than until timeout_time has been
   reached. The behavior is undefined if Mutex does not satisfy TimedLockable.

Parameters
   other - another unique_lock to initialize the state with
   m - mutex to associate with the lock and optionally acquire ownership
                      of
   t - tag parameter used to select constructors with different locking
                      strategies
   timeout_duration - maximum duration to block for
   timeout_time - maximum time point to block until

Example
// Run this code
 #include <iostream>
 #include <mutex>
 #include <thread>
 #include <utility>
 #include <vector>
 std::mutex m_a, m_b, m_c;
 int a, b, c = 1;
 void update()
 {
     { // Note: std::lock_guard or atomic<int> can be used instead
         std::unique_lock<std::mutex> lk(m_a);
         ++a;
     }
     { // Note: see std::lock and std::scoped_lock for details and alternatives
         std::unique_lock<std::mutex> lk_b(m_b, std::defer_lock);
         std::unique_lock<std::mutex> lk_c(m_c, std::defer_lock);
         std::lock(lk_b, lk_c);
         b = std::exchange(c, b + c);
     }
 }
 int main()
 {
     std::vector<std::thread> threads;
     for (unsigned i = 0; i < 12; ++i)
         threads.emplace_back(update);
     for (auto& i : threads)
         i.join();
     std::cout << a << "'th and " << a + 1 << "'th Fibonacci numbers: "
               << b << " and " << c << '\n';
 }

Output:
 12'th and 13'th Fibonacci numbers: 144 and 233