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std::unique...:unique_lock(3) C++ Standard Libarystd::unique...: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.	 Addi-
       tionally:
	  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  an  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_un-
       til(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	Timed-
       Lockable.

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 <thread>
	#include <vector>
	#include <mutex>
	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 al-
       ternatives
	      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

http://cppreference.com		  2022.07.31	 std::unique...:unique_lock(3)

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