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std::unique_lock::lock(3)     C++ Standard Libary    std::unique_lock::lock(3)

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

Synopsis
	  void lock();	(since C++11)

	  Locks	 (i.e.,	 takes ownership of) the associated mutex. Effectively
       calls
	  mutex()->lock().

Parameters
	  (none)

Return value
	  (none)

Exceptions
	    * Any exceptions thrown by mutex()->lock()

	    * If there is no associated	mutex, std::system_error with an error
       code of
	      std::errc::operation_not_permitted

	    * If the mutex is already locked by	 this  unique_lock  (in	 other
       words, owns_lock	is
	      true), std::system_error with an error code of
	      std::errc::resource_deadlock_would_occur

Example
	  The  following  example uses lock to re-acquire a mutex that was un-
       locked.

       // Run this code

	#include <mutex>
	#include <thread>
	#include <iostream>
	#include <vector>
	#include <chrono>

	int main()
	{
	    int	counter	= 0;
	    std::mutex counter_mutex;
	    std::vector<std::thread> threads;

	    auto worker_task = [&](int id) {
		std::unique_lock<std::mutex> lock(counter_mutex);
		++counter;
		std::cout << id	<< ", initial counter: " << counter << '\n';
		lock.unlock();

		// don't hold the lock while we	simulate an  expensive	opera-
       tion
		std::this_thread::sleep_for(std::chrono::seconds(1));

		lock.lock();
		++counter;
		std::cout << id	<< ", final counter: " << counter << '\n';
	    };

	    for	(int i = 0; i <	10; ++i) threads.emplace_back(worker_task, i);

	    for	(auto &thread :	threads) thread.join();
	}

Possible output:
	0, initial counter: 1
	1, initial counter: 2
	2, initial counter: 3
	3, initial counter: 4
	4, initial counter: 5
	5, initial counter: 6
	6, initial counter: 7
	7, initial counter: 8
	8, initial counter: 9
	9, initial counter: 10
	6, final counter: 11
	3, final counter: 12
	4, final counter: 13
	2, final counter: 14
	5, final counter: 15
	0, final counter: 16
	1, final counter: 17
	7, final counter: 18
	9, final counter: 19
	8, final counter: 20

See also
		   tries to lock (i.e.,	takes ownership	of) the	associated mu-
       tex without
	  try_lock blocking
		   (public member function)
	  unlock   unlocks (i.e., releases ownership of) the associated	mutex
		   (public member function)

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

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