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 unlocked. // Run this code
 #include <chrono>
 #include <iostream>
 #include <mutex>
 #include <thread>
 #include <vector>
 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 operation
         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 mutex without
   try_lock blocking
            (public member function)
   unlock unlocks (i.e., releases ownership of) the associated mutex
            (public member function)