std::unique_lock<Mutex>::lock (3) - Linux Manuals

std::unique_lock<Mutex>::lock: std::unique_lock<Mutex>::lock

NAME

std::unique_lock<Mutex>::lock - std::unique_lock<Mutex>::lock

Synopsis

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

Locks 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 <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 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