Std::unique_lock::lock (3) Linux Manual Page

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

std::unique_lock::lock (3) Linux Manual Page