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# std::unique_lock::lock

From cppreference.com
< cpp‎ | thread‎ | unique lock

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

Locks the associated mutex. Effectively calls mutex()->lock().

std::system_error is thrown if there is no associated mutex or if the mutex is already locked.

### [edit] Parameters

(none)

### [edit] Return value

(none)

### [edit] 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, std::system_error with an error code of std::errc::resource_deadlock_would_occur

### [edit] 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

### [edit] See also

try_lock | tries to lock the associated mutex, returns if the mutex is not available (public member function) [edit]
unlock | unlocks the associated mutex (public member function) [edit]

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