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 <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