std::unique_lock::lock

 

C++

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std::unique_lock

Member functions

unique_lock::unique_lock

unique_lock::~unique_lock

unique_lock::operator=

Locking

unique_lock::lock

unique_lock::try_lock

unique_lock::try_lock_for

unique_lock::try_lock_until

unique_lock::unlock

Modifiers

unique_lock::swap

unique_lock::release

Observers

unique_lock::mutex

unique_lock::owns_lock

unique_lock::operator bool

Non-member functions

std::swap

 

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.

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, 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

See also

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