std::unique_lock::lock

C++

Language

Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

Thread support library

Threads
thread (C++11)
hardware_destructive_interference_sizehardware_constructive_interference_size (C++17)(C++17)
this_thread namespace
get_id (C++11) yield (C++11)	sleep_for (C++11) sleep_until (C++11)
Mutual exclusion
mutex (C++11) recursive_mutex (C++11) shared_mutex (C++17)	timed_mutex (C++11) recursive_timed_mutex (C++11) shared_timed_mutex (C++14)
Generic lock management
lock_guard (C++11) scoped_lock (C++17) unique_lock (C++11) shared_lock (C++14) defer_lock_ttry_to_lock_tadopt_lock_t (C++11)(C++11)(C++11)	lock (C++11) try_lock (C++11) defer_locktry_to_lockadopt_lock (C++11)(C++11)(C++11) once_flag (C++11) call_once (C++11)
Condition variables
condition_variable (C++11)
condition_variable_any (C++11)
notify_all_at_thread_exit (C++11)
cv_status (C++11)
Futures
promise (C++11) future (C++11) shared_future (C++11) packaged_task (C++11) async (C++11)	launch (C++11) future_status (C++11) future_error (C++11) future_category (C++11) future_errc (C++11)

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().

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

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)