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std::unique_lock

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Thread support library
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this_thread namespace
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unique_lock
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Condition variables
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Defined in header <mutex>
template< class Mutex >
class unique_lock;
(since C++11)

The class unique_lock is a general-purpose mutex ownership wrapper allowing deferred locking, timed locking, recursive locking, transfer of lock ownership, and use with condition variables.

The unique_lock class is non-copyable, but it is movable. The supplied Mutex type shall implement the BasicLockable concept.

Contents

Member types

Type Definition
mutex_type Mutex

Member functions

constructs a unique_lock, optionally locking the supplied mutex
(public member function) [edit]
unlocks the associated mutex, if owned
(public member function) [edit]
unlocks the mutex, if owned, and acquires ownership of another
(public member function) [edit]
Locking
locks the associated mutex
(public member function) [edit]
tries to lock the associated mutex, returns if the mutex is not available
(public member function) [edit]
attempts to lock the associated TimedLockable mutex, returns if the mutex has been unavailable for the specified time duration
(public member function) [edit]
tries to lock the associated TimedLockable mutex, returns if the mutex has been unavailable until specified time point has been reached
(public member function) [edit]
unlocks the associated mutex
(public member function) [edit]
Modifiers
swaps state with another std::unique_lock
(public member function) [edit]
disassociates the associated mutex without unlocking it
(public member function) [edit]
Observers
returns a pointer to the associated mutex
(public member function) [edit]
tests whether the lock owns its associated mutex
(public member function) [edit]
tests whether the lock owns its associated mutex
(public member function) [edit]

Non-member functions

specialization of std::swap for unique_lock
(function template) [edit]

Example

#include <mutex>
#include <thread>
#include <chrono>
 
struct Box {
    explicit Box(int num) : num_things{num} {}
 
    int num_things;
    std::mutex m;
};
 
void transfer(Box &from, Box &to, int num)
{
    // don't actually take the locks yet
    std::unique_lock<std::mutex> lock1(from.m, std::defer_lock);
    std::unique_lock<std::mutex> lock2(to.m, std::defer_lock);
 
    // lock both unique_locks without deadlock
    std::lock(lock1, lock2);
 
    from.num_things -= num;
    to.num_things += num;
 
    lock1.unlock();
    lock2.unlock();
}
 
int main()
{
    Box acc1(100);
    Box acc2(50);
 
    std::thread t1(transfer, std::ref(acc1), std::ref(acc2), 10);
    std::thread t2(transfer, std::ref(acc2), std::ref(acc1), 5);
 
    t1.join();
    t2.join();
}