# std::unique_lock

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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, time-constrained attempts at locking, recursive locking, transfer of lock ownership, and use with condition variables.

The class unique_lock is movable, but not copyable -- it meets the requirements of MoveConstructible and MoveAssignable but not of CopyConstructible or CopyAssignable.

The class unique_lock meets the BasicLockable requirements. If Mutex meets the Lockable requirements, unique_lock also meets the Lockable requirements (ex.: can be used in std::lock); if Mutex meets the TimedLockable requirements, unique_lock also meets the TimedLockable requirements.

### Template parameters

Mutex | - | the type of the mutex to lock. The type must meet the BasicLockable requirements

### Member types

Type | Definition
`mutex_type` | Mutex

### Member functions

(constructor) | constructs a `unique_lock`, optionally locking (i.e., taking ownership of) the supplied mutex (public member function)
(destructor) | unlocks (i.e., releases ownership of) the associated mutex, if owned (public member function)
operator= | unlocks (i.e., releases ownership of) the mutex, if owned, and acquires ownership of another (public member function)
##### Locking
lock | locks (i.e., takes ownership of) the associated mutex (public member function)
try_lock | tries to lock (i.e., takes ownership of) the associated mutex without blocking (public member function)
try_lock_for | attempts to lock (i.e., takes ownership of) the associated TimedLockable mutex, returns if the mutex has been unavailable for the specified time duration (public member function)
try_lock_until | tries to lock (i.e., takes ownership of) the associated TimedLockable mutex, returns if the mutex has been unavailable until specified time point has been reached (public member function)
unlock | unlocks (i.e., releases ownership of) the associated mutex (public member function)
##### Modifiers
swap | swaps state with another std::unique_lock (public member function)
release | disassociates the associated mutex without unlocking (i.e., releasing ownership of) it (public member function)
##### Observers
mutex | returns a pointer to the associated mutex (public member function)
owns_lock | tests whether the lock owns (i.e., has locked) its associated mutex (public member function)
operator bool | tests whether the lock owns (i.e., has locked) its associated mutex (public member function)

### Non-member functions

std::swap(std::unique_lock) (C++11) | specialization of std::swap for `unique_lock` (function template)

### Example

Run this code

    #include <mutex>
    #include <thread>
    #include <iostream>

    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 lock1{from.m, std::defer_lock};
        std::unique_lock lock2{to.m, std::defer_lock};

        // lock both unique_locks without deadlock
        std::lock(lock1, lock2);

        from.num_things -= num;
        to.num_things += num;

        // 'from.m' and 'to.m' mutexes unlocked in 'unique_lock' dtors
    }

    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();

        std::cout << "acc1: " << acc1.num_things << "\n"
                     "acc2: " << acc2.num_things << '\n';
    }

Output:

    acc1: 95
    acc2: 55

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 2981 | C++17 | redundant deduction guide from `unique_lock<Mutex>` was provided | removed

### See also

lock (C++11) | locks specified mutexes, blocks if any are unavailable (function template)
lock_guard (C++11) | implements a strictly scope-based mutex ownership wrapper (class template)
scoped_lock (C++17) | deadlock-avoiding RAII wrapper for multiple mutexes (class template)
mutex (C++11) | provides basic mutual exclusion facility (class)

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