std::unique_lock

 

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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

swap(std::unique_lock)

 

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 movable, but not copyable -- it meets the requirements of Template:concept and Template:concept but not of Template:concept or Template:concept.

Template parameters

Mutex

-

the type of the mutex to lock. The type must meet the Template:concept 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)

specializes the std::swap algorithm (function template)

Example

Run this code

#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();
}