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Difference between revisions of "cpp/thread/unique lock"

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< cpp‎ | thread
(See also)
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Revision as of 14:06, 22 December 2016

 
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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, 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 Template:concept and Template:concept but not of Template:concept or Template:concept.

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

Contents

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

 constructs a unique_lock, optionally locking (i.e., taking ownership of) the supplied mutex
(public member function) [edit]
unlocks (i.e., releases ownership of) the associated mutex, if owned
(public member function) [edit]
unlocks (i.e., releases ownership of) the mutex, if owned, and acquires ownership of another
(public member function) [edit]
Locking
locks (i.e., takes ownership of) the associated mutex
(public member function) [edit]
tries to lock (i.e., takes ownership of) the associated mutex without blocking
(public member function) [edit]
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) [edit]
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) [edit]
unlocks (i.e., releases ownership of) 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 (i.e., releasing ownership of) it
(public member function) [edit]
Observers
returns a pointer to the associated mutex
(public member function) [edit]
tests whether the lock owns (i.e., has locked) its associated mutex
(public member function) [edit]
tests whether the lock owns (i.e., has locked) its associated mutex
(public member function) [edit]

Non-member functions

(C++11)
 specializes the std::swap algorithm
(function template) [edit]

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;
 
    // '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();
}

See also

(C++11)
 implements a strictly scope-based mutex ownership wrapper
(class template) [edit]
(C++17)
 deadlock-avoiding RAII wrapper for multiple mutexes
(class template) [edit]
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