std::lock

 

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Defined in header <mutex>
template< class Lockable1, class Lockable2, class LockableN... > void lock( Lockable1& lock1, Lockable2& lock2, LockableN& lockn... );		(since C++11)

Locks the given Lockable objects lock1, lock2, ..., lockn using a deadlock avoidance algorithm to avoid deadlock.

The objects are locked by an unspecified series of calls to lock, try_lock, unlock. If a call to lock or unlock results in an exception, unlock is called for any locked objects before rethrowing.

Parameters

lock1, lock2, ... , lockn

-

the Lockable objects to lock

Return value

(none)

Notes

Boost provides a version of this function that takes a sequence of Lockable objects defined by a pair of iterators.

Example

The following example uses std::lock to lock pairs of mutexes without deadlock.

Run this code

#include <mutex>
#include <thread>
#include <iostream>
#include <vector>
#include <functional>
#include <chrono>
 
struct Employee {
    Employee(int id) : id(id) {}
    int id;
    std::vector<int> lunch_partners;
    std::mutex m;
};
 
void send_mail(Employee &e1, Employee &e2)
{
    // simulate a time-consuming messaging operation
    std::this_thread::sleep_for(std::chrono::seconds(1));
}
 
void assign_lunch_partner(Employee &e1, Employee &e2)
{
    // use std::lock to acquire two locks without worrying about 
    // other calls to assign_lunch_partner deadlocking us
    std::lock(e1.m, e2.m);
 
    e1.lunch_partners.push_back(e2.id);
    e2.lunch_partners.push_back(e1.id);
 
    e1.m.unlock();
    e2.m.unlock();
 
    send_mail(e1, e2);
    send_mail(e2, e1);
}
 
int main()
{
    Employee alice(0), bob(1), christina(2), dave(3);
 
    // assign in parallel threads because mailing users about lunch assignments
    // takes a long time
    std::vector<std::thread> threads;
    threads.emplace_back(assign_lunch_partner, std::ref(alice), std::ref(bob));
    threads.emplace_back(assign_lunch_partner, std::ref(christina), std::ref(bob));
    threads.emplace_back(assign_lunch_partner, std::ref(christina), std::ref(alice));
    threads.emplace_back(assign_lunch_partner, std::ref(dave), std::ref(bob));
 
    for (auto &thread : threads) thread.join();
}

See also

try_lock (C++11)

attempts to obtain ownership of mutexes via repeated calls to try_lock (function template)