std::lock(3)

NAME
std::lock - std::lock

Synopsis
   Defined in header <mutex>
   template< class Lockable1, class Lockable2, class... LockableN > (since C++11)
   void lock( Lockable1& lock1, Lockable2& lock2, LockableN&... lockn );
   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, and
   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.
   std::scoped_lock offers a RAII wrapper for this function, and is generally preferred
   to a naked call to std::lock.

Example
   The following example uses std::lock to lock pairs of mutexes without deadlock. // Run this code
 #include <chrono>
 #include <functional>
 #include <iostream>
 #include <mutex>
 #include <string>
 #include <thread>
 #include <vector>
 struct Employee
 {
     Employee(std::string id) : id(id) {}
     std::string id;
     std::vector<std::string> lunch_partners;
     std::mutex m;
     std::string output() const
     {
         std::string ret = "Employee " + id + " has lunch partners: ";
         for (auto n{lunch_partners.size()}; const auto& partner : lunch_partners)
             ret += partner + (--n ? ", " : "");
         return ret;
     }
 };
 void send_mail(Employee&, Employee&)
 {
     // Simulate a time-consuming messaging operation
     std::this_thread::sleep_for(std::chrono::milliseconds(696));
 }
 void assign_lunch_partner(Employee& e1, Employee& e2)
 {
     static std::mutex io_mutex;
     {
         std::lock_guard<std::mutex> lk(io_mutex);
         std::cout << e1.id << " and " << e2.id << " are waiting for locks" << std::endl;
     }
     // Use std::lock to acquire two locks without worrying about
     // other calls to assign_lunch_partner deadlocking us
     {
         std::lock(e1.m, e2.m);
         std::lock_guard<std::mutex> lk1(e1.m, std::adopt_lock);
         std::lock_guard<std::mutex> lk2(e2.m, std::adopt_lock);
     // Equivalent code (if unique_locks are needed, e.g. for condition variables)
     // std::unique_lock<std::mutex> lk1(e1.m, std::defer_lock);
     // std::unique_lock<std::mutex> lk2(e2.m, std::defer_lock);
     // std::lock(lk1, lk2);
     // Superior solution available in C++17
     // std::scoped_lock lk(e1.m, e2.m);
         {
             std::lock_guard<std::mutex> lk(io_mutex);
             std::cout << e1.id << " and " << e2.id << " got locks" << std::endl;
         }
         e1.lunch_partners.push_back(e2.id);
         e2.lunch_partners.push_back(e1.id);
     }
     send_mail(e1, e2);
     send_mail(e2, e1);
 }
 int main()
 {
     Employee alice("Alice"), bob("Bob"), christina("Christina"), dave("Dave");
     // 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();
     std::cout << alice.output() << '\n'
               << bob.output() << '\n'
               << christina.output() << '\n'
               << dave.output() << '\n';
 }

Possible output:
 Alice and Bob are waiting for locks
 Alice and Bob got locks
 Christina and Bob are waiting for locks
 Christina and Bob got locks
 Christina and Alice are waiting for locks
 Dave and Bob are waiting for locks
 Dave and Bob got locks
 Christina and Alice got locks
 Employee Alice has lunch partners: Bob, Christina
 Employee Bob has lunch partners: Alice, Christina, Dave
 Employee Christina has lunch partners: Bob, Alice
 Employee Dave has lunch partners: Bob

See also
   unique_lock implements movable mutex ownership wrapper
   (C++11) (class template)
   try_lock attempts to obtain ownership of mutexes via repeated calls to try_lock
   (C++11) (function template)
   scoped_lock deadlock-avoiding RAII wrapper for multiple mutexes
   (C++17) (class template)