mutex/example6.cpp

// unique_lock constructor example
#include <iostream>       // std::cout
#include <thread>         // std::thread
#include <mutex>          // std::mutex, std::lock, std::unique_lock
                                     // std::adopt_lock, std::defer_lock
std::mutex foo,bar;

void task_a () {
 std::lock (foo,bar);         // simultaneous lock (prevents deadlock)
 std::unique_lock<std::mutex> lck1 (foo,std::adopt_lock);
 std::unique_lock<std::mutex> lck2 (bar,std::adopt_lock);
 std::cout << "task a\n";
 // (unlocked automatically on destruction of lck1 and lck2)
}

void task_b () {
 // foo.lock(); bar.lock(); // replaced by:
 std::unique_lock<std::mutex> lck1, lck2;
 lck1 = std::unique_lock<std::mutex>(bar,std::defer_lock);
 lck2 = std::unique_lock<std::mutex>(foo,std::defer_lock);
 std::lock (lck1,lck2);       // simultaneous lock (prevents deadlock)
 std::cout << "task b\n";
 // (unlocked automatically on destruction of lck1 and lck2)
}

int main ()
{
 std::thread th1 (task_a);
 std::thread th2 (task_b);

 th1.join();
 th2.join();

 return 0;
}