Is unique_ptr thread safe?

Question

Is unique_ptr thread safe? Is it impossible for the code below to print same number twice?

#include <memory> #include <string> #include <thread> #include <cstdio>  using namespace std;  int main() {     unique_ptr<int> work;      thread t1([&] {         while (true) {             const unique_ptr<int> localWork = move(work);             if (localWork)                 printf("thread1: %d\n", *localWork);             this_thread::yield();         }     });      thread t2([&] {         while (true) {             const unique_ptr<int> localWork = move(work);             if (localWork)                 printf("thread2: %d\n", *localWork);             this_thread::yield();         }     });      for (int i = 0; ; i++) {         work.reset(new int(i));          while (work)             this_thread::yield();     }      return 0; } 

Answer 1

unique_ptr is thread safe when used correctly. You broke the unwritten rule: Thou shalt never pass unique_ptr between threads by reference.

The philosophy behind unique_ptr is that it has a single (unique) owner at all times. Because of that, you can always pass it safely between threads without synchronization -- but you have to pass it by value, not by reference. Once you create aliases to a unique_ptr, you lose the uniqueness property and all bets are off. Unfortunately C++ can't guarantee uniqueness, so you are left with a convention that you have to follow religiously. Don't create aliases to a unique_ptr!