Lock-free swap of two unique_ptr<T>

Question

Swapping two unique_ptrs is not guaranteed to be threadsafe.

std::unique_ptr<T> a, b;
std::swap(a, b); // not threadsafe

Since I need atomic pointer swaps and since I like the ownership handling of unique_ptr, is there a simple way to combine them both?

---

Edit: If this is not possible, I am open for alternatives. I at least want to do something like this:

threadshared_unique_ptr<T> global;

void f() {
   threadlocal_unique_ptr<T> local(new T(...));
   local.swap_content(global); // atomically for global
}

What is the idiomatic way of doing this in C++11?

Answer 1

Lock-free swapping of two pointers

It seems there is no general lock-free solution for this problem. To do this, you need a possibility to atomically write new values into two non-continous memory locations. This is called DCAS, but it is not available in Intel processors.

Lock-free transfer of ownership

This one is possible, as it is only needed to atomically save new value into global and receive its old value. My first idea was to use CAS operation. Take a look at the following code to get an idea:

std::atomic<T*> global;

void f() {
   T* local = new T;
   T* temp = nullptr;
   do {
       temp = global;                                                   // 1
   } while(!std::atomic_compare_exchange_weak(&global, &temp, local));  // 2

   delete temp;
}

Steps

1. Remember current global pointer in temp
2. Save local to global if global is still equal to temp (it wasn't changed by other thread). Try again if this is not true.

Actually, CAS is overkill there, as we do not do anything special with old global value before it is changed. So, we just can use atomic exchange operation:

std::atomic<T*> global;

void f() {
   T* local = new T;
   T* temp = std::atomic_exchange(&global, local);
   delete temp;
}

See Jonathan's answer for even more short and elegant solution.

Anyway, you will have to write your own smart pointer. You can't use this trick with standard unique_ptr.