Thread safe local variable

Question

void HelloWorld()
{
   static std::atomic<short> static_counter = 0;
   short val = ++static_counter; // or val = static_counter++;
}

If this function is called from two threads,

Can the local variable val be 1 in both threads? or (0 if static_counter++ is used?)

Answer 1

Can the local variable val be 1 in both threads?

No. ++static_counter is equivalent to:

 fetch_add(1)+1

which cannot return same value for two (or more) threads because fetch_add is executed atomically.

Answer 2

No. The only way val could have the same value in both threads is if the two atomic operations overlapped. By definition, atomic operations cannot overlap.