std::async function running serially

Question

When using std::async with launch::async in a for loop, my code runs serially in the same thread, as if each async call waits for the previous before launching. In the notes for

Answer 1

As you noticed yourself, the future d-tor of future returned by std::async blocks and waits for the async operation to finish (for the future to become ready). In your case, cache object goes out of scope at each of the loop iterations and thus gets destructed, together with the future it holds, so you see the mentioned effect.