C++ / boost : how to wait for an async task to complete?

I have a thread pool which execute tasks asynchronously. But I need to wait for a certain task to complete before proceeding (running the task in current thread is not allowed, the task must be run by a worker thread).

What's the easiest way to achieve this using C++11 or Boost?

  pool.enqueue([]() {
    std::this_thread::sleep_for(2s); // task 1
    // notify task 1 completion???
    std::this_thread::sleep_for(2s); // task 2
  });
  // wait until task 1 is complete???