How to use a future without waiting for it?

The following example is taken from a C++ async tutorial:

#include <future>
#include <iostream>
#include <vector>

int twice(int m) { return 2 * m; }

int main() {
  std::vector<std::future<int>> futures;
  for(int i = 0; i < 10; ++i) { futures.push_back (std::async(twice, i)); }

  //retrive and print the value stored in the future
  for(auto &e : futures) { std::cout << e.get() << std::endl; }
  return 0;
}

How can I use the result of a future without waiting for it? Ie I would like to do something like this:

  int sum = 0;
  for(auto &e : futures) { sum += someLengthyCalculation(e.get()); }

I could pass a reference to the future to someLengthyCalculation, but at some point I have to call get to retrieve the value, thus I dont know how to write it without waiting for the first element being completed, before the next one can start summing.