How to use a future without waiting for it?

Question

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.

Belgae

Are you looking for something like then and when_all or when_any continuations? – Pharisaism

Answer 1

You are right that the current future library is not complete yet. What we miss is a way to indicate 'when future x is ready, start operation f'. Here's a nice post about that.

What you may want is a map/reduce implementation: upon each future's completion, you want to start adding it to the accumulator (reduce).

You can use a library for that - it's not very simple to build it yourself :). One of the libraries that's gaining traction is RxCpp - and they have a post on map/reduce.

Potomac

Answer 2

The design of futures lends itself to this kind of solution, where you create more futures representing the calculated values:

  std::vector<std::future<int>> calculated_futures;

  for (auto &e : futures) {
      calculated_futures.push_back(
          std::async([&e]{ return someLengthyCalculation(e.get()); })
      );
  }

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

Godmother

Maybe that is more of the answer OP is helped with... Hint: std::transform futures into calculated_futures to better express intent. – Potomac