Async programming: std::future and std::async

Worked example

Read it before you run it

#include <future>
// Header for std::async, std::future and std::promise.
double slow_plan(double goal) { return goal * 2.0; }
// Stand-in for a slow planner.
auto pending = std::async(std::launch::async, slow_plan, 3.0);
// Start slow_plan(3.0) on another thread immediately.
std::cout << "planning...\n";
// This thread keeps working in the meantime.
double path = pending.get();
// Wait for the result (6.0); an exception would be rethrown here.
std::promise<int> p;
// The sending end of a one-time result.
std::future<int> f = p.get_future();
// The receiving end tied to the promise.
std::thread t{[&p] { p.set_value(42); }};
// Another thread fulfils the promise.
std::cout << f.get() << '\n';
// Blocks until the value arrives, then prints 42.
t.join();
// Always join the thread before it is destroyed.

Practice code

Now type it yourself

Copy the starter into practice.cpp and replace every TODO with your own code. Build and run it: g++ -std=c++17 -Wall -Wextra -Wpedantic practice.cpp -o practice -pthread && ./practice

#include <future>
#include <iostream>

int main() {
  // TODO: start a std::async job (std::launch::async) that returns 6 * 7, then get() the result
  std::cout << answer << '\n';
  return 0;
}

42

#include <future>
#include <iostream>

int main() {
  auto pending = std::async(std::launch::async, [] { return 6 * 7; });
  int answer = pending.get();
  std::cout << answer << '\n';
  return 0;
}