async_future_promise.cpp

#include <functional>
#include <future>
#include <iostream>

int factorialSharedFuture(std::shared_future<int> f) {
  int value = 1;
  int n = f.get();
  for (int i = 1; i <= n; i++)
    value = i * value;
  return value;
}

int factorialFuture(std::future<int> &f) {
  int value = 1;
  int n = f.get();
  for (int i = 1; i <= n; i++)
    value = i * value;
  return value;
}

int factorial(int input) {
  int value = 1;
  for (int i = 1; i <= input; i++)
    value = i * value;
  return value;
}

int main() {
  {
    int input = 8;
    int output;
    std::future<int> future_thread = std::async(factorial, input);
    output = future_thread.get();
    std::cout << output << std::endl;
  }

  {
    int input = 8;
    int output;
    std::future<int> future_function =
        std::async(std::launch::async, factorial, input);
    output = future_function.get();
    std::cout << output << std::endl;
  }

  {
    int input = 8;
    int output;
    std::promise<int> p;
    std::future<int> inputFuture = p.get_future();
    std::future<int> future_promise_factorial =
        std::async(std::launch::async, factorialFuture, std::ref(inputFuture));
    // some code here
    p.set_value(input);
    // p.set_exception(std::make_exception_ptr(std::runtime_error("Value
    // couldn't be assigned")));
    output = future_promise_factorial.get();
    std::cout << output << std::endl;
  }

  {
    std::promise<int> p;
    std::future inputFuture = p.get_future();
    std::shared_future<int> inputFutureShared = inputFuture.share();
    std::future<int> future_promise_factorial1 = std::async(
        std::launch::async, factorialSharedFuture, inputFutureShared);
    std::future<int> future_promise_factorial2 = std::async(
        std::launch::async, factorialSharedFuture, inputFutureShared);
    std::future<int> future_promise_factorial3 = std::async(
        std::launch::async, factorialSharedFuture, inputFutureShared);
  }
}