How to do it...

We will implement a simple application that invokes a long-running function in a separate thread and waits for its result. While the function is running, the application can keep working on other calculations.

1. In your ~/test working directory, create a subdirectory called async.
2. Use your favorite text editor to create an async.cpp file in the async subdirectory.
3. Put the code of our application into the async.cpp file, starting from the common headers and our long-running function:

#include <chrono>
#include <future>
#include <iostream>

int calculate (int x) {
  auto start = std::chrono::system_clock::now();
  std::cout << "Start calculation\n";
  std::this_thread::sleep_for(std::chrono::seconds(1));
  auto delta = std::chrono::system_clock::now() - start;
  auto ms = std::chrono::duration_cast<std::chrono::milliseconds>(delta);
  std::cout << "Done in " << ms.count() << " ms\n";
  return x*x;
}

4. Next, add the test function, which invokes the long-running function:

void test(int value, int worktime) {
  std::cout << "Request result of calculations for " << value << std::endl;
  std::future<int> fut = std::async (calculate, value);
  std::cout << "Keep working for " << worktime << " ms" << std::endl;
  std::this_thread::sleep_for(std::chrono::milliseconds(worktime));
  auto start = std::chrono::system_clock::now();
  std::cout << "Waiting for result" << std::endl;
  int result = fut.get();
  auto delta = std::chrono::system_clock::now() - start;
  auto ms = std::chrono::duration_cast<std::chrono::milliseconds>(delta);

  std::cout << "Result is " << result
            << ", waited for " << ms.count() << " ms"
            << std::endl << std::endl;
}

5. Finally, add a main minimalistic function:

int main ()
{
  test(5, 400);
  test(8, 1200);
  return 0;
}

6. Create a file called CMakeLists.txt in the loop subdirectory, with the following content:

cmake_minimum_required(VERSION 3.5.1)
project(async)
add_executable(async async.cpp)

set(CMAKE_SYSTEM_NAME Linux)
set(CMAKE_SYSTEM_PROCESSOR arm)

SET(CMAKE_CXX_FLAGS "--std=c++14")
target_link_libraries(async pthread -static-libstdc++)

set(CMAKE_CXX_COMPILER /usr/bin/arm-linux-gnueabi-g++)

You can build and run the application.