When to use promise over async or packaged_task?

Question

When should I use std::promise over std::async or std::packaged_task? Can you give me practical examples of when to use each one of them?

Answer 1

std::async

std::async is a neat and easy way to get a std::future, but:

• It does not always it start a new thread; pass std::launch::async as a first parameter to force it.

  auto f = std::async( std::launch::async, func );
  

• The std::~future destructor can block until the new thread finishes

  auto sleep = [](int s) { std::this_thread::sleep_for(std::chrono::seconds(s)); };
  
  {
      auto f = std::async( std::launch::async, sleep, 5 );
  }
  

  Normally we expect that only .get() or .wait() blocks, but for a std::future returned from std::async, the destructor also may block, so be careful not to block your main thread just by forgetting about it.

• If the std::future is stored in a temporary-life object, the std::async call will block immediately, so the following block will take 10 seconds if you remove the auto f = initializations:

  auto sleep = [](int s) { std::this_thread::sleep_for(std::chrono::seconds(s)); };
  
  {
      auto f1 = std::async( std::launch::async, sleep, 5 );
      auto f2 = std::async( std::launch::async, sleep, 5 );
  }
  

std::packaged_task

std::packaged_task by itself has nothing to do with threads: it is just a functor and a related std::future. Consider the following:

auto task = [](int i) { std::this_thread::sleep_for(std::chrono::seconds(5)); return i+100; };

std::packaged_task< int(int) > package{ task };
std::future<int> f = package.get_future();
package(1);
std::cout << f.get() << "\n";

Here we just run the task by package(1), and after it returns, f is ready so no blocking on .get().

There is a feature of std::packaged_task that makes it very useful for threads. Instead of just a function, you can initialize std::thread with a std::packaged_task which gives a really nice way of getting to the 'std::future'. Consider the following:

std::packaged_task< int(int) > package{ task };
std::future<int> f = package.get_future();
std::thread t { std::move(package), 5 };

std::cout << f.get() << "\n";       //block here until t finishes

t.join();

Because std::packaged_task is not copyable, you must move it to new thread with std::move.

std::promise

std::promise is a powerful mechanism. For example, you can pass a value to new thread without need of any additional synchronization.

auto task = [](std::future<int> i) {
    std::cout << i.get() << std::flush;
};

std::promise<int> p;
std::thread t{ task, p.get_future() };

std::this_thread::sleep_for(std::chrono::seconds(5));
p.set_value(5);

t.join();

New thread will wait for us on .get()

---

So, in general, answering your question:

• Use std::async only for simple things, e.g. to make some call non-blocking, but bear in mind the comments on blocking above.

• Use std::packaged_task to easily get to a std::future, and run it as a separate thread

  std::thread{ std::move(package), param }.detach();
  

  or

  std::thread t { std::move(package), param };
  

• Use std::promise when you need more control over the future.

See also std::shared_future and on passing exceptions between threads std::promise::set_exception