What is the difference between packaged_task and async

Question

While working with the threaded model of C++11, I noticed that

std::packaged_task<int(int,int)> task([](int a, int b) { return a + b; });
auto f = task.get_future();
task(2,3);
std::cout << f.get() << '\n';

and

auto f = std::async(std::launch::async, 
    [](int a, int b) { return a + b; }, 2, 3);
std::cout << f.get() << '\n';

seem to do exactly the same thing. I understand that there could be a major difference if I ran std::async with std::launch::deferred, but is there one in this case?

What is the difference between these two approaches, and more importantly, in what use cases should I use one over the other?

Answer 1

Actually the example you just gave shows the differences if you use a rather long function, such as

//! sleeps for one second and returns 1
auto sleep = [](){
    std::this_thread::sleep_for(std::chrono::seconds(1));
    return 1;
};

Packaged task

A packaged_task won't start on it's own, you have to invoke it:

std::packaged_task<int()> task(sleep);

auto f = task.get_future();
task(); // invoke the function

// You have to wait until task returns. Since task calls sleep
// you will have to wait at least 1 second.
std::cout << "You can see this after 1 second\n";

// However, f.get() will be available, since task has already finished.
std::cout << f.get() << std::endl;

std::async

On the other hand, std::async with launch::async will try to run the task in a different thread:

auto f = std::async(std::launch::async, sleep);
std::cout << "You can see this immediately!\n";

// However, the value of the future will be available after sleep has finished
// so f.get() can block up to 1 second.
std::cout << f.get() << "This will be shown after a second!\n";

Drawback

But before you try to use async for everything, keep in mind that the returned future has a special shared state, which demands that future::~future blocks:

std::async(do_work1); // ~future blocks
std::async(do_work2); // ~future blocks

/* output: (assuming that do_work* log their progress)
    do_work1() started;
    do_work1() stopped;
    do_work2() started;
    do_work2() stopped;
*/

So if you want real asynchronous you need to keep the returned future, or if you don't care for the result if the circumstances change:

{
    auto pizza = std::async(get_pizza);
    /* ... */
    if(need_to_go)
        return;          // ~future will block
    else
       eat(pizza.get());
}   

For more information on this, see Herb Sutter's article async and ~future, which describes the problem, and Scott Meyer's std::futures from std::async aren't special, which describes the insights. Also do note that this behavior was specified in C++14 and up, but also commonly implemented in C++11.

Further differences

By using std::async you cannot run your task on a specific thread anymore, where std::packaged_task can be moved to other threads.

std::packaged_task<int(int,int)> task(...);
auto f = task.get_future();
std::thread myThread(std::move(task),2,3);

std::cout << f.get() << "\n";

Also, a packaged_task needs to be invoked before you call f.get(), otherwise you program will freeze as the future will never become ready:

std::packaged_task<int(int,int)> task(...);
auto f = task.get_future();
std::cout << f.get() << "\n"; // oops!
task(2,3);

TL;DR

Use std::async if you want some things done and don't really care when they're done, and std::packaged_task if you want to wrap up things in order to move them to other threads or call them later. Or, to quote Christian:

In the end a std::packaged_task is just a lower level feature for implementing std::async (which is why it can do more than std::async if used together with other lower level stuff, like std::thread). Simply spoken a std::packaged_task is a std::function linked to a std::future and std::async wraps and calls a std::packaged_task (possibly in a different thread).