What is the difference between packaged_task and async

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

//! sleeps for one second and returns 1auto 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 blocksstd::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).

Packaged Task vs async

p> Packaged task holds a task [function or function object] and future/promise pair. When the task executes a return statement, it causes set_value(..) on the packaged_task's promise.

a> Given Future, promise and package task we can create simple tasks without worrying too much about threads [thread is just something we give to run a task].

However we need to consider how many threads to use or whether a task is best run on the current thread or on another etc.Such descisions can be handled by a thread launcher called async(), that decides whether to create a new a thread or recycle an old one or simply run the task on the current thread. It returns a future .

"The class template std::packaged_task wraps any callable target (function, lambda expression, bind expression, or another function object) so that it can be invoked asynchronously. Its return value or exception thrown is stored in a shared state which can be accessed through std::future objects."

"The template function async runs the function f asynchronously (potentially in a separate thread) and returns a std::future that will eventually hold the result of that function call."