When to use promise over async or packaged_task?

问题

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?

回答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();

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

回答2:

A promise is used to store a value that was calculated using e.g. a std::async. See http://en.cppreference.com/w/cpp/thread/promise

I can imagine you wonder about the difference between std::packaged_task and std::async (in the most common approach, std::async starts a separate thread NOW to run function/lambda/etc with a (likely) expensive calculation. A std::packaged_task is used to wrap a function/lambda/etc with the current values of arguments so you can LATER run it, either synchronously or in a separate thread).

Both std::packaged_task and std::async provide a std::future that will contain the RESULT of the wrapped function/lambda/etc once run. Internally, the std::future uses a std::promise to hold that result.

来源：https://stackoverflow.com/questions/17729924/when-to-use-promise-over-async-or-packaged-task

标签

c++

asynchronous

promise

future

packaged-task