I'm confusing myself with difference between a future and a promise.
Obviously, they have different methods and stuff, but what is the actual use case?
Is it?:
Future and Promise are the two separate sides of an asynchronous operation.
std::promise is used by the "producer/writer" of the asynchronous operation.
std::future is used by the "consumer/reader" of the asynchronous operation.
The reason it is separated into these two separate "interfaces" is to hide the "write/set" functionality from the "consumer/reader".
auto promise = std::promise<std::string>();
auto producer = std::thread([&]
{
promise.set_value("Hello World");
});
auto future = promise.get_future();
auto consumer = std::thread([&]
{
std::cout << future.get();
});
producer.join();
consumer.join();
One (incomplete) way to implement std::async using std::promise could be:
template<typename F>
auto async(F&& func) -> std::future<decltype(func())>
{
typedef decltype(func()) result_type;
auto promise = std::promise<result_type>();
auto future = promise.get_future();
std::thread(std::bind([=](std::promise<result_type>& promise)
{
try
{
promise.set_value(func()); // Note: Will not work with std::promise<void>. Needs some meta-template programming which is out of scope for this question.
}
catch(...)
{
promise.set_exception(std::current_exception());
}
}, std::move(promise))).detach();
return std::move(future);
}
Using std::packaged_task which is a helper (i.e. it basically does what we were doing above) around std::promise you could do the following which is more complete and possibly faster:
template<typename F>
auto async(F&& func) -> std::future<decltype(func())>
{
auto task = std::packaged_task<decltype(func())()>(std::forward<F>(func));
auto future = task.get_future();
std::thread(std::move(task)).detach();
return std::move(future);
}
Note that this is slightly different from std::async where the returned std::future will when destructed actually block until the thread is finished.
