问题
What is the difference in the following:
std::async(my_function);
and
std::async(std::launch::async, my_function);
What is the difference in using the pilicy std::launch::async in this case?? Does the first option not launch the function asynchronously anyway??
回答1:
The first one is equivalent to passing launch::async | launch::deferred, in which case it is up to the implementation whether it is launched asynchronously or merely deferred (called when a non-timed waiting function like get() is called on the returned future).
The idea is that by default, the implementation can choose to defer if creating a new thread isn't going to be a performance gain. To force a new thread to be created, pass launch::async only.
来源：https://stackoverflow.com/questions/28659215/is-stdlaunchasync-policy-needed