How to check if a std::async task is finished?

Use future::wait_for(). You can specify a timeout, and after that, get a status code.

Example:

task.wait_for(std::chrono::seconds(1));

This will return future_status::ready, future_status::deferred or future_status::timeout, so you know the operation's status. You can also specify a timeout of 0 to have the check return immediately as soon as possible.