One of the things that keeps cropping up in the ability to notify another function that something has finished. In the past, I would use condition_variable and wait on that.

There are a couple of ways I have used to do this.

Have a look at Klemens' experimental library

https://github.com/klemens-morgenstern/sam

It contains an async condition variable fully compatible with asio

The other way is to use an asio steady_timer. You would cancel the timer in order to signal the condition. The waiter would see this as an operation_aborted error, which would indicate, "you have been signalled".

You can set the timer to expires_at(std::chrono::steady_clock::max()) to indicate that the wait should be indefinite.