I think the idea is that you're using some external library (e.g. database drivers) which do not use asio but returns a std::future. You can't just "not use std

I think the idea is that you're using some external library (e.g. database drivers) which do not use asio but returns a std::future. You can't just "not use std::future" if that's what your library uses without fully rewriting your external library. The other option is as you mention polling using a timer, but I don't see how that's better, I'd rather move the work off of the event loop to a thread. And you then have to do the "latency vs. CPU time" tradeoff dance, trying to judge how often to poll vs. how much latency you're willing to accept.