We recently open sourced our C++ core library, and after speaking a bit with @alfred_bratterud and @amirmc, we thought that some of you might be interested in this. We're using it for our storage platform at Docker.
Basically, it provides different ways to approach asynchronism (using coroutines), networking, formatting, serialization, logging, RPCs, etc. You'll find more details in the github repo directly: https://github.com/infinit/elle
Don't hesitate if you have any questions/feedback!