Since I started writing Owl in 2012, several excellent libraries have been released. Many of these libraries are header only and use only a fraction of the setup compared to the libraries I used originally. The following three come immediately to mind:

fmt

A formatting library that allows you to do things like std::string s = fmt::format("{0}{1}{0}", "abra", "cad");. Nearly as fast, if not faster, than standard std::cout or boost::format.

spdlog

“Fast C++ logging library.” Uses fmt-type syntax. Supports standard log sinks such as the console, text files and text files with rollovers.

JSON for Modern C++

Very simple and easy to use JSON functionality using intuitive syntax and operations. This is already used in arcc.