Currently, there is no C++ admin SDK (
see this page). The
Firebase C++ SDK is intended for client use and the
desktop workflow (what you'd need for a server running anything other than iOS or Android) is currently beta. Although you could probably get really tricky with a special user who you allow to bypass various security rules, this will probably be pretty difficult to maintain.
If C++ is a requirement on your server, you could consider calling one of the admin SDKs from C code. My personal preference would be to use the Go admin SDK using
cgo to bounce between C++ and Go logic (I choose this because it's relatively full featured, has an easy to learn foreign function interface, Go will feel familiar to C developers, and the runtime is really lightweight if you don't have the other languages setup).
The JavaScript SDK will net you the most features, but communicating with the Node runtime directly can get a little complicated (and with your desire to use C++ on the server, I assume that you have some performance or safety requirements that are better addressed by the other SDKs).