I wrote a Matrix client library in Qt about three weeks ago, and it was really braindead simple. JSON in, JSON out. HTTP codes tell you what response you got.

I wrote a Matrix client library in Qt about three weeks ago, and it was really braindead simple. JSON in, JSON out. HTTP codes tell you what response you got. TLS on an https connection for secure communications. WebRTC for voice calls, and you just query the API for the signaling server to use.