Support forum for Nagios Core, Nagios Plugins, NCPA, NRPE, NSCA, NDOUtils and more. Engage with the community of users including those using the open source solutions.
Nagios Core is written in C, and I do not recall seeing any C++ on a casual glance.
The editor you use is entirely dependent on your own preferences. I personally prefer to use vim, some people swear by emacs. If you want fancy features like code-completion you might look into Sublime Text or a full IDE (Integrated Development Environment). You could ask 50 people and get 50 different answers, however the common themes are usually syntax highlighting, multi-file support, and useful keyboard shortcuts.
You are very welcome. If you have a specific question about the Core codebase you are welcome to start a new thread for the new question. I will be closing this one now.