I’m sorry people downvoted you without explaining why you deserve them. I’m guessing people think your question isn’t educated enough? But in any case, I’ll try to get you started. I am not educated enough to answer your question, but I can show you how you can answer it yourself and probably learn a lot more than you would have gotten from here.
https://github.com/vim/vim/blob/master/src/README.txt -The readme for the vim source code, which is all written in C. Not exactly C++, but the better you are at C, the better you are at certain facets of C++. And if you look at the list of source files in the readme along with their short descriptions, you do kind of get a skeleton.
Notepad++ actually is written in C++, but I suspect the GUI overhead would make it significantly harder to trace. Still, if you want, https://github.com/notepad-plus-plus/notepad-plus-plus/tree/master/PowerEditor/src