Code in C / C ++ for learning, where can I see the source code?

Question

C++ / C#

It is curious to see and learn how the code is implemented, for educational purposes. There are enough examples, of course, but I would like something from real life, using the example of a billing program, without frills. Working with a simple client database of clients, accounting for actions, loading external data. Or, as an example, an application for accounting for the sale of any product, as well as a certain customer base, the product of which is recorded, sales to customers. The main thing is that the essence is realized.

Answer 1

https://github.com/search?l=C%2B%2B&q=billing&type...
Did this option cross your mind?

Answer 2

Look like Alexandrescu. There it is quite in the spirit and with a smell. https://ru.wikipedia.org/wiki/Loki

Answer 3

I asked myself the same question once. My colleagues advised me to watch Doom sources.