Depends on what Operating System you are talking about - Linux or Windows or Mac OS?? Do you really mean that you want the "implementation code" of an Operating System??

I would imagine that if you are trying to learn about an Operating System the actual code would be far to advanced and more or less useless to a beginner. Have you ever tried to read someone else's program code without any other documentation - even a simple program.

Maybe you mean that you want to use C++ with MS Windows or similar Operating System then try to find out about Windows API. If you use google you will find many references to information on Windows API and sample code using it.

Alan