What happens if main does not return 0?

If main returns a non-zero value you will get a Run Time Error. However, with a C++ compiler this should not happen
as long as you (a) declare main to return int, and (b) do not explicitly return a non-zero value. If you declare main to have a non-int return type you should get a compile error. It never hurts to have an explicit return 0 at the end of main, though.