A file shall not depend on declarations that are only indirectly included by means of a nested #include directive in some other included header file. Otherwise, it would be unnecessarily difficult to change dependencies of a header file, even if it already was self-contained. It would also be somewhat less obvious which files are the actual users of specific headers.

Exception:an exception to this is the inclusion of "stdafx.h" in Visual Studio. In this header file a lot of files are combined to speed up the compilation process.