Includes predate C++ by a bit. And there is nothing that specifically forbids such a thing; it's just the preprocessor literally inserting (and interpreting) th

Includes predate C++ by a bit. And there is nothing that specifically forbids such a thing; it's just the preprocessor literally inserting (and interpreting) the file specified. Now, normally there are include guards (or #pragma once) to guard against including the same header file twice. But of course that's only necessary because the language specification doesn't say that including the same file twice is disallowed. One can surely construct something where including the same file twice does make sense, e.g. in the light of new #defines in between the two inclusions: // foo.h #ifdef FOO ... #else ... #endif // foo.c #include "foo.h" #define FOO #include "foo.h" Mind you, this is probably not a good idea, at all. But some people might build such things. A similar method that springs to mind is Android's native build system ndk-build, which is a thickly-veiled GNU make where you basically include targets that do something with a lot of variables you set beforehand. And you can do so multiple times in a row. Maybe the included file uses a defined symbol to emit some generated code based on that symbol and you might want to do that with several symbols. Macros are usually preferred for that, but nothing stops you from using an included file.