I don't know why this is downvoted. This is exactly how it is. The first time the compiler reads the include file, it remembers where the corresponding #endif

I don't know why this is downvoted. This is exactly how it is. The first time the compiler reads the include file, it remembers where the corresponding #endif is for each #if. The next time the file is included, the compiler can skip over the whole included file. There is no performance advantage from outsourcing the conditional to every including location. Putting an ifndef guard at the location where the file located is not workable. It's terrible boilerplate. To the relatively simple '#include <A.h>' another variable that must be synchronized with dependencies gets added. Plus 2-3 lines for the conditional. At each including location.