title: Can main.cpp use the class which is included by the file main.cpp include?
body: <p>If there're three files, let's say main.cpp, a.h and b.h. main.cpp includes a.h and a.h includes b.h, but main.cpp does not include b.h directly. class b is declared in b.h.</p>
  <p>May I ask if we can use class b in main.cpp directly right now?</p>