I have a.h a.cpp b.h and a library for b. The thing is, if I choose to include b.h in either a.h or a.cpp, it will let complier return "was not declared in this scope" for every line of a.h. The same program works fine on another computer, although Makefile might be different. I have to admit, I think I may have change some settings, but I forgot which one I changed. Is there anybody have any idea? I am using linux and gcc.