I created two classes, Student and Course.
They are friend classes.
Each class has fields of the other class, meaning:
Student has a field vector <course> and Course has a field vector <Student>.
I have compilation error only in Course.h but not in Student.h.
I also tried #include to each other in both of them and it did not work either.
Adding the codes both of the classes if anyone sees something I don't see: