compiler cannot recognize my class in c++ - cyclic dependency

having this base class:

Core.hpp:

#ifndef C3_CORE_HPP
#define C3_CORE_HPP

#include <c3/utils/Str.hpp>
#include <c3/utils/Vec.hpp>
#include <c3/school/Student.hpp>

class Core {
public:
    Core() = default;
    explicit Core(std::istream&in);
    virtual ~Core();

    virtual double grade() const;

    const Str &getName() const;
    double getMidterm() const;
    double getFinal() const;
    const Vec<double> &getHomeworks() const;

protected:
    Vec<double> homeworks;

    virtual std::istream &read(std::istream &in);
    virtual Core *clone() const;

    std::istream &read_common(std::istream &in);

private:
    Str name;
    double midterm{}, final{};

    friend class Student;
};

std::istream &read_hw(std::istream &in, Vec<double> &hws);

#endif //C3_CORE_HP

and Grad.hpp:

#ifndef C3_GRAD_HPP
#define C3_GRAD_HPP

#include <c3/school/Core.hpp>

class Grad: public Core {
public:
    Grad() = default;
    explicit Grad(std::istream &in);

    std::istream &read(std::istream &in) override;
    double grade() const override;

protected:
    Grad *clone() const override;

private:
    double thesis{};
};


#endif //C3_GRAD_HPP

(The code is created according to book accelerated C++ by Andrew Koenig)

Now this gets me error:

In file included from /home/shepherd/Desktop/cpp/cpp0book/c3/./c3/school/Student.hpp:8,
                 from /home/shepherd/Desktop/cpp/cpp0book/c3/./c3/school/Core.hpp:10,
                 from /home/shepherd/Desktop/cpp/cpp0book/c3/c3/main.cpp:4:
/home/shepherd/Desktop/cpp/cpp0book/c3/./c3/school/Grad.hpp:10:25: error: expected class-name before ‘{’ token
   10 | class Grad: public Core {
      |                         ^
/home/shepherd/Desktop/cpp/cpp0book/c3/./c3/school/Grad.hpp:15:19: error: ‘std::istream& Grad::read(std::istream&)’ marked ‘override’, but does not override
   15 |     std::istream &read(std::istream &in) override;
      |                   ^~~~
/home/shepherd/Desktop/cpp/cpp0book/c3/./c3/school/Grad.hpp:16:12: error: ‘double Grad::grade() const’ marked ‘override’, but does not override
   16 |     double grade() const override;
      |            ^~~~~
/home/shepherd/Desktop/cpp/cpp0book/c3/./c3/school/Grad.hpp:19:11: error: ‘Grad* Grad::clone() const’ marked ‘override’, but does not override
   19 |     Grad *clone() const override;
      |           ^~~~~
In file included from /home/shepherd/Desktop/cpp/cpp0book/c3/./c3/school/Core.hpp:10,
                 from /home/shepherd/Desktop/cpp/cpp0book/c3/c3/main.cpp:4:
/home/shepherd/Desktop/cpp/cpp0book/c3/./c3/school/Student.hpp:26:5: error: ‘Core’ does not name a type
   26 |     Core *cp{};
      |     ^~~~
gmake[2]: *** [CMakeFiles/c3.dir/build.make:76: CMakeFiles/c3.dir/c3/main.cpp.o] Error 1
gmake[1]: *** [CMakeFiles/Makefile2:83: CMakeFiles/c3.dir/all] Error 2
gmake: *** [Makefile:91: all] Error 2

The first error is

error: expected class-name before ‘{’ token
   10 | class Grad: public Core {

Which seems to me the compiler cannot recognize the Core class even when included. So why cannot compiler recognize my base class?

using this directory structure: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1204r0.html

github repo: https://github.com/Herdsmann/student_project.git

As i said in the comment, the problem is due to cyclic dependency. In particular, your

Student.hpp includes --> Grad.hpp which in turn includes --> Core.hpp which finally includes --> Student.hpp

So as you can see from above, you ended up where you started, namely at Student.hpp. This is why it is called cyclic dependency.

To solve this just remove the #include <c3/school/Student.hpp> from Core.hpp. This is because for the friend declaration friend class Student, you don't need to forward declare or include the Student class.

So the modified/correct Core.hpp file looks like this:

#ifndef C3_CORE_HPP
#define C3_CORE_HPP

#include <c3/utils/Str.hpp>
#include <c3/utils/Vec.hpp>
//note i have removed the include header from here

class Core {
  //other members here as before

private:
    Str name;
    double midterm{}, final{};

    friend class Student;//THIS WORKS WITHOUT INCLUDING Student.hpp
};

std::istream &read_hw(std::istream &in, Vec<double> &hws);

#endif //C3_CORE_HPP