編譯器無法識別C++中的類

具有這個基類：

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

（代碼是根據安德魯·克尼格的加速書C++生成的）

現在，我得到了一個錯誤：

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

第一個錯誤是

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

在我看來，即使包含Core類，編譯器也無法識別它。那么為什么編譯器不能識別我的基類呢？

使用此目錄結構：http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1204r0.html

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