编译器无法识别我在 C++ 中的类

Question

拥有这个基类：

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

和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

（代码根据书accelerated C++ by Andrew Koenig创建）

现在这让我出错了：

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

第一个错误是

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

在我看来，即使包含 Core 类，编译器也无法识别。那么为什么编译器不能识别我的基类呢？

使用这个目录结构： http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1204r0.html

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

Answer 1

正如我在评论中所说，问题是由于循环依赖。特别是，您的

Student.hpp 包括 --> Grad.hpp 又包括 --> Core.hpp 最后包括 --> Student.hpp

因此，正如您从上面看到的那样，您最终到达了起点，即 Student.cpp。这就是为什么它被称为循环依赖。

要解决，只需从 Core.hpp 中删除 #include <c3/school/Student.hpp>。这是因为对于朋友声明friend class Student，您不需要转发声明或包含Student 类。

所以修改/正确的 Core.hpp 文件如下所示：

#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

Answer 2

在 git 存储库中，文件 main.cpp 包含 Core.hpp，其中包含 Student.hpp 以获取朋友类 Student 的定义。在文件Student.hpp 中包含标题Grad.hpp，它本身包含Core.hpp。现在，由于Core.hpp 在包含Student.hpp 之前定义了包含保护C3_CORE_HPP，所以当Grad.hpp 包含Core.hpp 时，预处理器会跳过Core.hpp 的内容。这就是为什么在定义类 Grad 时，编译器不知道类 Core。

要解决此问题，您可以在 Core.hpp 中预定义类 Student 并省略包含：

Core.hpp：

#ifndef C3_CORE_HPP
#define C3_CORE_HPP

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

class Student;

class Core {

    // [...]

    friend class Student;
};

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

#endif //C3_CORE_HP