继承的类C＆＃x2B;＆＃x2B;，虚拟函数无法实现

Question

我正在尝试使用返回Tuition_Fees+Library_fines的方法来创建一个班级学生的毕业生（后者是私有的，我们可以通过在父级中公开的get和set函数访问它）。我收到以下消息：“缺失的VTable通常意味着第一个非内部虚拟成员函数没有定义。”这证明有一个虚拟函数无法实现。这是我的标题，实施和主代码，我找不到错误。有什么建议可以解决吗？谢谢 ！

#include "Student.hpp"


class GraduateStudent : public Student {
private:
        double library_fines;
public:
    GraduateStudent();
    GraduateStudent(std::string name, double fines, double fees, bool fullTime);
    bool fullTime;

    virtual double MoneyOwed() const;
    virtual ~GraduateStudent() {} ;
};


#include "GraduateStudent.hpp"
#include "Student.hpp"
#include <iostream>


GraduateStudent::GraduateStudent() : Student()
{
     bool fullTime; 
}



double GraduateStudent::MoneyOwed() const
{
       return tuition_fees + GetLibraryFines();
}

int main(int argc, char* argv[])
{
      Student* stu1= new GraduateStudent;
      Student* stu2= new GraduateStudent;

//computing the owes
      stu1->tuition_fees= 1000.0;
      stu1->SetLibraryFines(10);
      stu2->SetLibraryFines(1500);
      stu2->tuition_fees= 1000.0;


std::cout << "Fees for Student 1=" << stu1-> MoneyOwed()<<"\n";
std::cout << "Fees for Student 2=" << stu2-> MoneyOwed()<<"\n";


delete stu1;
delete stu2;



return 0;
}

当我测试父班时，一切正常。但是我更喜欢在这里分享父班的标题：

#include <string>
#include <iostream>

class Student {
public:
    Student();
    Student(std::string name, double fines, double fees);

    std::string name;
    double tuition_fees;
    virtual double MoneyOwed() const;
    virtual ~Student() {};

    void SetLibraryFines(double amount);
    double GetLibraryFines() const;
        friend std::ostream& operator<<(std::ostream& out, Student const& obj);

    private:
        double library_fines;
    };