无法使静态函数递增并返回其值

Question

基本上，该程序创建课程，在该课程中最多创建30名学生，召集每个学生的出勤率，并询问已创建的课程总数。我在最后一部分遇到了麻烦。

我在Student类中编写的getID（）函数有效，但是当我与Course类中的howMany（）函数进行类似操作时，出现错误。具体来说，我在Course.cpp的第23行（其中numInstances被告知在Course构造函数中增加一个）和Course.h的第28行（其中howMany（ ） 被定义为）。

我怀疑这与包含静态变量的事实有关。

Student.h

##ifndef STUDENT_H
#define STUDENT_H
#include <iostream>
#include <string>
using std::string;

//This class describes a Student

class Student
{
    protected:
        //data
        string name;
        int ID;

    public:
        virtual string answer();      //allows polymorph
        Student();
        Student(string theName, int theID);
        string toString();
        string getName();
        int getID(){ return ID; }
};

#endif //STUDENT_H

Student.cpp

#include <iostream>
#include <string>
using namespace std;

#include "Student.h"

//This class describes a Student

    //default constructor
    Student::Student()
    {
        name = "unknown";
        ID = 0;
    }

    //parameterized constructor
    Student::Student(string theName, int theID)
    {
        name = theName;
        ID = theID;
    }

    //toString - returns String representation of the instance
    string Student::toString()
    {
        string msg = name + ":     \t";
        msg += to_string(ID);
        return msg;
    }

    //answer - returns how the Student instance answers during attendance
    string Student::answer()
    {
        return "\"Here\"";  //note use of escape character here...
    }

    //getName - returns the name
    string Student::getName()
    {
        return name;
    }

Course.h

#ifndef COURSE_H
#define COURSE_H
#include <iostream>
#include <string>
using std::string;

#include "Student.h"

const int MAX_STUDENTS = 30;

    class Course
    {
    private:
        //data
        string name;
        int maxCount;
        int count;
        Student *roster[MAX_STUDENTS];

    public:
        static int numInstances;  //initialized in Course.cpp
        Course();
        string toString();
        void takeAttendance();
        void addStudent(Student *newStudent);
        static int howMany() { return numInstances; }
    };

#endif

Course.cpp

#include <iostream>
#include <string>
#include <typeinfo>
using namespace std;

#include "Course.h"

    static int numInstances = 0;


    //default constructor - sets the maxCount to 30,
    //                      initializes name/count, creates array,
    //                      updates numInstances.
    Course::Course()
    {
        name = "unknown";
        maxCount = MAX_STUDENTS; //30
        count = 0;
        *roster[maxCount];
        numInstances++;
    }

Driver.cpp

#include <iostream>
#include <string>
using std::string;
using std::cout;
using std::cin;
using std::endl;

#include "Student.h"
#include "Course.h"


//This is the driver class for the program

    int main()
    {
        //create a Student
        Student firstStudent("Joe", 13);
        cout << "----- First Student is: \n";
        cout << firstStudent.toString() << endl;

        //tell it to return its ID - print it.
        cout << "\n----- Ask the first Student for its ID: \n";
        cout << firstStudent.getID() << endl;

        cout << "\n----- Creating a Course and telling it to add some Students.  Printing it:\n";
        //create a new Course
        Course algebra;  //max of 30 students by default

        //tell it to add some Students
        algebra.addStudent(&firstStudent);
        algebra.addStudent(new Student());
        algebra.addStudent(new Student("Peter", 21));
        algebra.addStudent(new Student("Janet", 43));
        algebra.addStudent(new Student("Jim", 243));
        algebra.addStudent(new Student("Al", 3));
        algebra.addStudent(new Student());
        algebra.addStudent(new Student("Jenn", 23));
        algebra.addStudent(new Student("Jose", 43));
        algebra.addStudent(new Student("Grant", 7));

        //print the string representation of the course to see what is there
        cout << algebra.toString() << endl;

        //tell the course to take attendance
        cout << "\n----- telling the course to take attendance\n";
        algebra.takeAttendance();

        //create two more new Courses
        Course history;
        Course spanish;

        //ask the Course class how many have been created (static)
        cout << "\n----- Asking the Course class how many courses have been created: \n";
        cout << Course::howMany() << endl;


        cin.get();
        return 0;

    }//end main()

代码转储的歉意。我试图减少线路，但遇到了更多错误，因此为了避免混淆，我保留了很多错误。问题（我相信）在Course.h和Course.cpp文件中。所制作课程的输出应为3。