不完整的類型是不允許的錯誤

Question

Here is my code 

#include<iostream> 
#include<string> 
using namespace std; 
class GradeBook; // GradeBook class definition 
{ 
public: 
    // constructor initializes courseName 
    GradeBook(string name) 
    { 
     setCourseName(name); 
    } 
    void setCourseName(string name) 
    { 
     courseName = name; 
    } 
    string getCourseName() 
    { 
     return courseName; 
    } 
    void displayMessage() 
    { 
     cout << "Welcome to the grade book for\n" << getCourseName() << "!" << endl; 
    } 
private: 
    string courseName; 
}; 

int main() 
{ 

    GradeBook gradeBook1("CS101 Introduction to C++ Programming"); 
    GradeBook gradeBook2("Visual C++ introduction"); 

    cout << "gradeBook1 created for course: " << gradeBook1.getCourseName() 
     << "\ngradeBook2 created for course: " << gradeBook2.getCourseName() 
     << endl; 
    return 0; 
} 

Answer 1

刪除後class Gradebook和一切都應該罰款。