Incomplete type is not allowed(here is my code). Please help me :      #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()
    {`enter code here`
    	
    	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;
    }