C++中使用無效不完全型的

Question

我有以下文件：main.cpp中，class.cpp，class.hpp，student.cpp，student.hpp

下面是代碼：

的main.cpp ：

#include <iostream> 
#include <vector> 
#include <string> 

#include "class.hpp" 


using namespace std; 


int main(){ 
    vector<Student> students; 
    vector<Class> classes; 

    Class B4(4, 'B'); 
    classes.push_back(B4); 

    Student john("John", "Boss", B4); 
    students.push_back(john); 
    Student marc("Marc", "Solo", B4); 
    students.push_back(marc); 

    marc.printInfo(); 
    B4.addStudent(marc); 

    return 0; 
} 

class.hpp：

#include <vector> 
#include "student.hpp" 

using namespace std; 

class Class{ 
private: 
    int number; 
    char letter; 
    vector<Student> students; 

public: 
    Class(int, char); 

    void addStudent(Student &); 
    void printClass(); 
}; 

class.cpp：

#include <iostream> 
#include "class.hpp" 


Class::Class(int number, char letter){ 
    this -> number = number; 
    this -> letter = letter; 
} 


void Class::printClass(){ 
    cout << this -> number << "." << this -> letter << endl; 
} 


void Class::addStudent(Student & student){ 
    this -> students . push_back(student); 
} 

student.hpp：

#include <string> 

using namespace std; 

class Class; 

class Student{ 
private:  
    string name; 
    string surname; 
    Class* classStudent; 

public: 
    Student(string name, string surname, Class &); 

    void printInfo(); 
}; 

student.cpp：

#include <iostream> 
#include "student.hpp" 

using namespace std; 

Student::Student(string name, string surname, Class & classStudent){ 
    this -> name   = name; 
    this -> surname   = surname; 
    this -> classStudent = &classStudent; 
} 

// printInfo 
void Student::printInfo(){ 
    cout << "Name: " << this -> name << "\nSurname: " << this -> surname << 
        "\nClass:" << this -> classStudent -> printClass() << endl; 
} 

當我編譯這些文件我得到以下錯誤：

student.cpp: In member function ‘void Student::printInfo()’: 
student.cpp:15:41: error: invalid use of incomplete type ‘class Class’ 
     "\nClass:" << this -> classStudent -> printClass() << endl; 
             ^
In file included from student.cpp:2:0: 
student.hpp:5:7: error: forward declaration of ‘class Class’ 
class Class; 
    ^

我從來沒有來像這樣的情況：我需要在第二節課中使用第一節課，在第一節節中使用第二節課。

Answer 1

後

#include <iostream> 
#include "student.hpp" 

添加

#include "class.hpp" 

在student.cpp，或編譯器將不知道什麼Class是。