C++ 友元

#include <iostream>
#include <string>

using namespace std;
//提前声明类
class student;

class grade
{
private :
int _grade;
public :
grade(int grade):_grade(grade){}

void display(student &);
};

//可以当做一个普通类
class student
{
public:
int _num;
string _name;
//构造函数和普通函数除了返回值得区别
student(int num,string name):_num(num),_name(name){}
//要调用另外一个类中函数  可以申明为友元函数  不过将类对象传递过去
friend void grade::display(student &);
};
//友元函数的定义
void grade::display(student &two)
{
cout<<"student  name  num  grade: "<<two._name<<"  "<<two._num<<"  "<<this->_grade<<endl;
};

int main(){
student stu(100,"buyingfei");
grade gra(100);
gra.display(stu);
return 0;
}

#include <iostream>
#include <string>

using namespace std;

//可以当做一个普通类
class student
{
private:
int _num;
string _name;
public:
friend class grade;  //声明为友元类  则在本类中可以调用声明友元类中成员
//构造函数和普通函数除了返回值得区别
student(int num,string name):_num(num),_name(name){}
//传递参数是另外类的引用  声明函数加 ;  不要{}
void display(grade &);
};

class grade
{
private :
int _grade;
public :
grade(int grade):_grade(grade){};
int get(){
return this->_grade;
}

};
//友元函数的定义
void student::display(grade &two)
{
cout<<"student  name  num grade: "<<this->_name<<"  "<<this->_num<< "   "<<two.get()<<endl;
};

int main(){
student stu(100,"buyingfei");
grade gra(100);
stu.display(gra);
return 0;
}