学步园

友元机制允许一个类将对其非共有成员的访问权授予指定的函数或类

#include <string>  
#include <iostream>  
using namespace std;    
 
 class B;
 
 class A{
     private:
         int x;
     public:
         A();
         void display(B &);
 };
 
 class C;
 
 class B{
     private:
         int y;
         int z;
     public:
         B();
         B(int, int);
     friend void A::display(B &);//友元成员函数
     friend void display(B &);//友元函数
     friend class C;//友元类
 };
 
 class C{
     private:
         int sum;
         void calc(B &);
     public:
         C();
         void display(B &);
 };
 
 //必须在友元关系的类后进行定义
 void display(B &v)//友元函数
 {
     cout << v.y << " " << v.z << endl;
 }
 
 A::A()
 {
     this->x = 0;
 }
 
 void A::display(B &v)//友元成员函数
 {
     this->x = v.y + v.z;
     cout << this->x << endl;
 }
 
 B::B()
 {
     this->y = 0;
     this->z = 0;
 }
 
 B::B(int y, int z)
 {
     this->y = y;
     this->z = z;
 }
 
 C::C()
 {
     sum = 0;
 }
 
 void C::display(B &v)
 {
     this->calc(v);
     cout << sum << " = " << v.y << " + " << v.z << endl;
 }
 
 void C::calc(B &v)
 {
     sum = v.y + v.z;
 }
 
 int main()
 {
     A a;
     B b(2, 3);
     display(b);
     a.display(b);
     C c;
     c.display(b);
     
     return 0;
 }

输出：

pateo@pateo-B86N53X:~/work/study$ g++ main.cc -o main
pateo@pateo-B86N53X:~/work/study$ ./main
2 3
5
5 = 2 + 3