The basic concept of friend element in C + +

Friend Basic concept: 1, a general function is declared as a class of friend function 2, a Class A of some member functions declared as a Class B friend function 3, a Class A is declared a Class B friend class. The role of friends: You can access all member variables and member methods in class B, whether public,protected or private. 1, to declare a general function show as a friend function of a class

#include <iostream>using namespace std;class Test{  friend void show(const Test &t);public:  Test(int d = 0) : data(d){}private:  void fun()const{    cout << "fun:" << data << endl;  }  int data;};//友元，即可以访问私有成员变量，也可以访问私有方法                 void show(const Test &t){  cout << "friend of Test:" << t.data << endl;  t.fun();}int main(){  Test t(100);  show(t);  return 0;}

2, the fun member function of a Class A is declared as a friend function of class test, but the FUN1 function of Class A is not a friend function of class test, so the private member of test cannot be accessed in fun1.

#include <iostream>using namespace std;class Test;class A{ public:  A(int d = 0) : data(d){}  void fun(const Test &t);  void fun1(const Test &t); private:  int data;};class Test{  friend void A::fun(const Test &t);public:  Test(int d = 10) : data(d){}private:  int data;};void A::fun(const Test &t){  cout << t.data << endl;}void A::fun1(const Test &t){  //编译不能通过，因为fun1不是class Test的友元函数。                 //cout << t.data <<endl;                                         }int main(){  Test t(100);  A a;  a.fun(t);}

3, Class B is declared as a friend of class test, so all public member functions of Class B are friend functions of class test.

#include <iostream>using namespace std;class Test;class B{public:  void fun1(const Test &t);  void fun2(const Test &t);};class Test{  friend class B;public:  Test(int d = 0) : data(d){}private:  void pri()const{    cout << "pri" << endl;  }  int data;};void B::fun1(const Test &t){  cout << t.data << endl;}void B::fun2(const Test &t){  t.pri();}int main(){  Test t(10);  B b;  b.fun1(t);  b.fun2(t);}

The basic concept of friend element in C + +