友元(一)

#include <iostream>using namespace std;class B;class A{private:int x,y;public:A(int i,int j) {x=i;y=j;}int sum(B b);};class B{private:int z;public:B(int i=0) {z=i;}friend int A::sum(B b);};int A::sum(B b) {return x+y+b.z;}void main(){A a(2,3);B b(4);cout<<a.sum(b)<<endl;}