友元函数

#include<iostream.h>
class B
{private:
int mm,nn;
public:
 B(int i,int j);
 friend class A;//声明友元类
};
B::B(int i,int j)
{mm=i;
nn=j;
}
class A
{private:
 int ax,ay;
public:
 A(int i,int j);
 friend int sum(A);//声明友元函数
 int sumB(B b);
};
A::A(int i,int j)
{ax=i;
ay=j;
}
int sum(A a)
{return (a.ax+a.ay);//访问类对象的私有成员ax和ay
}
int A::sumB(B b)
{return (b.mm+b.nn);//访问类对象b的私有成员mm和 nn.
}
int main()
{B b(4,5);
A a(5,10);
cout<<sum(a)<<endl;
cout<<a.sumB(b)<<endl;
return 0;
}