本文通过一个C++示例程序展示了如何在一个类中声明另一个类为友元类，并允许特定成员函数访问其私有成员。同时，还展示了一个外部类的成员函数作为友元的情况。
/*
5. Create three classes. The first class contains private data,
and grants friendship to the entire second class and to a
member function of the third class. In main( ),
demonstrate that all of these work correctly.
*/
#include<iostream>
using namespace std;
//先声明A，确保C::cfun(A*)可以被正确声明
class A;
//C的定义必须在A的前面，否则就无法friend void C::cfun(A*)
class C {
public:
void cfun(A*);
};
class A {
int a;
public:
friend class B;
friend void C::cfun(A*);
};
class B {
int b;
};
//C::cfun(A*)的定义
void C::cfun (A* t) {
cout << t << endl;
}
5. Create three classes. The first class contains private data,
and grants friendship to the entire second class and to a
member function of the third class. In main( ),
demonstrate that all of these work correctly.
*/
#include<iostream>
using namespace std;
//先声明A，确保C::cfun(A*)可以被正确声明
class A;
//C的定义必须在A的前面，否则就无法friend void C::cfun(A*)
class C {
public:
void cfun(A*);
};
class A {
int a;
public:
friend class B;
friend void C::cfun(A*);
};
class B {
int b;
};
//C::cfun(A*)的定义
void C::cfun (A* t) {
cout << t << endl;
}
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