C++友元机制详解-优快云博客

#include <iostream>
using namespace std;

class A;

class B
{
    friend A;        // 友元类

public:
    B( int b ) { this->b = b; }

public:
    void funB( A * p );

private:
    int b;
};

class A
{
public:
    A( int a ) { this->a = a; }

public:
    friend void fun( const A & a );        // 友元函数
    friend void B::funB( A * p );          // 将B中的成员函数声明为A的友元

    void Display( B * p );

private:
    int a;
};

void fun( const A & a )
{
    cout << a.a << endl;
}

void B::funB( A * p )
{
    cout << p->a << endl;
}

void A::Display( B * p )
{
    cout << a << endl;
    cout << p->b << endl;          // 类A中用到了类B中的私有变量，
                                   // 因为A是B的“朋友”
}

int main()
{
    A a(10);
    B b(20);
    a.Display( &b );
    return 0;
}