友元关系是类之间的一种特殊关系，这种关系不仅允许友元类访问对方的public方法和属性，还允许友元访问对方的protected和private方法和属性。
实例1：友元关系访问保护量
1 #include <iostream>
2 #include <string>
3
4 using namespace std;
5 class Lovers//爱人关系,基类
6 {
7 public:
8 Lovers(string theName);//
9 void kiss(Lovers *lover);
10 void ask(Lovers *lover, string something);
11
12 protected:
13 string name;
14
15 friend class Others; //祸根，交友不慎。。。。 (友元关系),则Others可以访问name
16 };
17
18 class Boyfriend:public Lovers//Boyfriend子类，继承Lovers基类
19 {
20 public:
21 Boyfriend(string theName);
22 };
23 class Girlfriend:public Lovers//girlfriend子类，继承Lovers基类
24 {
25 public:
26 Girlfriend(string theName);
27 };
28
29 class Others//Others类 ,并没继承Lovers基类
30 {
31 public:
32 Others(string theName);
33 void kiss(Lovers *lover);
34
35 protected:
36 string name;
37 };
38
39 Lovers::Lovers(string theName)//Lovers类的构造函数
40 {
41 name = theName;
42 }
43 void Lovers::kiss(Lovers *lover)//Lovers *lover表示定义Lovers类的一个对象*lover
44 {
45 cout << name << "亲亲我们家的" << lover->name << endl;
46 }
47 void Lovers::ask(Lovers *lover,string something)
48 {
49 cout << "宝贝儿" << lover->name << "帮我" << something << endl;
50 }
51 Boyfriend::Boyfriend(string theName):Lovers(theName)
52 {
53 }
54 Girlfriend::Girlfriend(string theName):Lovers(theName)
55 {
56 }
57 Others::Others(string theName)
58 {
59 name = theName;
60 }
61 void Others::kiss(Lovers *lover)
62 {
63 cout << name << "亲一下" << lover->name << endl;
64 }
65 int main()
66 {
67 Boyfriend boyfriend("A君");
68 Girlfriend girlfriend("B妞");
69
70 Others others("路人甲");
71
72 girlfriend.kiss(&boyfriend);
73 girlfriend.ask(&boyfriend,"洗衣服啦");
74
75 cout << "\n当当当当！传说中的路人甲登场啦。。。。\n";
76 others.kiss(&girlfriend);
77
78 return 0;
79 }
