成员函数作为友元

　　成员函数作为友元,只能是一种顺序.因为我们必须明确指出该成员函数属于哪个类,只有类的声明而无定义,无法在友元声明中指出该成员函数所属的类.以window_mgr类的成员函数clear作为Screen类的友元为例.

必须先定义window_mgr类,而不能先定义Screen类(此时会对window_mgr作前向声明).

 1 #include <iostream>
 2 #include <string>
 3 
 4 using std::string;
 5 using std::cout;
 6 using std::endl;
 7 
 8 #if 0//正确的形式
 9 class window_mgr {
10 public:
11     void clear();
12 };
13 
14 class Screen {
15 friend void window_mgr::clear();
16 private:
17     string contents;
18 public:
19     Screen() = default;
20     Screen(char c) : contents(10, c) {}
21 };
22 
23 void window_mgr::clear()
24 {
25     Screen myScreen('b');
26     cout << "清理之前:" << endl;
27     cout << myScreen.contents << endl;
28     myScreen.contents = "";
29     cout << "清理之后:" << endl;
30     cout << myScreen.contents << endl;
31     cout << "================================" << endl;
32 }
33 
34 int main()
35 {
36     window_mgr w;
37     w.clear();
38     return 0;
39 }
40 
41 
42 #else if                //!!!测试 : 错误的方式.
43 class window_mgr;
44 //void window_mgr::clear();
45 class Screen {
46     friend void window_mgr::clear();//只有声明而无定义:无法访问window_mgr的成员函数clear.
47 private:
48     string contents;
49 public:
50     Screen() = default;
51     Screen(char c) : contents(10, c) {}
52 };
53 
54 class window_mgr {
55 public:
56     void clear();
57 };
58 
59 void window_mgr::clear()
60 {
61     Screen myScreen('b');
62     cout << "清理之前:" << endl;
63     cout << myScreen.contents << endl;
64     myScreen.contents = "";
65     cout << "清理之后:" << endl;
66     cout << myScreen.contents << endl;
67     cout << "================================" << endl;
68 }
69 
70 int main()
71 {
72     window_mgr w;
73     w.clear();
74     return 0;
75 }
76 #endif

　　启用42行后面的代码,编译器(VS2015)给出的错误提示如下:

　　从46行的错误提示可以看出,尽管只是用作友元声明,未定义的类(有前向声明)也无法访问其成员.

不是尽力而为,而是全力以赴.