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).

#include <iostream>
#include <string>

using std::string;
using std::cout;
using std::endl;

#if 0//正确的形式
class window_mgr {
public:
    void clear();
};

class Screen {
friend void window_mgr::clear();
private:
    string contents;
public:
    Screen() = default;
    Screen(char c) : contents(10, c) {}
};

void window_mgr::clear()
{
    Screen myScreen('b');
    cout << "清理之前:" << endl;
    cout << myScreen.contents << endl;
    myScreen.contents = "";
    cout << "清理之后:" << endl;
    cout << myScreen.contents << endl;
    cout << "================================" << endl;
}

int main()
{
    window_mgr w;
    w.clear();
    return 0;
}


#else if                //!!!测试 : 错误的方式.
class window_mgr;
//void window_mgr::clear();
class Screen {
    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.
private:
    string contents;
public:
    Screen() = default;
    Screen(char c) : contents(10, c) {}
};

class window_mgr {
public:
    void clear();
};

void window_mgr::clear()
{
    Screen myScreen('b');
    cout << "清理之前:" << endl;
    cout << myScreen.contents << endl;
    myScreen.contents = "";
    cout << "清理之后:" << endl;
    cout << myScreen.contents << endl;
    cout << "================================" << endl;
}

int main()
{
    window_mgr w;
    w.clear();
    return 0;
}
#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.