会员功能朋友 [英] Member function a friend

问题描述

我一直在尝试一些例子（C ++ Primer by Stanley Lippman）
，我明白一个类可以让另一个类成为其朋友（访问一些私有成员）。现在我正在阅读一个成员函数是一个朋友，我尝试这个例子

I've been trying some examples in a book (C++ Primer by Stanley Lippman) and I understand that a class can make another class its friend (to access some private members). Now I'm reading about a member function being a friend and I try the example

class Screen
{
public:
    friend void Window_mgr::clear();

    typedef std::string::size_type pos;

    Screen () = default;
    Screen (pos ht, pos wd, char c) : height (ht), width (wd),
                                      contents (ht * wd, c) { }

private:
    void do_display (std::ostream &os) const
    {
        os << contents;
    }

    pos cursor = 0;
    pos height = 0, width = 0;
    pos test_num = 100, test_num2 = 222;;
    std::string contents = "contents";
   };

  class Window_mgr {
 public:
     using ScreenIndex = std::vector<Screen>::size_type;
     void clear (ScreenIndex);

 private:
     std::vector <Screen> screens {Screen (24, 80, ' ')};
 };

 void Window_mgr::clear(ScreenIndex i)
{
Screen &s = screens[i];
s.contents = std::string(s.height * s.width, ' ');
}

但会产生编译器错误，说

but it produces a compiler error saying

Window_mgr尚未声明

Window_mgr has not been declared

，然后我阅读：

•首先定义Window_mgr类，它声明但不能定义，清除。屏幕必须在声明之前清除才能使用Screen的成员。

• First, define the Window_mgr class, which declares, but cannot define, clear. Screen must be declared before clear can use the members of Screen.

•接下来，定义Screen屏幕，包括一个明确的朋友声明。

• Next, define class Screen, including a friend declaration for clear.

•最后，定义清除，现在可以在屏幕中引用成员。

• Finally, define clear, which can now refer to the members in Screen.

我不明白这部分 - 有人可以解释吗？

I don't understand this part -- can someone explain?

推荐答案

当编译器获得 friend void Window_mgr :: clear（）; 它不知道 Window_mgr 是什么，因为它还没有看到。你需要重新排序一些东西让它工作。首先你转发声明屏幕，然后你有 Window_mgr

When the compiler gets to friend void Window_mgr::clear(); it has no idea what Window_mgr is as it has not seen that yet. You need to reorder things around a little to get this to work. first you forward declare Screen and then you have you Window_mgr

class Screen;

class Window_mgr {
public:
    using ScreenIndex = std::vector<Screen>::size_type;
    void clear(ScreenIndex);
    Window_mgr();

private:
    std::vector <Screen> screens;  // don't initialize here as we don't know what a screen actually is yet
    //std::vector <Screen> screens {Screen (24, 80, ' ')}; can't do this as we don't what a Screen is here
};

然后你可以让你 Screen class

Then you can have you Screen class

class Screen
{
public:
    friend void Window_mgr::clear(ScreenIndex);

    typedef std::string::size_type pos;

    Screen() = default;
    Screen(pos ht, pos wd, char c) : height(ht), width(wd),
        contents(ht * wd, c) { }

private:
    void do_display(std::ostream &os) const
    {
        os << contents;
    }

    pos cursor = 0;
    pos height = 0, width = 0;
    pos test_num = 100, test_num2 = 222;
    std::string contents = "contents";
};

然后你可以拥有 Window_mgr 使用屏幕

Window_mgr::Window_mgr() : screens{ Screen(24, 80, ' ') } {}

void Window_mgr::clear(ScreenIndex i)
{
    Screen &s = screens[i];
    s.contents = std::string(s.height * s.width, ' ');
}

你可以看到它在这 live example

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