c++ - 为什么这个函数只被调用一次

Question

我正在阅读 The C++ Programming Language 3rd edition .在第 15 章(第 398 页)中，它给出了一个小例子。

class Window {
public:
    virtual void draw() { //sth }
};
class Window_with_border : public virtual Window {
public:
    void own_draw() { //sth }
    void draw() {Window::draw(); own_draw();}
};
class Window_with_menu : public virtual Window {
public:
    void own_draw() { //sth }
    void draw() {Window::draw(); own_draw(); }
};
class Clock : public Window_with_border, public Window_with_menu {
public:
    void own_draw() { //sth }
    void draw() {Window::draw(); Window_with_border::draw(); Window_with_menu::draw(); own_draw(); }
}; 

然后作者说draw()现在可以使用 own_draw() 编写函数功能，以便任何 draw() 的调用者得到 Window::draw()恰好调用一次。

但我认为，当您定义一个 Clock 对象并调用 draw() 时，它会被调用 3 次。像这样

Clock obj;
obj.draw() //The Window::draw() function got invoked three times?

我说的对吗？

更新:抱歉，我看错了代码。正确的代码是这样的:

class Clock : public Window_with_border, public Window_with_menu {
public:
    void own_draw() { //sth }
    void draw() {Window::draw(); Window_with_border::own_draw(); Window_with_menu::own_draw(); own_draw(); }
}; 

所以我错了

Answer 1

更长的解释:这正是作者所说的 - Window::draw() 被调用了三次:

1. 窗口::绘制();
2. Window_with_border::draw();
3. Window_with_menu::draw();

你可以省略第一个并像这样重写它:

class Clock : public Window_with_border, public Window_with_menu {
public:
    void own_draw() { //sth }
    void draw() {
        Window_with_border::draw();
        Window_with_menu::draw();
        own_draw(); }
};

但还是两次，所以最后应该是:

class Clock : public Window_with_border, public Window_with_menu {
public:
    void own_draw() { //sth }
    void draw() {
        Window::draw();
        Window_with_border::own_draw();
        Window_with_menu::own_draw();
        own_draw(); }
};

或者:

class Clock : public Window_with_border, public Window_with_menu {
public:
    void own_draw() { //sth }
    void draw() {
        Window_with_border::draw();
        Window_with_menu::own_draw();
        own_draw(); }
};