c++ - 继承/多态——子类的调用方法

Question

在下面，我怎样才能使程序使用 MainMenuScreen 中的 draw 方法而不是 GameScreen 中的方法？

class GameScreen {
public:
    virtual void GameScreen::draw() {
        cout << "Drawing the wrong one..." << endl;
    }
};

class MainMenuScreen : public GameScreen {
public:
    void MainMenuScreen::draw() {
        cout << "Drawing the right one..." << endl;
    }
};

class ScreenManager {
public:
    list<GameScreen> screens;

    // assume a MainMenuScreen gets added to the list

    void ScreenManager::draw()
    {
        for ( list<GameScreen>::iterator screen = screens.begin(); 
              screen != screens.end(); screen++ )
        {
            screen->draw(); /* here it uses the draw method from GameScreen, 
                               but I want it to use the draw method from              
                               MainMenuScreen */
        }
    }
};

PS:我不想让 GameScreen::draw 纯虚拟，​​所以请提出其他建议。

Answer 1

how can I make it such that the program uses the draw method from MainMenuScreen instead of the one from GameScreen?

你不能，除非你在实际类型为 MainMenuScreen 的指针或引用上调用它。

 list<GameScreen> screens;

声明对象的列表，而不是指针或引用。如果向其中添加 MainMenuScreen 对象，由于对象切片，它们将丢失类型信息，并且多态性将不起作用。你需要:

 list<GameScreen*> screens;

或者，更好的是:

 list<shared_ptr<GameScreen> > screens;