Design Pattern --- Chain of Responsibility

class Window
{
protected:
    Window *m_successor;
    bool m_msg;

public:
    Window(Window *successor, bool msg) : m_successor(successor), m_msg(msg) {}
public:
    // Interface.
    virtual void Msg()
    {
        if (m_msg)
        {
            showMsg();
        }
        else
        {
            if (m_successor != nullptr)
            {
                m_successor->Msg();
            }
        }
    }

private:
    // logic.
    virtual void showMsg()
    {
        cout <<"Window." <<endl;
    }
};
class Dialog : public Window
{
public:
    Dialog(Window *successor, bool msg) : Window(successor, msg) {}

private:
    // logic.
    virtual void showMsg() override
    {
        cout <<"Dialog." <<endl;
    }
};
class Button : public Window
{
public:
    Button(Window *successor, bool msg) : Window(successor, msg) {}

private:
    // logic.
    virtual void showMsg() override
    {
        cout <<"Button." <<endl;
    }
};

int main(int argc, char *argv[])
{
    Window app(nullptr, true);
    Dialog dlg(&app, true);            // false.
    Button btn(&dlg, false);

    btn.Msg();                        // 链式向上查找， 直到有一个能处理请求为止。非常适合消息处理机制。

    return 0;
}