《Effective C++》——条款20：宁以pass-by-reference-to-const替换pass-by-value

class Window
{
public:
    ...
    std::string name()const;     //返回窗口名称
    virtual void display()const; //显示窗口和其内容
};
class WindowWithScrollBars:public Window
{
public:
    ...
    virtual void display()const;
};

void printNameAndDisplay(Window w)//不正确，参数可能被切割
{
    std::cout << w.name();
    w.display();
}
WindowWithScrollBars wwsb;
printNameAndDisplay(wwsb);//将调用Window::display();

void printNameAndDisplay(const Window& w)
{
    std::cout << w.name();
    w.display();
}
WindowWithScrollBars wwsb;
printNameAndDisplay(wwsb);//将调用WindowWithScrollBars::display();