c++ 操作符重载 io操作符重载
1.io操作符的重载函数必须为非成员函数，否则左操作数最能是该类类型的对象:
class Shop_item { friend std::istream& operator>>(std::istream&, Shop_item&); friend std::ostream& operator<<(std::ostream&, Shop_item&);public: Shop_item& operator+=(const Shop_item&);}