说明：  在linux文件系统下，实现虚函数，简单地说，那些被virtual关键字修饰的成员函数，就是虚函数。虚函数的作用，用专业术语来解释就是实现多态性（Polymorphism），多态性是将接口与实现进行分离；用形象的语言来解释就是实现以共同的方法，但因个体差异而采用不同的策略
(In linux file system, the virtual function implementation, easy to say that those who have been modified by the keyword virtual member function is virtual function. The role of virtual function, using professional terminology to explain is to achieve polymorphism (Polymorphism), polymorphism is the separation of interface and implementation with the image of the language to explain that in order to achieve a common methodology, but due to individual differences and different strategy)