这有效地隐藏了基类中的虚函数。
This effectively hides the virtual function in the base class.
当你通过基类的指针来调用虚函数，技术也是一样的。
When you invoke calls through a pointer to the base class, the technique is the same.
当抽象类从基类继承虚方法时，抽象类可以使用抽象方法重写该虚方法。
When an abstract class inherits a virtual method from a base class, the abstract class can override the virtual method with an abstract method.
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