由抽象类别衍生出来的类别必须实作纯虚拟成员函式，否则它们也是抽象类别。

Classes derived from the abstract class must implement the pure virtual member function or they, too, are abstract classes.

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由抽象类别衍生出来的类别必须实作纯虚拟成员函式，否则它们也是抽象类别。

Classes derived from the abstract class must implement the pure virtual member function or they, too, are abstract classes.

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