In a base class, abstract class is defined by declaring pure virtual function, a derived class must define all inherited pure virtual functions of its base classes to be concrete.
在基类中可以通过将虚函数声明为纯虚函数的手段来定义抽象类，强制派生类在自己的类定义中实现虚函数的具体功能。
Classes derived from the abstract class must implement the pure virtual member function or they, too, are abstract classes.
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.
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。
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