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。

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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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