类包含或者继承一个或多个纯虚函数，这样的类就是抽象基类。

A class containing (or inheriting) one or more pure virtual functions is an abstract base class.

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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。

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.

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