该类库将包含两个基类宣布为抽象的（它们不能被实例化） ，但我会利用这些基类，用于继承。
The library will contain two base classes declared as abstract (they can't be instantiated), but I will use them as base classes for building application-specific classes through inheritance.
基类中的私有成员被继承，但不能从派生类访问。
Private members in base classes are inherited but are not accessible from derived classes.
基类中的私有成员被继承，但不能从派生类访问。
Private members in base classes are inherited but are not accessible from derived classes.
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