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。
A function that is virtual in a base class is implicitly virtual in its derived classes. When a derived class overrides a virtual, the parameters in the base and derived classes must match exactly.
子对象多重继承自多个基类，每个基类都有虚函数，那么子类对象的内存布局是什么样的？
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