class hierarchy:interface&implementation

1) The logical conclusion of this line of thought is a system represented to users as a hierarchy of
abstract classes and implemented by a classical hierarchy. In other words:

• Use abstract classes to support interface inheritance (§3.2.3, §20.1).
• Use base classes with implementations of virtual functions to support implementation inheritance (§3.2.3, §20.1).

多重继承：

• 使用抽象基类提供接口表示层；
• 使用额外的基类及多态机制提供实现层；