Marking the base class as "private" allowed the derived class to inherit from it without actually becoming one of them.
将基类标记为 “私有” 允许派生类从该基类继承而来，而无需实际成为 一个基类。
The customer class can be an abstract base class for a family of different customer types, such as private or business customer.
客户类可以是用于许多不同客户类型的抽象基类，比如说私有客户或业务客户。
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