If a class that uses an abstraction has to downcast to a specific subclass in one section, you're not following the Open Closed Principle.

如果在某一部分中，一个抽象了的类必须向下转换为特定的子类，那么你就没有遵循开闭原则。

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At the top of the VFS is a common API abstraction of functions such as open, close, read, and write.

在VFS上面，是对诸如open、close、read和write之类的函数的一个通用API抽象。

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TCP further provides the port abstraction, which enables a host to open multiple TCP connections in parallel.

TCP 进一步提供了端口抽取功能，这使得主机可以并行地打开多个 TCP 连接。

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The scheme will see United Utilities decommission a number of water intakes along the two rivers, refurbish those that remain and open two new abstraction plants.

BBC: River Brennand