If there is no use of the virtual key word, those sub instances of the derived class instance are basically perfectly normal instances of a base class.
如果没有使用的virtual关键字，这些子派生类实例的实例基本上是很正常的一个基类的实例。
In the processing of information retrieval we base our work on word frequency statistics and analysis of key sentences, and retrieve information from these to fill event frames using rules.
信息抽取阶段对原文进行词频统计，关键词句分析，并在此基础上利用规则抽取相应信息填充摘要框架。
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