每个表类需要将对数据库连接的引用存储在它的_ connection成员中。

Each of your table classes needs to store a reference to a database connection in its _connection member.

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定义表的类还有一组定义表字段的成员。

A class that defines a table also has a set of members that define the table's fields.

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有一个为每个类的虚表，其中包含每个成员的功能的实现的指针列表。

There's a virtual table for each class, which contains a list of Pointers to the implementation of each member function.

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