对这个动态类我想做的最后一个改进，是用成员变量访问字段，而不是用笨重的get _和set _操作符。
One final improvement I want to make on this dynamic class is to use member variables to access the fields, instead of the clunky get_ and set_ operators.
“matches” 操作符只能应用于协议或者字符串类型的协议字段。
The "matches" operator is only implemented for protocols and for protocol fields with a text string representation.
操作符表还声明索引方法在为一个索引字段选定该操作符表的时候要使用的支持过程。
The operator class also specifies the support procedures to be used by the index method when the operator class is selected for an index column.
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