通常是由于持久化框架在编译期或者运行期修改类的定义，添加某些所需信息以减少持久化实体时的麻烦。

Usually the problem occurs because the persistence frameworks will alter classes during compilation or runtime, to include information they require, to ease the pain related to persisting entities.

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细节请见保存一个非常量值时的编译期陷阱。

See Compile time trap when saving a non-const value for details.

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