Plain old data structures are appropriate when there is a part of a system where it should be clearly indicated that the detailed logic for data manipulation and integrity are elsewhere. PODs are often found at the boundaries of a system, where information is being moved to and from other systems or persistent storage and the problem domain logic that is found in other parts of the system is not relevant. For example, PODs would be convenient for representing the field values of objects that are being constructed from external data, in a part of the system where the semantic checks and interpretations needed for valid objects have not yet been applied.
參考資料：
http://en.wikipedia.org/wiki/Plain_Old_Data_Structures
http://www.cplusplus.com/forum/beginner/5980/
http://blog.wanwan722.com/2008/10/podplain-old-data-and-nonpod.html
http://www.informit.com/guides/content.aspx?g=cplusplus&seqNum=32
http://bbs.csdn.net/topics/290079341
http://www.cppfans.org/1431.html