c++中struct和class的区别 参考 https://stackoverflow.com/questions/92859/what-are-the-differences-between-struct-and-class-in-c https://www.fluentcpp.com/2017/06/13/the-real-difference-between-struct-class/ https://blogs.mentor.com/colinwalls/blog/2014/06/02/struct-vs-class-in-c/