[c++] 命名空间

参考：

• https://blog.csdn.net/fengbingchun/article/details/63685373
• http://c.biancheng.net/view/2193.html
• https://blog.csdn.net/weixin_43469680/article/details/88863207
• https://blog.csdn.net/dgyanyong/article/details/46517467
• https://www.zhihu.com/question/267494533?sort=created
• https://bbs.csdn.net/topics/60203039
• https://www.cnblogs.com/zhoug2020/p/5972439.html