stringstream clear

clear后内存仍然增长，并不清空内容内容释放空间。用stringstream的str方法清空。

https://blog.csdn.net/l631068264/article/details/25115917

https://blog.csdn.net/chenlei0630/article/details/39643887

clear是用来清除标志位的

https://www.cnblogs.com/elenno/p/stringstream_clear.html