1.evpp
https://blog.csdn.net/zieckey/article/details/70245013
2.enable_shared_from_this
shared_ptr接收裸指针，更改引用计数。
shared_ptr接收shared_ptr，更改引用计数。
在类的内部获取shared_ptr需要shared_from_this。
https://www.zhihu.com/question/30957800
https://baike.baidu.com/item/shared_ptr/2708164?fr=aladdin
https://blog.csdn.net/caoshangpa/article/details/79392878