https://analyticspace.wordpress.com/2016/07/page/3/
https://zhougy0717.github.io/2018/03/06/C++%E8%8F%B1%E5%BD%A2%E7%BB%A7%E6%89%BF/
https://www.oschina.net/translate/cpp-virtual-inheritance?cmp&p=2#