https://www.cnblogs.com/duwenxing/p/7476469.html
•总结
| 父类中的访问属性 | 继承方式 | 子类中的访问属性 |
| private | public/protected/private | 不允许访问 |
| public | public | public |
| public | protected | protected |
| public | private | private |
| protected | public | protected |
| protected | protected | protected |
| protected | private | private |