所有权设计：raw pointer、reference、unique_ptr、shared_ptr 怎么选 | C++
本章覆盖：owner vs observer；non-null borrow；nullable borrow；exclusive ownership；shared ownership；API parameter；return value；member field；factory；tree / graph；async callback；所有权越简单越好；shared ownership 必须有业务理由。
本章知识点
- owner vs observer
- non-null borrow
- nullable borrow
- exclusive ownership
- shared ownership
- API parameter
- return value
- member field
- factory
- tree / graph
- async callback
- 所有权越简单越好
- shared ownership 必须有业务理由