Function Object 与 operator() | STL
本章覆盖：functor；class with `operator()`；state；reusable callable；template friendliness；inline opportunity；comparator object；hash object；historical STL design；lambda 本质上生成闭包对象的连接。
本章知识点
- functor
- class with
operator() - state
- reusable callable
- template friendliness
- inline opportunity
- comparator object
- hash object
- historical STL design
- lambda 本质上生成闭包对象的连接