std::chrono::year_month_weekday::year, std::chrono::year_month_weekday::month, std::chrono::year_month_weekday::weekday, std::chrono::year_month_weekday::index, std::chrono::year_month_weekday::weekday_indexed
来自cppreference.com
                    
                                        
                    < cpp | chrono | year month weekday
                    
                                                            
                    |   constexpr std::chrono::year year() const noexcept;  | 
(1) | (C++20 起) | 
|   constexpr std::chrono::month month() const noexcept;  | 
(2) | (C++20 起) | 
|   constexpr std::chrono::weekday weekday() const noexcept;  | 
(3) | (C++20 起) | 
|   constexpr unsigned index() const noexcept;  | 
(4) | (C++20 起) | 
|   constexpr std::chrono::weekday_indexed weekday_indexed() const noexcept;  | 
(5) | (C++20 起) | 
返回存储于此 year_month_weekday 对象中的域的值。
返回值
1) 返回存储的 std::chrono::year 值。
2) 返回存储的 std::chrono::month 值。
3) 返回存储的 std::chrono::weekday 值。
4) 返回存储的星期之日下标。
5) weekday()[index()] 。