说明：  字符串类的设计与实现，包括使用堆分配存储表示实现字符串的存储，串赋值，串比较，求串长，串连接，求子串，清空子串操作
(Design and implementation of the string class, including the use of heap-allocated memory representation for storage string, string assignment, string comparison, seeking string length, string concatenation, Praying string, empty substring operation)