在string :: clear函数的描述中,它说：
@H_301_3@clear: Erases the contents of the string,which becomes an empty string (with a length of 0 characters).@H_301_3@在list :: clear函数的描述中,它说：
@H_301_3@clear: Removes all elements from the list container (which are destroyed),and leaving the container with a size of 0.@H_301_3@清除是否覆盖字符串和列表的内存或只是释放它们？
