假设:string s = "0123456789";
string sub1 =s.substr(5);//只有一个数字5表示从下标为5开始一直到结尾:sub1 = "56789"
string sub2 =s.substr(5, 3);//从下标为5开始截取长度为3位:sub2 = "567"
假设:string s = "0123456789";
string sub1 =s.substr(5);//只有一个数字5表示从下标为5开始一直到结尾:sub1 = "56789"
string sub2 =s.substr(5, 3);//从下标为5开始截取长度为3位:sub2 = "567"