c++一个问题，求解答

编写一个字符串加密类CEncryptInfo, 输入字符串和秘钥串加密(加密函数)，加密算法自选（可异或、变位、加减乘除），输出加密串，再用相同的秘钥串解密（解密函数），输出解密串，对外接口函数string encrypt(string strOri, string code); string decrypt(string strEncrypt, string code);

#include <iostream>
using namespace std;
enum Mode
{
    SUM,
    XOR
};
class CEncryptInfo
{
private:
    Mode mode;

public:
    CEncryptInfo(Mode m) : mode(m){};

    string encrypt(string strOri, string code)
    {
        string strAim = strOri;
        int max = code.length();
        int s = 0;
        if (mode == SUM)
        {
            for (int i = 0; i < strAim.length(); i++)
            {
                strAim[i] = strAim[i] + code[s++];
                if (s >= max)
                    s = 0;
            }
            return strAim;
        }
        return "todo";
    }
    string decrypt(string strEncrypt, string code)
    {
        string strAim = strEncrypt;
        int max = code.length();
        int s = 0;
        if (mode == SUM)
        {
            for (int i = 0; i < strAim.length(); i++)
            {
                strAim[i] = strAim[i] - code[s++];
                if (s >= max)
                    s = 0;
            }
            return strAim;
        }
        return "todo";
    }
};
int main(int argc, char const *argv[])
{
    CEncryptInfo *engine = new CEncryptInfo(SUM);
    string code = "test", key = "key";
    string encode = engine->encrypt(code, key);
    string decode = engine->decrypt(encode, key);
    cout << encode << endl
         << decode << endl;
}