引用变量的函数调用问题

#include<iostream>
#include<string>
using namespace std;
string version1(const string &s1, const string &s2);
const string &version2(string &s1, const string &s2);
const string &version3(string &s1, const string &s2);

int main()
{
    string input;
    string copy;
    string result;

    cout << "输入字符串: ";
    getline(cin, input);
    copy = input;
    cout << "输入的字符串: " << input << endl;
    result = version1(input, "***");
    cout << "你的弦增强了: " << result << endl;
    cout << "你原来的弦: " << input << endl;

    result = version2(input, "###");
    cout << "你的弦增强了: " << result << endl;
    cout << "你原来的弦: " << input << endl;

    cout << "重置原始字符串.\n";
    input = copy;
    version3(input, "@@@");
    cout << "你的弦增强了: " << result << endl;
    cout << "你原来的弦: " << input << endl;


    system("pause");
    return 0;
}


string version1(const string &s1, const string s2)
{
    string temp;
    temp = s2 + s1 + s2;
    return temp;
}

const string &version2(string &s1, const string &s2)
{
    s1 = s2 + s1 + s2;
    return s1;
}

const string &version3(string &s1, const string &s2)
{
    string temp;
    temp = s2 + s1 + s2;
    return s1;
}




错误提示:


    3    IntelliSense:  有多个 重载函数 "version1" 实例与参数列表匹配:
            函数 "version1(const std::string &s1, const std::string &s2)"
            函数 "version1(const std::string &s1, std::string s2)"
            参数类型为:  (std::string, const char [4])    f:\2013_C++\测试\测试\程序清单8.7.cpp    18    11    测试