c++ prime plus 有一道例题不懂 菜鸟请教

#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<<"Enter a string : ";
    getline(cin,input);
    copy=input;
    cout<<"Your string as entered: "<<input<<endl;
    result=version1(input,"***");
    cout<<"Your string enhanced: "<<result<<endl;
    cout<<"Your original string: "<<input<<endl;

    result=version2(input,"###");
    cout<<"Your string enhanced: "<<result<<endl;
    cout<<"Your original string: "<<input<<endl;
    cout<<"Resetting original string.\n";
    input =copy;
    result=version3(input,"@@@");
    cout<<"Your string enhanced: "<<result<<endl;
    cout<<"Your original string: "<<input<<endl;
    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 temp;
}