C++难题(高手进来)

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