单数怎么变复数啊？

//make_plural函数是将单词的单数变复数的，请问各位该怎么改这个程序啊？
#include <iostream>
#include <string>
using namespace std;
int main()
{
    string make_plural(string &);
    string str;
    cout<<make_plural("success")<<endl;
    cout<<make_plural("failure")<<endl;
    return 0;
}
string make_plural(string &str)
{
    string str1;
    str1=str+'s';
    return str1;
}