how to replace a string

#include<iostream>
#include<string>
using namespace std;

void replaceword(string str, string s1,string s2)
{
  size_t n=0;
  while((n=str.find(s1,n))!=string::npos)
  {
    str.replace(n,s1.size(),s2);
    n+=s2.size();
  }
  cout<<str<<endl;
}

int main()
{
  string str,s1,s2;
  getline(cin,str);
  getline(cin,s1);
  getline(cin,s2);
  replaceword(str,s1,s2);
  return 0;
}