Help add commas to int on console output

I am passing a whole number integer to this function, it converts the
integer to a string, then I want to add commas so a number like
1234567 shows as 1,234,567.  I am a little stuck, I belive the loop is
reading the string from right to left which led me to try decrementing
the loop, but a number like 12345 shows up like 123,45.  Or is there a
better way all together to achive ths.

string toStr(int &i)
{
    std::string s;
    std::stringstream out;
    out << i;
    s = out.str();
    for(size_t x = s.size(); x > 0; x--){
        if(x != s.size() && x%3 == 0)
             s.insert(x,",");
    }
     return s;
}