whats the exact rule in function declaration in c++

I want to know is it mandatory to have constant arguments in the functions. Recently I had in interview and wrote a code like below

int numofsubstring(string s1, string s2)
{
   int nCount =0;
   size_t pos;
   while((pos = s1.find(s2)) != string::npos)
   {
     nCount++;
     s1 = s1.substr(s1.find(s2)+s2.length());
   }
    return nCount;
}  //number of times s2 is present in s1

The panel was expecting const string&s1 ,const string&s2 but i didn't wrote bcz of substr functionality usage...

can any one guide what's the standardised format to write the code.... the panel was insisting to improve the arguments in the function.