Cannot convert ‘boost::multiprecision::cpp_int

Giving this error on compilation:- no matching function for call to ‘to_string(boost::multiprecision::cpp_int&)’ string s = to_string(i);

#include <boost/lexical_cast.hpp>
#include <boost/multiprecision/cpp_int.hpp>
namespace mp = boost::multiprecision;
using namespace std;

#define int long long int

int main()
{
   mp::cpp_int l,i;
   for(i=l;i<r;i++)
    {
        string s = to_string(i); 
     }

return 0;
}