classes containing operator() for std::sort, and virtual functions

In the following code example, I define several Comparator classes
which contain different compare functions to use with std::sort. I
have a Sorter class that gets passed a Comparator and is supposed to
sort using the specific Comparator that's passed to it.

But it always uses the Comparator base class and not the derived class
that it is supposed to use, although the functions are virtual.

How can I make the code work? You might argue that a Sorter class is
not needed, but I want one because I have more data and operations
related to sorting that I would like to put there.

#include <iostream>
#include <vector>

class Comparator {
public:
  virtual bool operator() (const int& i1, const int& i2) const {
    return i1 < i2;
  }
};

// would like to make it abstract base class, but that doesn't compile
class ReverseComparator : public Comparator {
public:
  virtual bool operator() (const int& i1, const int& i2) const {
    return i2 < i1;
  };
};

class Sorter {
public:
  Sorter(std::vector<int>& v, Comparator& c){
    sort(v.begin(), v.end(), c);
  }
};

int main(int argc, char** argv){
  std::vector<int> v;
  v.push_back(3);
  v.push_back(1);
  v.push_back(2);
  ReverseComparator c;
  Sorter s(v, c);
  for(std::vector<int>::const_iterator it = v.begin(); it != v.end(); +
+it){
    std::cout << *it << std::endl; // prints 1,2,3, instead of 3,2,1
  }
  return EXIT_SUCCESS;
}