Singleton template / private constructor for Singleton instances

Referencing the source below, I'm trying to prevent the case where the end user can create an instance of foo without utilzing Singleton::Instance.  IOW:  I"m trying to prevent this:
  foo f;
To do this I thought about making the constructor of foo protected but that's problematic for the singleton class.  I'm not sure how to achieve the objective without making foo a friend of Singleton which is not acceptable.  How can I achieve my objective?

# include <iostream>
# include <numeric>

template <typename T>
class Singleton
{
  //friend class foo; //not acceptable
protected:
   Singleton() {}
   virtual ~Singleton() {}
public:
   static T& Instance() 
   {
     static T obj ; 
     return ( obj ) ;
   }
private:
   Singleton(const Singleton &s);
   Singleton &operator = (const Singleton &s);
};

class foo : public Singleton < foo > 
{
  int x ; 
protected:
  //foo() {}
public :
  void SetX ( int const in ) 
  { x = in ; }
  int GetX () const { return ( x ) ; }
};

int main() 
{
  foo f; 
  f.SetX ( 5 ) ;
  foo& ff = foo::Instance();
  std::cout << ff.GetX() << std::endl;
  std::cin.get();
}