template <class T>
struct Tmp
 {  T& that;
    Tmp(T& t):that(t){}
    template <class C> operator C();
 };
template <class I>
 Tmp<I> convert(I& i)
 { return Tmp<I>(i);
 }


template <>
 Tmp<int>::operator double ()
 {  return that*2;
 }

int main(int argc, char **argv)
{  int a=9;
   double d=convert(a);