Peano numbers with constexpr metaprogramming (yet another implementation)

peano.cpp

template<typename T>
struct S { 
  typedef T Previous;
  enum { ToInt = T::ToInt + 1 };
};

struct Z { enum { ToInt = 0 }; };

template<typename T>
struct ToType {
  typedef T TypeValue;
};

template<>
struct ToType<Z> {
  typedef Z TypeValue;
};

template<typename T>
struct ToType<S<T>> {
  typedef S<typename ToType<T>::TypeValue> TypeValue;
};

#define P(n) n::Previous
#define TO_INT(n) n::ToInt

int k = TO_INT(P( S < S < Z > > ));
int main(int argc, char** argv) {
  int i = k;
}