The second step of implementing Fibonacci with C++ Metaprogramming, involving lazy-evaluation.

Cpp_Fibonacci_NonWorkingSkeleton_V2.cpp

#include <iostream>
using namespace std;

template <class First, class Second> struct Add;

template <int First, int Second>
struct Add<
	std::integral_constant<int, First>, 
	std::integral_constant<int, Second>>
{
	typedef int value_type;
	typedef std::integral_constant<value_type, (First + Second)> type;
	static const value_type value = type::value;
	value_type operator() () const { return value; }
};

template <class First, class Second> struct Subtract;

template <int First, int Second>
struct Subtract<
	std::integral_constant<int, First>,
	std::integral_constant<int, Second>>
{
	typedef int value_type;
	typedef std::integral_constant<value_type, (First - Second)> type;
	static const value_type value = type::value;
	value_type operator() () const { return value; }
};

template <class CValueOne, class CValueTwo> struct Equals; // Prototype

template <int First, int Second>
struct Equals<
	std::integral_constant<int, First>, 
	std::integral_constant<int, Second>>
	: std::false_type
{
	// Inherited from std::false_type
};

template <int First>
struct Equals<
	std::integral_constant<int, First>, 
	std::integral_constant<int, First>>
	: std::true_type
{
	// Inherited from std::true_type
};

template <class First, class Second> struct Less; // Prototype

template <int First, int Second>
struct Less<
	std::integral_constant<int, First>,
	std::integral_constant<int, Second>>
	: std::integral_constant<bool, (First < Second)>
{
	// Inherited from std::integral_constant<bool, constexpr>
};

template <bool Cond, class EvalIfTrue, class EvalIfFalse>
struct ConditionalEval;

template <class EvalIfTrue, class EvalIfFalse>
struct ConditionalEval<false, EvalIfTrue, EvalIfFalse>
{
	typedef typename EvalIfFalse::type type;
};

template <class EvalIfTrue, class EvalIfFalse>
struct ConditionalEval<true, EvalIfTrue, EvalIfFalse>
{
	typedef typename EvalIfTrue::type type;
};

template <class CIndex>
struct Fibonacci
{
	struct details
	{
		typedef std::integral_constant<int, 0> N0;
		typedef std::integral_constant<int, 1> N1;
		typedef std::integral_constant<int, 2> N2;
		typedef std::integral_constant<int, 42> Nuniverse;
	};
	typedef typename ConditionalEval<
		Less<typename CIndex::type, typename details::N2>::value,
		typename details::N1,
		/* typename details::Nuniverse */
		Fibonacci<Subtract<typename CIndex::type, typename details::N2>>
		>::type type;
};

int main() 
{
	typedef std::integral_constant<int, 0> N0;
	typedef std::integral_constant<int, 1> N1;
	typedef std::integral_constant<int, 2> N2;
	typedef std::integral_constant<int, 3> N3;
	typedef std::integral_constant<int, 4> N4;
	typedef std::integral_constant<int, 5> N5;
	std::cout << "N0 gives: " << Fibonacci<N0>::type::value << std::endl;
	std::cout << "N1 gives: " << Fibonacci<N1>::type::value << std::endl;
	std::cout << "N2 gives: " << Fibonacci<N2>::type::value << std::endl;
	std::cout << "N3 gives: " << Fibonacci<N3>::type::value << std::endl;
	std::cout << "N4 gives: " << Fibonacci<N4>::type::value << std::endl;
	std::cout << "N5 gives: " << Fibonacci<N4>::type::value << std::endl;
	return 0;
}