Compile-time linked list and key-value store

test11.cpp

#include <iostream>
#include <type_traits>
using namespace std;

struct Nil {
  typedef Nil Head;
  typedef Nil Tail;
};

template <typename H, typename T = Nil> struct Lst {
  typedef H Head;
  typedef T Tail;
  static const int Length = 1 + T::Length;
};

template <typename H> struct Lst <H, Nil> {
  typedef H Head;
  typedef Nil Tail;
  static const int Length = 1;
};

template <typename L, int I> struct LstIndex {
  typedef typename LstIndex<typename L::Tail, I-1>::Value Value;
};

template <typename L> struct LstIndex <L, 0> {
  typedef typename L::Head Value;
};

template <int N> struct Int {
  static const int value = N;
};

template <int K, int V> struct IntPair {
  static const int key = K;
  static const int value = V;
};

template <int QK, typename KVs> struct KeySearch :
  conditional<KVs::Head::key == QK, typename KVs::Head, KeySearch<QK, typename KVs::Tail>>::type {};

template <int QK> struct KeySearch <QK, Nil> : Nil {};

typedef Lst< IntPair<4, 8>,
        Lst< IntPair<6, 12>,
        Lst< IntPair<1, 2>
        >>> Foo;

int main(int argc, char** argv) {
  cout << Foo::Length << ' '
       << Foo::Tail::Length << ' '
       << Foo::Tail::Tail::Length
       << endl;

  cout << Foo::Head::key << ' '
       << Foo::Tail::Head::key << ' '
       << Foo::Tail::Tail::Head::key
       << endl;

  cout << Foo::Head::value << ' '
       << Foo::Tail::Head::value << ' '
       << Foo::Tail::Tail::Head::value
       << endl;

  cout << LstIndex<Foo, 0>::Value::value << ' '
       << LstIndex<Foo, 1>::Value::value << ' '
       << LstIndex<Foo, 2>::Value::value
       << endl;

  cout << KeySearch<1, Foo>::value << ' '
       << KeySearch<4, Foo>::value << ' '
       << KeySearch<6, Foo>::value
       << endl;

  return 0;
}