Question

I was implenting a TypeList based on Loki, based on reading from:

http://aszt.inf.elte.hu/~gsd/halado_cpp/ch06s09.html

and saw this code from the site for IndexOf (to find the index of a type in the typelist):

template <class T>
struct IndexOf< NullType, T>
{
    enum { value = -1 };
};

template <class T, class Tail>
struct IndexOf< Typelist<Head, Tail>, T>
{
private:
    enum { temp = IndexOf<Tail, T>::value };
public:
    enum { value = (temp == -1) ? -1 : 1+temp };
};

It doesn’t seem like that would work because nowhere do I see something comparing T as the list is recursively traversed. In my implementation it looks like this:

template<typename Tlist, typename T>
struct IndexOf
{
private:
  static const int temp = IndexOf<typename Tlist::Tail, T>::value;
public:
  static const int value = (temp == -1) ? -1 : 1 + temp;
};

template<typename T>
struct IndexOf<NullType, T>
{
  static const int value = -1;
};

and, in fact, always returns -1. If I think of it, imagine one has TypeList; then Tail will be NullType, so temp will be -1 by the specialization and then value will be -1..even if Head was char and I would have expected zero. What am I missing here?

Thanks

---

My implementation of Typelist is merely:

template<typename H, typename T>
struct Typelist
{
  typedef H Head;
  typedef T Tail;
};

I’m guessing this isn’t Lokis, but With Joel’s answer I got this working for me:

template<typename Head, typename Tail, typename T>
struct IndexOfImpl
{
private:
  static const int temp = IndexOfImpl<typename Tail::Head, typename Tail::Tail, T>::value;
public:
  static const int value = (temp == -1) ? -1 : temp + 1;
};

template<typename T, typename Tail>
struct IndexOfImpl<T, Tail, T>
{
  static const int value = 0;
};

template<typename T>
struct IndexOfImpl<T, NullType, T>
{
  static const int value = 0;
};

template<typename Head, typename T>
struct IndexOfImpl<Head, NullType, T>
{
  static const int value = -1;
};

template<typename Tlist, typename T>
struct IndexOf
{
 public:
  static const int value = IndexOfImpl<typename Tlist::Head, typename Tlist::Tail, T>::value;
};

Answer 1

Should be :

template <class T>
struct IndexOf< NullType, T>
{
    enum { value = -1 };
};

template <class T, class Head, class Tail>
struct IndexOf< Typelist<Head, Tail>, T>
{
private:
    enum { temp = IndexOf<Tail, T>::value };
public:
    enum { value = (temp == -1) ? -1 : 1+temp };
};

template <class T, class Tail>
struct IndexOf< Typelist<T, Tail>, T>
{
public:
    enum { value = 0 };
};

You try to find T at some Head in the recursive encoding of the list.