how do I fix this c++ typelist template compile error?

Question

(from reading chapter 3 of modern c++ design)

typelist.hpp:

class NullType {};

struct EmptyType {};


template <class T, class U>
struct Typelist
{
  typedef T Head;
  typedef U Tail;
};

#define TYPELIST_1(T1) Typelist<T1, NullType>
#define TYPELIST_2(T1, T2) Typelist<T1, TYPELIST_1(T2) >
#define TYPELIST_3(T1, T2, T3) Typelist<T1, TYPELIST_2(T2, T3) >
#define TYPELIST_4(T1, T2, T3, T4) Typelist<T1, TYPELIST_3(T2, T3, T4) >
#define TYPELIST_5(T1, T2, T3, T4, T5) Typelist<T1, TYPELIST_4(T2, T3, T4, T5) >
#define TYPELIST_6(T1, T2, T3, T4, T5, T6) Typelist<T1, TYPELIST_5(T2, T3, T4, T5, T6) >


namespace TL
{
  template <class TList> struct Length;
  template <> struct Length<NullType>
  {
    enum { value = 0 };
  };

  template <class T, class U>
    struct Length< Typelist<T, U> >
    {
      enum { value = 1 + Length<U>::value };
    };


  template <class Head, class Tail>
    struct TypeAt<Typelist<Head, Tail>, 0>
    {
      typedef Head Result;
    };

  template <class Head, class Tail, unsigned int i>
    struct TypeAt<Typelist<Head, Tail>, i>
    {
      typedef typename TypeAt<Tail, i-1>::Result Result;
    };

}

main.cpp

#include "typelist.hpp"

Typelist<int, double> foo;

int main() {
}

g++ main.cpp typelist.hpp:37: error: ‘TypeAt’ is not a template typelist.hpp:43: error: type/value mismatch at argument 2 in template parameter list for ‘template struct TL::TypeAt’ typelist.hpp:43: error: expected a type, got ‘i’

Why do I get this error? How do I fix this?

Answer 1

Typelist is designed to be used recursively: It expects its second templater parameter to be either another Typelist, or NullType (signaling the end of recursion).

To define foo, you should write:

TYPELIST_2(int, double) foo;

or, if you want to avoid macros:

Typelist<int, Typelist<double, NullType> > foo;

Answer 2

Looks like you're missing a forward declaration.

This is a partial specialization:

template <class Head, class Tail>
struct TypeAt<Typelist<Head, Tail>, 0>

But the compiler has no idea what it's a specialization of. Add this before it:

template <class List, unsigned Index>
struct TypeAt;

This let's the compiler know: "There is a class TypeAt which has two template parameters." So now when you specialize it, the compiler knows what class you're talking about.

---

Note, your usage of Typelist is incorrect. These algorithm's are sentinel-terminated. This means, like C-strings, they expect the data to be concluded with a special value. In our case, this is NullType.

So, take Éric's advice. (i.e. hint: if you found his answer helpful, up-vote it.)