이 문제를 어떻게 해결해야 하나요 c++템플목록은 컴파일한 오류?

Question

(에서 독서 3 장의 현대적인 c++디자인)

목록.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<class Head, class Tail> struct TL::TypeAt’
typelist.hpp:43: error:   expected a type, got ‘i’

표시되는 이유는 무엇입니까이 오류가?이 문제를 어떻게 해결해야 하나요?

Answer 1

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.)

Answer 2

목록을 재귀적으로 사용:그것이 기대하는 두 번째 templater 매개 변수 중 하나가 될 또 다른목록,또는 NullType(신호의 끝을 재귀).

을 정의하 foo 이메일로 문의하시기 바랍니다:

TYPELIST_2(int, double) foo;

또는,당신이 원하는 경우를 방지하는 매크로:

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