TypeList 之 Replace,把类型列表中的 T 类型换成 U 类型

#include <cstdlib>
#include <iostream>

using namespace std;
class NullType;
namespace TL
{
  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)>
  //-----------------------------------------------------------------
  //Length
  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};
  };
  //---------------------------------------------------------------
  //利用索引查找对象
  template<class T,int U>struct TypeAt;
  template<class head,class tail>
  struct TypeAt<TypeList<head,tail>,0>
  {
    typedef head Result;
  };
  template<class head,class tail,int i>
  struct TypeAt<TypeList<head,tail>,i>
  {
    typedef typename TypeAt<tail,i - 1>::Result Result;
  };
  //---------------------------------------------------------------
  //indexof
  template<class TList,class T>struct IndexOf;
  template<class T>
  struct IndexOf<NullType,T>
  {
    enum{value = -1};
  };
  template<class Tail,class T>
  struct IndexOf<TypeList<T,Tail>,T>
  {
    enum{value = 0};
  };
  template<class Head,class Tail,class T>
  struct IndexOf<TypeList<Head,Tail>,T>
  {
  private:
    enum{temp = IndexOf<Tail,T>::value};
  public:
    enum{value = temp == -1 ? -1 : 1 + temp};
  };
  //---------------------------------------------------------------
  //Append
  template<class TList,class T>struct Append;
  template<>
  struct Append<NullType,NullType>
  {
    typedef NullType Result;
  };
  template<class T>
  struct Append<NullType,T>
  {
    typedef TYPELIST_1(T) Result;
  };
  template<class head,class Tail>
  struct Append<NullType,TypeList<head,Tail> >
  {
    typedef TypeList<head,Tail> Result;
  };
  template<class head,class Tail,class T>
  struct Append<TypeList<head,Tail>,T>
  {
    typedef TypeList<head,typename Append<Tail,T>::Result> Result;
  };
  //---------------------------------------------------------------
  //Erase
  template<class TList,class T>struct Erase;
  template<class T>
  struct Erase<NullType,T>
  {
    typedef NullType Result;
  };
  template<class T,class tail>
  struct Erase<TypeList<T,tail>,T>
  {
    typedef tail Result;
  };
  template<class head,class tail,class T>
  struct Erase<TypeList<head,tail>,T>
  {
    typedef TypeList<head,typename Erase<tail,T>::Result> Result;
  };
  //---------------------------------------------------------------
  //NoDuplicate
  template<class TList>struct NoDuplicate;
  template<>struct NoDuplicate<NullType>
  {
    typedef NullType Result;
  };
  template<class head,class tail>
  struct NoDuplicate<TypeList<head,tail> >
  {
  private:
    typedef typename NoDuplicate<tail>::Result L1;
    typedef typename Erase<L1,head>::Result L2;
  public:
    typedef TypeList<head,L2> Result;
  };
  //---------------------------------------------------------------
  //TypeList 之 Replace,把类型列表中的 T 类型换成 U 类型
  template<class TList,class T,class U>struct Replace;
  template<class T,class U>
  struct Replace<NullType,T,U>
  {
    typedef NullType Result;
  };
  template<class T,class U,class tail>
  struct Replace<TypeList<T,tail>,T,U>
  {
    typedef TypeList<U,tail> Result;
  };
  template<class head,class tail,class T,class U>
  struct Replace<TypeList<head,tail>,T,U>
  {
    typedef TypeList<head,typename Replace<tail,T,U>::Result> Result;
  };
}
using namespace TL;
int main(int argc, char *argv[])
{
    typedef TYPELIST_4(int,float,char,double) Test1;
    Test1::head n = 1011;
    cout<<"替换类型之前是整数类型,其值 = "<<n<<endl;
    typedef Replace<Test1,int,unsigned char>::Result TestRes;
    TestRes::head cc[] = "现在变成 char 了";
    cout<<"替换类型之后是char类型,其值 = "<<cc<<endl;
    system("PAUSE");
    return EXIT_SUCCESS;
}