[C++模板技术] TypeList(2)

1、随机访问

template <typename TList,unsigned int index>
struct TypeAt;

template <typename Head,typename Tail>
struct TypeAt<TypeList<Head,Tail>,0>
{
using Result = Head;
};

template <typename Head,typename Tail,unsigned int index>
struct TypeAt<TypeList<Head, Tail>, index>
{
using Result = typename TypeAt<Tail, index - 1>::Result;
};

2、类别索引查找

template <typename TList,typename T>
struct IndexOf;

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

template<typename T, typename Tail>
struct IndexOf<TypeList<T, Tail>, T>
{
enum
{
value = 0
};
};

template <typename Head,typename Tail,typename T>
struct IndexOf<TypeList<Head, Tail>, T>
{
private:
enum
{
temp = IndexOf<Tail, T>::value
};
public:
enum
{
value = temp == -1 ? -1 : temp + 1
};
};

3、附加元素

template<typename TList,typename T>
struct Append;

template<>
struct Append<NullType, NullType>
{
using Result = NullType;
};

template<typename T>
struct Append<NullType, T>
{
using Result = TypeList<T, NullType>;
};

template<typename Head,typename Tail>
struct Append<NullType, TypeList<Head, Tail>>
{
using Result = TypeList<Head, Tail>;
};

template<typename Head,typename Tail,typename T>
struct Append<TypeList<Head, Tail>, T>
{
using Result = TypeList<Head, typename Append<Tail, T>::Result>;
};

4、删除特定类型

template<typename TList,typename T>
struct Erase;

template<typename T>
struct Erase<NullType, T>
{
using Result = NullType;
};

template<typename Tail,typename T>
struct Erase<TypeList<T, Tail>, T>
{
using Result = Tail;
};

template<typename Head,typename Tail,typename T>
struct Erase<TypeList<Head, Tail>, T>
{
using Result = TypeList<Head, typename Erase<Tail, T>::Result>;
};