将任意类型映射到一个唯一整数（C++模板实现TypeList）

template <class Prev, class This>
class TypeList
{
public:
enum
{
position = (Prev::position) + 1,
};
};

template <>
class TypeList<void, void>
{
public:
enum
{
position = 0,
};
};

#include <iostream>

int main()
{
typedef TypeList< void, void> base;  // base
typedef TypeList< base, double> t2;  // position is unique id for double
typedef TypeList< t2, char > t3; // position is unique id for char

std::cout << "T1 Posn: " << base::position << std::endl;
std::cout << "T2 Posn: " << t2::position << std::endl;
std::cout << "T3 Posn: " << t3::position << std::endl;
}