模板元编程，使用可变模板：编译器错误(Template Meta-programming, with Variadic Templates: compiler error)

 我第一次尝试variadic模板元编程，并一直得到一个编译器错误，我一直无法追查。 
 
 我遵循这个页面上的“元组”示例（尽管我将对象称为ItemSet ） 
 
 ItemSet部分编译得很好： 
 
template<typename...Ts> class ItemSet { };

template<typename item_T, typename ...Ts>
class ItemSet<item_T, Ts...> : public ItemSet<Ts...>
{
public:
    ItemSet(item_T t, Ts... item_types) : ItemSet<Ts...>(item_types...), tail(t) { }

protected:
    item_T tail;
};





template <typename...M> struct type_holder;

template<typename T, typename ...M>
struct type_holder<0, ItemSet<T, M...>>
{                          // ERROR: M parameter pack expects a type template argument.
    typedef T type;
};

template <int k, typename T, typename ...M>
struct type_holder<k, ItemSet<T, M...>>
{
    typedef typename type_holder<k - 1, ItemSet<M...>>::type type;
};




int main()
{
    ItemSet<int, string, string, double> person(0, "Aaron", "Belenky", 29.492);
}
 
 但是，在注释掉的代码中，我在type_holder的声明处得到编译器错误。 我已经在相同的语法上尝试了一些变体，但始终存在相同的问题。 
 
 我正在使用Microsoft Visual Studio 2013，它应该完全支持模板编程和变量模板。 
 
 你知道编译器错误是什么，你能解释一下吗？ 

I'm attempting variadic template meta programming for the first time and consistently getting a compiler error that I have not been able to track down.
 
I'm following the "tuple" example on this page (although I'm calling my object an ItemSet)
 
The ItemSet part compiles just fine:
 
template<typename...Ts> class ItemSet { };

template<typename item_T, typename ...Ts>
class ItemSet<item_T, Ts...> : public ItemSet<Ts...>
{
public:
    ItemSet(item_T t, Ts... item_types) : ItemSet<Ts...>(item_types...), tail(t) { }

protected:
    item_T tail;
};





template <typename...M> struct type_holder;

template<typename T, typename ...M>
struct type_holder<0, ItemSet<T, M...>>
{                          // ERROR: M parameter pack expects a type template argument.
    typedef T type;
};

template <int k, typename T, typename ...M>
struct type_holder<k, ItemSet<T, M...>>
{
    typedef typename type_holder<k - 1, ItemSet<M...>>::type type;
};




int main()
{
    ItemSet<int, string, string, double> person(0, "Aaron", "Belenky", 29.492);
}
 
However, in the commented out code, I get the compiler error at the declaration of type_holder. I've tried a bunch of variations on the same syntax, but always with the same issue.
 
I'm using Microsoft Visual Studio 2013, which is supposed to have full support for Template programming and Variadic Templates.
 
Do you understand what the compiler error is, and can you explain it to me?

原文：https://stackoverflow.com/questions/27487470