为什么我收到错误“参数包'F'必须位于模板参数列表的末尾”

Question

I have code that uses a variadic template and I'm trying to understand where to put the ellipsis. In the below code I put them, like the error says, at the end of the template parameter list. But I still get errors. What am I doing wrong?

template <typename T> struct S {

    void operator << (const T &) {}

};

template <template <typename, typename...> class ... F, typename T = int>
struct N : S<F<T>> ... {};

prog.cpp:10:82: error: parameter pack 'F' must be at the end of the template parameter list

Answer 1

You have another parameter, T , at the end of the list after F . As the error message says, the variadic pack must come at the end. Unfortunately, that makes it awkward to have both variadic and defaulted parameters in the same template.