如何按数字设置类型？

Question

我需要这样的东西

template<unsigned N>
class Class
{
    std::function<double(double, double ...)> _function1; // double N times
    std::function<double(double, ...)> _function2; // double N - 1 times
}

Answer 1

也许是：

template <typename R, typename T, size_t N>
struct MakeFunctionType {
  template <size_t>
  using SwallowIndex = T;

  template <size_t... Is>
  static std::function<R(SwallowIndex<Is>...)>
      MakeFunctionTypeHelper(std::index_sequence<Is...>);

  using type = decltype(MakeFunctionTypeHelper(
      std::make_index_sequence<N>{}));
};

demo