c – 将可变模板参数转换为其他类型

如何将类型从可变模板参数转换为另一种类型？

例如：

template <typename... T>
struct single
{
   std::tuple<T...> m_single;
};

template <typename... T>
struct sequences
{
   single<T...> get(size_t pos)
   {
       // I don't know how to convert here
       return std::make_tuple(std::get<0>(m_sequences)[pos]... std::get<N>(m_sequences)[pos]);
   }

   template <size_t Idx>
   std::vector<
      typename std::tuple_element<Idx,std::tuple<T...>>::type
      >
   get_sequence()
   {
      return std::get<Idx>(m_sequences);
   }

   std::tuple<T...> m_sequences; // std::tuple<std::vector<T...>> I don't know how to conver here
};

我想写：

sequences<int,double,double> seq;
single<int,double> sin = seq.get(10);

并且具有std :: tuple< std :: vector< int>,std :: vector< double>,std :: vector< double>>在结构序列中.并从中获得单身.

的std ::矢量<单<吨...>>对我来说是个坏主意,因为我需要一个完整的序列,并且很容易将它复制出来. 可能吗？ 非常感谢你.对不起,我的英语不好.

解决方法

您可以做的不仅仅是将可变参数包作为普通列表扩展：您也可以扩展表达式.因此,您可以将m_sequences作为元素的元组,而不是元素的元组：

template <typename... T>
struct sequences
{
   std::tuple<std::vector<T>...> m_sequences;
};

您还可以使用参数包进行漂亮的技巧,从矢量中选择适当的元素：

template<size_t ... Indices> struct indices_holder
{};

template<size_t index_to_add,typename Indices=indices_holder<> >
struct make_indices_impl;

template<size_t index_to_add,size_t...existing_indices>
struct make_indices_impl<index_to_add,indices_holder<existing_indices...> >
{
    typedef typename make_indices_impl<
        index_to_add-1,indices_holder<index_to_add-1,existing_indices...> >::type type;
};

template<size_t... existing_indices>
struct make_indices_impl<0,indices_holder<existing_indices...> >
{
    typedef indices_holder<existing_indices...>  type;
};

template<size_t max_index>
typename make_indices_impl<max_index>::type make_indices()
{
    return typename make_indices_impl<max_index>::type();
}



template <typename... T>
struct sequences
{
    std::tuple<std::vector<T>...> m_sequences;

    template<size_t... Indices>
    std::tuple<T...> get_impl(size_t pos,indices_holder<Indices...>)
    {
        return std::make_tuple(std::get<Indices>(m_sequences)[pos]...);
    }

    std::tuple<T...> get(size_t pos)
    {
        return get_impl(pos,make_indices<sizeof...(T)>());
    }
};