How to create a tuple of vectors of type deduced from a variadic template in C++17?

问题

I have implemented a collection class that converts a vector of tuples to a tuple of vectors (it is essentially an AOS to SOA conversion). This code works for this example of two template classes. I was trying to make it more generic by using variadic templates. In order to do that I need to create the type for the member variable m_col. In C++17, is it possible to convert a tuple to a tuple of vectors? So the type of the member variance m_col in this example will be generated automatically from template types.

template<class T1, class T2>
class Collection
{
    std::tuple<std::vector<T1>, std::vector<T2>> m_col;

public:
    void addRow(const std::tuple<T1, T2>& data)
    {
        std::get<0>(m_col).push_back(std::get<0>(data));
        std::get<1>(m_col).push_back(std::get<1>(data));
    } 

    void show()
    {
        std::cout << std::get<0>(m_col1).size() <<std::endl;
    }
};


int main()
{
    using data_t = std::tuple<int, double>;
    data_t data{1,1.0};
    using col_t = Collection<int, double>;
    col_t col;
    col.addRow(data);    
    col.show();
}

回答1:

You're using C++17 so... what about using template folding and some std::apply()?

I mean

template <typename ... Ts>
class Collection
 {
   private:
      std::tuple<std::vector<Ts>...> m_col;

   public:
      void addRow (std::tuple<Ts...> const & data)
       {
          std::apply([&](auto & ... vectors){
                     std::apply([&](auto & ... values){
                                (vectors.push_back(values), ...);},
                                data); },
                     m_col);        
       } 

      void show () const
       {
         std::apply([](auto & ... vectors){
             ((std::cout << vectors.size() << '\n'), ...);}, m_col);
       }
 };

回答2:

You might do with std::index_sequence:

template<class ... Ts>
class Collection
{
    std::tuple<std::vector<Ts>...> m_col;

private:

    template <std::size_t ... Is>
    void addRow(std::index_sequence<Is...>, const std::tuple<Ts...>& data)
    {
        (std::get<Is>(m_col).push_back(std::get<Is>(data)), ...);
    } 

public:
    void addRow(const std::tuple<Ts...>& data)
    {
        addRow(std::index_sequence_for<Ts...>(), data);
    } 

};

Demo

来源：https://stackoverflow.com/questions/53305395/how-to-create-a-tuple-of-vectors-of-type-deduced-from-a-variadic-template-in-c

标签

c++

c++17

variadic-templates

stdtuple

structure-of-arrays