Making a tuple of references out of another tuple by condition

Question

I have a templated structure ContainerInner that has a tuple with vectors of types from its variadic template and a structure Container that contains a tuple of ContainerInner, specified in its template. The code looks like this:

template<typename... Args>
struct ContainerInner
{
    std::tuple<std::vector<Args>...> m_elements;

    template<typename T>
    static constexpr bool containsOne()
    {
        return (std::is_same_v<T, Args> || ...);
    }

    template<typename... T>
    static constexpr bool contains()
    {
        return (containsOne<T>() && ...);
    }

    template<typename T>
    std::vector<T> &get()
    {
        return std::get<std::vector<T>>(m_elements);
    }
};

template<typename... T>
struct ArgList {
    template<typename... TTS>
    using add = ArgList<T..., ContainerInner<TTS...>>;
};
    
template<typename ListArgs>
struct Container;

template<typename... Args>
struct Container<ArgList<Args...>>
{
    std::tuple<Args...> m_containers;

    // It also crashes with std::tuple<Args&...> return type
    template<typename... T>
    inline auto getTuples()
    {
        auto tpl = std::make_tuple<Args&...>(std::get<Args>(m_containers)...);
        return tpl;
    }

    template<typename... T>
    ContainerInner<T...> &get()
    {
        return std::get<ContainerInner<T...>>(m_containers);
    }
};

using MyArgList = ArgList<>
    ::add<char, int>
    ::add<char, float>
    ::add<char, int, float>;

int main(int argc, char* args[])
{    
    Container <MyArgList> cnt;

    auto arr = cnt.getTuples<float>();

    std::get<ContainedType<char, int>>(arr).get<int>().push_back(3);

    std::cout << typeid(arr).name() << std::endl;
    std::cout << cnt.get<char, int>().get<int>().size() << std::endl;
}

I want to change getTuples method so that it would return a tuple with references to all ContainerInner from m_containers that contain specified types, which is a condition handled by static contains method. I thought I could use a concept like this:

template<typename InnerCnt, typename T>
concept ContainedType = 
    requires(T a, InnerCnt b) {
       b.contains<T>();
    };

to recursively iterate over variadic template and use std::tuple_cat to build final array, but didnt figure out how to do it. How can I solve this problem?

EDIT. I solved it, but probably not in the optimal way. I added Query wrapper over tuple with overloaded operator+ and used it for unfolding:

    template<typename... Args>
struct Query
{
    std::tuple<Args...> m_tpl;

    template<typename... TT>
    constexpr inline auto operator+(const Query<TT...> &rhs_)
    {
        return Query<Args..., TT...>(std::tuple_cat(m_tpl, rhs_.m_tpl));
    }

    template<typename... TT>
    constexpr inline auto &get()
    {
        return std::get<ContainerInner<TT...>&>(m_tpl);
    }
};

template<typename LST, typename CONTAINER_INNER>
constexpr inline auto getQueryElem(CONTAINER_INNER &t_)
{
    return Query<>();
}

template<typename LST, typename CONTAINER_INNER> requires Contained<CONTAINER_INNER, LST>
constexpr inline auto getQueryElem(CONTAINER_INNER &t_)
{
    return Query(std::tuple<CONTAINER_INNER&>(t_));
}


// Inside Container
template<typename... T>
constexpr inline auto getTuples()
{
    return (getQueryElem<Typelist<T...>>(std::get<Args>(m_containers)) + ...);
}

int main(int argc, char* args[]) {
  Container <MyArgList> cnt;
  auto arr = cnt.getTuples<int>();
  std::cout << typeid(arr).name() << std::endl;

  cnt.get<char, int>().get<int>().push_back(5);
  arr.get<char, int>().get<int>().push_back(-99);

  std::cout << cnt.get<char, int>().get<int>() << std::endl;
  std::cout << arr.get<char, int>().get<int>() << std::endl;
}

It seems to work, but i'm not sure how well does compiler optimize this, it might actually create temporary Queries during unfolding in runtime

Answer 1

Issues from getTuples():

• it is template but doesn't use its parameters. can be removed.
• std::make_tuple is intended to deduced its parameter, you want std::tuple directly:

auto getTuples()
{
    auto tpl = std::tuple<Args&...>(std::get<Args>(m_containers)...);
    return tpl;
}

Demo

or use std::tie:

auto getTuples()
{
    return std::tie(std::get<Args>(m_containers)...);
}

Demo

From your edit, it seems you want to filter (at compile time) some elements of the tuple, so using std::tuple_cat with function which return empty tuple or tuple with the element might help:

template <typename... Ts, typename... Us>
auto filter(ContainerInner<Us...>& c)
{
    if constexpr (ContainerInner<Us...>::template contains<Ts...>()) {
        return std::tie(c);
    } else {
        return std::tuple<>{};
    }
}

And then

template <typename... Ts>
constexpr auto getTuples()
{
    return std::tuple_cat(filter<Ts...>(std::get<Args>(m_containers))...);
})

Demo