Transform parameter pack types

Question

I have multiple classes which share the same public typedefs like

struct A { using Container = std::array<A, 3>; };
struct B { using Container = std::vector<B>; };
struct C { using Container = std::array<C, 5>; };

Now I have a class which gets a parameter pack containing only valid classes but it has to store a tuple of the containers. Pseudo-code:

template <typename... Modules>
struct Collector
{
std::tuple<Modules...::Container> mContainers;
};

Is there an elegant way to apply the ::Container during unpacking?

Answer 1

You can use a helper trait

template<typename T>
using ContainerOf = typename T::Container;

template <typename... Modules>
struct Collector
{
std::tuple<ContainerOf<Modules>...> mContainers;
};

alternatively, you can also inline this trait like so without a helper trait:

template <typename... Modules>
struct Collector
{
std::tuple<typename Modules::Container...> mContainers;
};