Converting Typelist to Parameterpack - GerdHirsch/GenericTools GitHub Wiki

With Loki::Typelist there are many tools available to manipulate lists of types i.e. Reverse<..>. To be able to use these tools for parameter packs, the packs should be convertible from and to Loki::Typelist. This is done by TList2TPack<typelist> := Typelist to Typepack.

using typelistABC = MakeTypelist<A, B, C>;
using packABC = TList2TPack<typelistABC>; // Typelist to Typepack

With this in place, all type-functions of Loki for Typelist can be used:

using packCBA = Reverse<packABC>;
using packCBA_2 = Reverse<A, B, C>;

The implementation of the type-function Reverse<..> is:

template<class ...Pack>
struct ReverseImpl{
    using typelist = MakeTypelist<Pack...>;
    using reverseList = typename Loki::TL::Reverse<typelist>::Result;
    using type = TList2TPack<reverseList>;
};
template<class ...Pack>
struct ReverseImpl<Typepack<Pack...>> : ReverseImpl<Pack...>{};

// Reverse<..> is the interface to the type-function
template<class ...pack>
using Reverse = typename ReverseImpl<pack...>::type;`

Note: Reverse<..> is not specialized for Typepack<..> but can be used with! The outcome of the call Reverse<packABC> above is a parameter pack with one argument: packABC of type Typepack<A, B, C> and this is delegated to ReverseImpl<..> which is specialized for that kind of type. Because ReverseImpl<..> is a type-function, the created type is always a Typepack<..> nevertheless the argument was a parameter ...pack or a Typepack<..>.

In Demo there are complete examples.