Concatenating tuples as types

Question

I'm trying to practice some template programming. Maybe there's a standard way to do this, and I would be thankful for such answers, but my main goal is to practice the template programming techniques, so I tried to implement it myself:

I need to concatenate multiple tuples, but as types, not like std::cat_tuple does it. So I need something like cat<std::tuple<int, float>, std::tuple<char, bool>, ...> to get std::tuple<int, float, char, bool, ...> as a type.

My current attempt failed with a is not a template error:

/* Concat tuples as types: */
template <typename first_t, typename... rest_t> struct cat {
    using type = typename _cat<first_t, typename cat<rest_t...>::type>::type;
                          ^^^^ cat is not a template
};
template <typename first_t, typename second_t>
struct cat<first_t, second_t> {
    using type = typename _cat<first_t, second_t>::type;
                          ^^^^ cat is not a template
};
// Concat two tuples:
template <typename, typename> struct _cat;
template <typename tuple_t, typename first_t, typename... rest_t>
struct _cat<tuple_t, std::tuple<first_t, rest_t...>> {
    using type = typename _cat<typename append<first_t, tuple_t>::type, std::tuple<rest_t...>>::type;
};
template <typename tuple_t, typename first_t>
struct _cat<tuple_t, std::tuple<first_t>> {
    using type = typename append<first_t, tuple_t>::type;
};
// Prepend element to tuple:
template <typename, typename> struct prepend;
template <typename elem_t, typename... tuple_elem_t>
struct prepend<elem_t, std::tuple<tuple_elem_t...>> {
    using type = std::tuple<elem_t, tuple_elem_t...>;
};
// Apppend element to tuple:
template <typename, typename> struct append;
template <typename elem_t, typename... tuple_elem_t>
struct append<elem_t, std::tuple<tuple_elem_t...>> {
    using type = std::tuple<tuple_elem_t..., elem_t>;
};

What may be causing the error?

Is this a good approach? It might be solved in a simpler way, but I wanted it to be multi-purpose (with the append/prepend operations etc.).

Answer 1

How about a one-liner direct template aliase:

template<typename ... input_t>
using tuple_cat_t=
decltype(std::tuple_cat(
    std::declval<input_t>()...
));


tuple_cat_t<
    std::tuple<int,float>,
    std::tuple<int>
    > test{1,1.0f,2};

Answer 2

After reordering the definition a little, your code works fine.

I don't think that there are any guidelines for template meta programming. Probably due to the fact that the C++ committee is enhancing TMP "aggressively" and too few people is using TMP.

Here is my version of Cat, it basically follows the same structure as yours:

template <class, class>
struct Cat;
template <class... First, class... Second>
struct Cat<std::tuple<First...>, std::tuple<Second...>> {
    using type = std::tuple<First..., Second...>;
};

Answer 3

Is too late to play?

I propose the following solution

template <typename T, typename ...>
struct cat
 { using type = T; };

template <template <typename ...> class C,
          typename ... Ts1, typename ... Ts2, typename ... Ts3>
struct cat<C<Ts1...>, C<Ts2...>, Ts3...>
   : public cat<C<Ts1..., Ts2...>, Ts3...>
 { };

Observe that this solution doesn't works only with a variadic list of std::tuple but also with a generic container of types. If a std::tuple-only solution is enough for you, you can simplify it as follows

template <typename T, typename ...>
struct cat
 { using type = T; };

template <typename ... Ts1, typename ... Ts2, typename ... Ts3>
struct cat<std::tuple<Ts1...>, std::tuple<Ts2...>, Ts3...>
   : public cat<std::tuple<Ts1..., Ts2...>, Ts3...>
 { };

You can test that works with

   using t1 = typename cat<std::tuple<int, float>,
                           std::tuple<char, bool>,
                           std::tuple<long, char, double>>::type;

   using t2 = std::tuple<int, float, char, bool, long, char, double>;

   static_assert(std::is_same<t1, t2>::value, "!");

-- EDIT --

As pointed by felix (thanks!) with my precedent solution we have that

std::is_same<int, typename cat<int>::type>::value == true

that is... cat<T>::type is defined also when T isn't a std::tuple.

This is a problem?

I don't know because I don't know how is used cat<T>::type.

Anyway... avoid it imposing that cat<Ts...>::type is defined only when all Ts... are type containers (with the same container), it's simple: the main version for cat become only declared but not defined

template <typename, typename ...> // or also template <typename...>
struct cat;

and is introduced an additional specialization with a single type (but only when it's a type container).

template <template <typename ...> class C, typename ... Ts1>
struct cat<C<Ts1...>>
 { using type = C<Ts1...>; };