可变参数模板：交错多个包

Question

给定任意数量的包，从每包中取出第一种，将它们放在一起。然后从每个包中取出第二种，将它们放在一起，等等......然后将它们全部合并。任何剩下的人都会在他们之间重复这个过程。例如，为了更好的可读性，使用整数来表示不同的类型，

InterlacePacks<Pack<1 2 3 4>, Pack<5 6 7>, Pack<8 9 10 11 12>>::type

会给

Pack<1 5 8 2 6 9 3 7 10 4 11 12>

仅当所有包装的尺寸相同时，以下代码才有效。当包装尺寸不同时，我现在完全无法处理 "left-overs"。到目前为止，这是我的代码。我会解释每个阶段，以便您了解我的计划：

#include <iostream>

// First a helper to remove the first N types from a pack:
template <int, typename> struct RemoveHead;

template <typename Pack>
struct RemoveHead<0, Pack> { using type = Pack; };

template <template <typename...> class P, typename First, typename... Rest>
struct RemoveHead<0, P<First, Rest...>> { using type = P<First, Rest...>; };

template <int N, template <typename...> class P, typename First, typename... Rest>
struct RemoveHead<N, P<First, Rest...>> : RemoveHead<N-1, P<Rest...>> {};

// Now a helper to merge multiple packs:
template <typename...> struct MergePacks;

template <typename Pack>
struct MergePacks<Pack> {
    using type = Pack;
};

// Final Pack type shall be the first one listed, if there are different pack types.
template <template <typename...> class P1, template <typename...> class P2, typename... Types1, typename... Types2, typename... Packs>
struct MergePacks<P1<Types1...>, P2<Types2...>, Packs...> : MergePacks<P1<Types1..., Types2...>, Packs...> {};

// First collect the first type from each pack:
template <typename, typename...> struct InterlacePacksHelper1;

template <template <typename...> class P, typename... Ts>
struct InterlacePacksHelper1<P<Ts...>> { using type = P<Ts...>; };

template <template <typename...> class P, template <typename...> class FirstPack, typename... Ts, typename First, typename... Rest, typename... Packs>
struct InterlacePacksHelper1<P<Ts...>, FirstPack<First, Rest...>, Packs...> : InterlacePacksHelper1<P<Ts..., First>, Packs...> {};

// Now remove the first type from each pack and repeat the process.  Use a parameter N as a counter, where N will start as the minimum size of the packs.
template <int, typename, typename...> struct InterlacePacksHelper;

template <template <typename...> class P, typename... Ts, typename... Packs>
struct InterlacePacksHelper<0, P<Ts...>, Packs...> { using type = P<Ts...>; };

template <int N, template <typename...> class P, typename... Ts, typename... Packs>
struct InterlacePacksHelper<N, P<Ts...>, Packs...> : InterlacePacksHelper<N-1,
    typename MergePacks<P<Ts...>, typename InterlacePacksHelper1<P<>, Packs...>::type>::type,
    typename RemoveHead<1, Packs>::type...> {};

// Now obtain the smallest pack size, given a list of packs.
template <int N, typename...> struct MinPackSize;

template <int N>
struct MinPackSize<N> : std::integral_constant<int, N> {};

template <int N, template <typename...> class P, typename... Types, typename... Packs>
struct MinPackSize<N, P<Types...>, Packs...> : std::integral_constant<int,
    (sizeof...(Types) < N) ? sizeof...(Types) : N> {}; 

// Finally, InterlacePacks itself.
template <typename...> struct InterlacePacks;

template <template <typename...> class P, typename... Ts, typename... Packs>
struct InterlacePacks<P<Ts...>, Packs...> : InterlacePacksHelper<MinPackSize<sizeof...(Ts), Packs...>::value, P<>, P<Ts...>, Packs...> {};

// test ----------------------------------------------------------------
template <typename...> struct Pack {};
template <typename...> struct Group {};
template <typename...> struct Wrap {};
struct Object {};  struct Blob {};

int main() {
    using TestPack1 = Pack<int, double, Object>;  // 3 types
    using TestPack2 = Group<double, std::string, int, short, long>;  // 5 types
    using TestPack3 = Wrap<char, short, Blob, std::string>;  // 4 types
    InterlacePacks<TestPack1, TestPack2, TestPack3>::type interlacedPack;
    std::cout << std::boolalpha << std::is_same< decltype(interlacedPack),
        Pack<int, double, char, double, std::string, short, Object, int, Blob> >::value << std::endl;  // true
//  Want it to be Pack<int, double, char, double, std::string, short, Object, int, Blob, short, std::string, long>
}

那么如何修复代码以便获得所需的输出

Pack<int, double, char, double, std::string, short, Object, int, Blob, short, std::string, long>

改为结果？

注意：我尝试使用 MaxPackSize 而不是 MinPackSize，但正如预期的那样无法编译。一种想法是在 MinPackSize 次迭代后丢弃空包并继续该过程，直到完成 MaxPackSize 次迭代（每次移除新的空包）。不过这是理论上的（还没有尝试过）：

template <typename, typename...> struct RemoveAllEmptyPacksHelper;

template <template <typename...> class P, typename... Packs> 
struct RemoveAllEmptyPacksHelper<P<Packs...>> : Identity<P<Packs...>> {};

template <template <typename...> class P, typename... CurrentPacks, template <typename...> class FirstPack, typename... Types, typename... Packs> 
struct RemoveAllEmptyPacksHelper<P<CurrentPacks...>, FirstPack<Types...>, Packs...> : 
    std::conditional<(sizeof...(Types) == 0),
    RemoveAllEmptyPacksHelper<P<CurrentPacks...>, Packs...>,
    RemoveAllEmptyPacksHelper<P<CurrentPacks..., FirstPack<Types...>>, Packs...>
    >::type {};

template <typename> struct RemoveAllEmptyPacks;

template <template <typename...> class P, typename... Packs> 
struct RemoveAllEmptyPacks<P<Packs...>> : RemoveAllEmptyPacksHelper<P<>, Packs...> {};

Answer 1

以下可能会有所帮助，它使用助手 class 并将结果汇总到第一个模板参数中：

template <typename...> struct InterlacePacksHelper;

// general case: take first parameter and en-queue the Pack for further computation
template <template <typename...> class PRes, typename... Ts,
          template <typename...> class P, typename U, typename... Us,
          typename... Packs>
struct InterlacePacksHelper<PRes<Ts...>, P<U, Us...>, Packs...>
{
    using type = typename InterlacePacksHelper<PRes<Ts..., U>, Packs..., P<Us...>>::type;
};

// final case
template <template <typename...> class PRes, typename... Ts>
struct InterlacePacksHelper<PRes<Ts...>>
{
    using type = PRes<Ts...>;
};

// Remove empty Pack.
template <template <typename...> class PRes, typename... Ts,
          template <typename...> class P,
          typename... Packs>
struct InterlacePacksHelper<PRes<Ts...>, P<>, Packs...>
{
    using type = typename InterlacePacksHelper<PRes<Ts...>, Packs...>::type;
};


// Finally, InterlacePacks itself.
template <typename...> struct InterlacePacks;

template <template <typename...> class P, typename... Ts, typename... Packs>
struct InterlacePacks<P<Ts...>, Packs...> : InterlacePacksHelper<P<>, P<Ts...>, Packs...>::type
{
    using type = typename InterlacePacksHelper<P<>, P<Ts...>, Packs...>::type;
};

Live demo

Answer 2

这是我迄今为止最短的 C++11 尝试：

template <class T, class...> struct interlace_ {using type = T;};
template <class... R,  template<class...> class T, class f, class... t, class... P>
struct interlace_<std::tuple<R...>, T<f, t...>, P...>
    : interlace_<std::tuple<R..., f>, P..., T<t...>> {};
template <class... R, template<class...> class T, class f, class... P>
struct interlace_<std::tuple<R...>, T<f>, P...>
    : interlace_<std::tuple<R..., f>, P...> {};

template <class... packs>
using interlace = interlace_<std::tuple<>, packs...>;

Demo。 P 代表包，R 是（当前）结果包，f 是第一个类型，t 是当前观察到的包的尾部。 T 是保存包的模板。

可变参数模板：交错多个包

Variadic templates: Interlacing multiple packs

c++

templates

variadic

c++11