c – 变体模板：交错多个包

在开车回家时,我想到了这个问题：给了任何数量的包装,从每个包装中取出第一个类型,把它们放在一起.然后从每个包的第二个类型,把它们放在一起等等…然后合并他们.任何一个流浪者都会重复这个过程.例如,使用整数表示不同类型以获得更好的可读性,

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>

如果所有的包只有相同的尺寸,以下代码可以工作.当包装尺寸不同时,我现在完全处于“左侧”状态.这是我的代码到目前为止.我解释每个阶段,以便你知道我的计划是什么：

#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... Rest>
struct RemoveHead<N,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... 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... Packs>
struct InterlacePacksHelper<0,P<Ts...>,Packs...> { using type = P<Ts...>; };

template <int N,typename... Packs>
struct InterlacePacksHelper<N,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,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... Packs>
struct InterlacePacks<P<Ts...>,Packs...> : InterlacePacksHelper<MinPackSize<sizeof...(Ts),Packs...>::value,P<>,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,Blob,std::string>;  // 4 types
    InterlacePacks<TestPack1,TestPack2,TestPack3>::type interlacedPack;
    std::cout << std::boolalpha << std::is_same< decltype(interlacedPack),Pack<int,char,Object,Blob> >::value << std::endl;  // true
//  Want it to be Pack<int,long>
}

那么如何修复代码,使所需的输出

Pack<int,long>

结果呢？

注意：我尝试使用MinPackSize的MaxPackSize intead,并且按预期不会编译.一个想法是在MinPackSize迭代后丢弃空包,并继续进行,直到MaxPackSize迭代完成(每次删除新的空包).这在理论上(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,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...> {};

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,T<f>,P...> {};

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