TMP：如何推广向量的笛卡尔积？

Question

对于整数向量的向量的笛卡尔积，有一个出色的 C++ 解决方案（实际上有 2 个解决方案：递归和非递归）。为了说明/简单起见，让我们只关注非递归版本。

我的问题是，如何使用模板概括此代码以获取如下所示的

std::tuple

同质向量：

{{2,5,9},{"foo","bar"}}

并生成

tuple

的齐次向量

{{2,"foo"},{2,"bar"},{5,"foo"},{5,"bar"},{9,"foo"},{9,"bar"}}

如果这让事情变得更容易，让我们假设输入中的内部向量都是同构的。所以像这样的输入是不允许：

{{5,"baz"}{'c',-2}}

编辑将输入从锯齿状向量更改为元组

Answer 1

更简单的递归解决方案。它将向量作为函数参数，而不是元组。此版本不构建临时元组，而是使用 lambda。现在它不再进行不必要的复制/移动，并且似乎已成功优化。

#include<tuple>
#include<vector>

// cross_imp(f, v...) means "do `f` for each element of cartesian product of v..."
template<typename F>
inline void cross_imp(F f) {
    f();
}
template<typename F, typename H, typename... Ts>
inline void cross_imp(F f, std::vector<H> const& h,
                           std::vector<Ts> const&... t) {
    for(H const& he: h)
        cross_imp([&](Ts const&... ts){
                      f(he, ts...);
                  }, t...);
}

template<typename... Ts>
std::vector<std::tuple<Ts...>> cross(std::vector<Ts> const&... in) {
    std::vector<std::tuple<Ts...>> res;
    cross_imp([&](Ts const&... ts){
                  res.emplace_back(ts...);
              }, in...);
    return res;
}

#include<iostream>

int main() {
    std::vector<int> is = {2,5,9};
    std::vector<char const*> cps = {"foo","bar"};
    std::vector<double> ds = {1.5, 3.14, 2.71};
    auto res = cross(is, cps, ds);
    for(auto& a: res) {
        std::cout << '{' << std::get<0>(a) << ',' <<
                            std::get<1>(a) << ',' <<
                            std::get<2>(a) << "}\n";
    }
}

Answer 2

我这样做已经有一段时间了，但这是第一次尝试。毫无疑问，它可以改进。

template<unsigned fixedIndex, class T>
class DynamicTupleGetter
{
    typedef typename std::tuple_element<fixedIndex, T>::type RetType;
public:
    static RetType get(unsigned dynIndex, const T& tupleInstance)
    {
        const RetType& ret = std::get<fixedIndex>(tupleInstance);

        if (fixedIndex == dynIndex)
            return ret;
        return DynamicTupleGetter<fixedIndex - 1, T>::get(dynIndex, tupleInstance);
    }

};

template<class T>
class DynamicTupleGetter<0, T>
{
    typedef typename std::tuple_element<0, T>::type RetType;
public:
    static RetType get(unsigned dynIndex, const T& tupleInstance)
    {
        assert(dynIndex == 0);
        return std::get<0>(tupleInstance);
    }
};
template<class Source>
struct Converter
{
    typedef typename std::tuple_element<0, Source>::type Zeroth;
    typedef typename std::tuple_element<1, Source>::type First;

    static const size_t size0 = std::tuple_size<Zeroth>::value;
    static const size_t size1 = std::tuple_size<First>::value;

    static const size_t  outerProductSize = size0 * size1;

    typedef typename std::tuple_element<0, Zeroth>::type BaseType0;
    typedef typename std::tuple_element<0, First>::type BaseType1;
    typedef typename std::tuple<BaseType0, BaseType1> EntryType;

    typedef std::array<EntryType, outerProductSize> DestinationType;

    DestinationType create(const Source& source)
    {
        Zeroth zeroth = std::get<0>(source);
        First first = std::get<1>(source);
        typedef typename DynamicTupleGetter<size0 -1, Zeroth> ZerothGetter;
        typedef typename DynamicTupleGetter<size1 -1, First> FirstGetter;
        DestinationType result;
        size_t resultIndex = 0;
        for(size_t i = 0; i < size0; ++i)
            for(size_t j = 0; j < size1; ++j)
            {
                std::get<0>(result[resultIndex]) = ZerothGetter::get(i, zeroth) ;        
                std::get<1>(result[resultIndex]) = FirstGetter::get(j, first); 
                ++resultIndex;
            }
            return result;
    }


};


template<class T>
void create(const T& source)
{
    Converter<T> converter;

    Converter<T>::DestinationType result = converter.create(source);

    std::cout << std::get<0>(std::get<3>(result)) << "," << std::get<1>(std::get<3>(result)) << std::endl;
}


auto intPart = std::make_tuple(2,5,9);
auto stringPart = std::make_tuple("foo","bar");
auto source = std::make_tuple(intPart, stringPart);

void f()
{
    create(source);
}