使用另一个创建新的增强融合序列

Question

给定一个融合序列 X，我想创建一个新的融合序列 Y，其实现将基于 X。特别是，我想创建一个 class 模板 make_fusion_conforming 所以

template <class X>
struct another_fusion_sequence : make_fusion_conforming <X> {};

将使 another_fusion_sequence<X> 符合融合，这样我就可以调用

auto sequence1 = another_fusion_sequence<X>(); //line 1
auto sequence2 = another_fusion_sequence<another_fusion_sequence<X>>(); // line 2
auto it1 = fusion::begin(sequence1);
auto it2 = fusion::begin(sequence2);

其中 it1 和 it2 将是序列第一个元素的迭代器（实际上是 X 序列的第一个元素）。请注意，第 2 行也是有效的，因为 another_fusion_sequence<X> 已经符合融合。

使用 boost 扩展文档和提供的 triple.cpp 演示，我想出了下面的内容。抱歉，它有点长，但它非常简单，因为 iterator 和 make_fusion_conforming 的实现只是委托给已经符合融合的序列。

#include <boost/fusion/iterator/iterator_facade.hpp>
#include <boost/fusion/iterator/value_of.hpp>
#include <boost/fusion/iterator/deref.hpp>
#include <boost/fusion/iterator/next.hpp>
#include <boost/fusion/iterator/prior.hpp>
#include <boost/fusion/iterator/distance.hpp>
#include <boost/fusion/include/begin.hpp>
#include <boost/fusion/include/end.hpp>
#include <boost/fusion/include/size.hpp>
#include <boost/fusion/include/category_of.hpp>
#include <boost/fusion/sequence/sequence_facade.hpp>

namespace fusion = boost::fusion;

// Sequence is the sequence I want to make fusion-conforming
// BaseIter is a fusion-conforming iterator
// the implementation of iterator below is based entirely on BaseIter
template <class Sequence, class BaseIter>
struct iterator
: fusion::iterator_facade<iterator<Sequence, BaseIter>, typename fusion::traits::category_of<BaseIter>::type> {

    using base_iter_type = BaseIter;

    BaseIter m_base_iter;

    iterator(BaseIter base_iter)
    : m_base_iter(base_iter){}

    template <class It>
    struct value_of {

        using type = typename fusion::result_of::value_of<typename It::base_iter_type>::type;
    };

    template <class It>
    struct deref {

        using type = typename fusion::result_of::deref<typename It::base_iter_type>::type;

        static type call(const It& iter) {
            return fusion::deref(iter.m_base_iter);
        }
    };

    template <class It>
    struct next {

        using type = typename fusion::result_of::next<typename It::base_iter_type>::type;

        static type call(const It& iter) {
            return fusion::next(iter.m_base_iter);
        }
    };

    template <class It>
    struct prior {

        using type = typename fusion::result_of::prior<typename It::base_iter_type>::type;

        static type call(const It& iter) {
            return fusion::prior(iter.m_base_iter);
        }
    };

    template <class It1, class It2>
    struct distance {

        using type = typename fusion::result_of::distance<typename It1::base_iter_type, typename It2::base_iter_type>::type;

        static type call(const It1& iter1, const It2& iter2) {
            return fusion::distance(iter1.m_base_iter, iter2.m_base_iter);
        }
    };

};

// Base is the fusion-conforming sequence
// Sequence is the sequence I want to make fusion-conforming
// once again, implementation is based entirely on the Base sequence
template <class Base, class Sequence>
struct make_fusion_conforming
: public Base
, public fusion::sequence_facade<Sequence, typename fusion::traits::category_of<Base>::type> {

    using Base::Base;

    template <class Seq>
    struct begin {

        using type = iterator<Seq, typename fusion::result_of::begin<Base>::type>;

        static type call(Seq& seq) {
            return type(fusion::begin(static_cast<Base&>(seq)));
        }
    };

    template <class Seq>
    struct end {

        using type = iterator<Seq, typename fusion::result_of::end<Base>::type>;

        static type call(Seq& seq) {
            return type(fusion::end(static_cast<Base&>(seq)));
        }
    };

    template <class Seq>
    struct size {
        using type = typename fusion::result_of::size<Base>::type;

        static type call(Seq& seq) {
            return fusion::size(static_cast<Base&>(seq));
        }
    };

    template <class Seq>
    struct empty {

        using type = typename fusion::result_of::empty<Base>::type;

        static type call(Seq& seq) {
            return fusion::empty(static_cast<Base&>(seq));
        }
    };

    template <class Seq, class N>
    struct at {

        using type = typename fusion::result_of::at<Base, N>::type;

        static type call(Seq& seq) {
            return fusion::at<N>(static_cast<Base&>(seq));
        }
    };

    template <class Seq, class N>
    struct value_at {

        using type = typename fusion::result_of::value_at<Base, N>::type;
    };
};

不幸的是，尝试获取开始迭代器失败...

#include <boost/fusion/include/vector.hpp>

template <class... T>
struct sequence1
: make_fusion_conforming< fusion::vector<T...>, sequence1<T...> > {

};

using seq1 = sequence1<int, float, double>;

// the below fails with:
// No type named 'type' in 'boost::fusion::result_of::begin<sequence1<int, float, double> >'
using test1 = fusion::result_of::begin<seq1>::type;

int main() {}

有人可以帮我解决这个问题吗？谢谢

Answer 1

问题似乎是 Boost.Fusion 的扩展机器无法识别 make_fusion_conforming 是 "sequence façade"。当使用内部操作（开始、结束、value_at 等）时，融合所做的第一件事是检查相关序列的标签。它首先检查您的类型是否具有 fusion_tag 关联类型（并将其作为其标签），然后检查它是否是 MPL 序列，如果检查失败，则确定该序列具有 non_fusion_tag 标签。为了让你所有的样板都能工作，sequence1 的标签需要是 sequence_facade_tag，你可以通过从 sequence_facade 派生但也可以从 Base 派生得到它（fusion::vector 在这种情况下有一个标签 vector_tag) 你已经使 fusion_tag 模棱两可，因此 tag_of<seq1>::type 变成了 non_fusion_tag。一个可能的解决方案是在 make_fusion_conforming 中添加一个 using fusion_tag = fusion::sequence_facade_tag; 来解决歧义（另一个可能是专门化 boost::fusion::traits::tag_of<sequence1<T...>>::type）。

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使用另一个创建新的增强融合序列

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