Boost::融合，特征和压缩转换

这个问题是从我对boost::fusion的另一个问题衍生出来的。这个想法是使用boost::fusion来迭代一个包含n维数组的大型c风格结构体。这些数组的计算由Eigen完成。通过使用boost::fusion，可以对整个C结构体进行简单的算术运算，如标量乘法或向量加法。

在处理二进制操作时，我使用boost::fusion::zip来形成单个序列，使用boost::fusion::for_each来迭代该序列。

boost::fusion::zip的问题是，它构建const序列，而我需要修改其中一个值(例如，返回值的加法)。因此，我最终使用const_cast来修改该值(Eigen向量)，但由于某种原因，我不能在add()函数中使用result_ref。为什么呢?

此外，是否有更好(或更简单)的方法来实现我想要做的事情?boost::fusion::zip可能不是最合适的，但我找不到任何其他简单的方法来做到这一点。

#include <iostream>
#include <boost/fusion/adapted/struct/adapt_struct.hpp>
#include <boost/fusion/include/adapt_struct.hpp>
#include <boost/fusion/algorithm/iteration/for_each.hpp>
#include <boost/fusion/include/for_each.hpp>
#include <boost/fusion/algorithm/transformation/zip.hpp>
#include <boost/fusion/include/zip.hpp>
#include <boost/bind.hpp>
#include <boost/fusion/container/vector/vector30.hpp>
#include <boost/fusion/include/vector30.hpp>
#include <boost/fusion/sequence/intrinsic/at_c.hpp>
#include <boost/fusion/include/at_c.hpp>
#include <boost/type_traits/remove_const.hpp>
#include <boost/type_traits/remove_reference.hpp>
#include <Eigen/Core>
template <class type_const_ref>
struct remove_const_ref
{
    typedef typename boost::remove_reference <type_const_ref>::type type_const;
    typedef typename boost::remove_const <type_const_ref>::type     type_ref;
    typedef typename boost::remove_const <type_const >::type        type;
};
namespace demo
{
template<typename T, int SIZE1, int SIZE2>
struct data
{
    T ar1[SIZE1][SIZE2];
    T ar2[SIZE1][SIZE2];
};
template<typename T>
struct EigenMap
{
    typedef Eigen::Map<Eigen::Matrix<T, Eigen::Dynamic, 1> > type;
};
template<typename T>
struct data_eigen
{
    template <int SIZE1, int SIZE2>
    data_eigen(data<T,SIZE1,SIZE2>& src)
        : ar1(typename EigenMap<T>::type(&src.ar1[0][0], SIZE1*SIZE2)),
          ar2(typename EigenMap<T>::type(&src.ar2[0][0], SIZE1*SIZE2))
    {
    }
    typename EigenMap<T>::type ar1;
    typename EigenMap<T>::type ar2;
};

struct print
{
    template<typename T>
    void operator()(const Eigen::Map<Eigen::Matrix<T, Eigen::Dynamic, 1> >& t) const
    {
        std::cout << t.transpose() << std::endl;
    }
};
struct scalarMult
{
    template<typename T, typename U>
    void operator()(T& t, U& u) const
    {
        t *= u;
    }
};
template <typename T>
struct add
{
    template <typename U>
    void operator() (const boost::fusion::vector3<U,U,U>& t) const
    {
        typedef typename remove_const_ref<U>::type_ref vector_ref;
        typedef typename remove_const_ref<U>::type     vector_type;
        // FIXME: find why we cannot use vector_ref
        vector_type result_ref = const_cast<vector_ref>(boost::fusion::at_c<2>(t));
        result_ref = boost::fusion::at_c<0>(t) + boost::fusion::at_c<1>(t);
    }
};
}
BOOST_FUSION_ADAPT_TPL_STRUCT
(
    (T),
    (demo::data_eigen) (T),
    (typename demo::EigenMap<T>::type, ar1)
    (typename demo::EigenMap<T>::type, ar2)
)
int main()
{
    typedef float REALTYPE;
    const int SIZE1 = 2;
    const int SIZE2 = 2;
    // Basic data structure with multidimensional arrays
    demo::data<REALTYPE, SIZE1, SIZE2> d1;
    for (unsigned int i = 0; i < SIZE1; ++i)
        for (unsigned int j = 0; j < SIZE2; ++j)
        {
            d1.ar1[i][j] = (i+1)*(j+1);
            d1.ar2[i][j] = i + j;
        }
    demo::data<REALTYPE, SIZE1, SIZE2> d2;
    demo::data<REALTYPE, SIZE1, SIZE2> d3;
    memset(&d3, 0, sizeof(demo::data<REALTYPE, SIZE1, SIZE2>));
    for (unsigned int i = 0; i < SIZE1; ++i)
        for (unsigned int j = 0; j < SIZE2; ++j)
        {
            d2.ar1[i][j] = 1.0;
            d2.ar2[i][j] = 2.0;
        }
    // Eigen::Map + BOOST_FUSION_ADAPT_TPL_STRUCT
    demo::data_eigen<REALTYPE> eig_d1(d1);
    demo::data_eigen<REALTYPE> eig_d2(d2);
    demo::data_eigen<REALTYPE> eig_d3(d3);
    std::cout << "d1:" << std::endl;
    boost::fusion::for_each(eig_d1, demo::print());
    std::cout << std::endl;
    std::cout << "d2:" << std::endl;
    boost::fusion::for_each(eig_d2, demo::print());
    std::cout << std::endl;
    boost::fusion::for_each(eig_d1, boost::bind<void>(demo::scalarMult(), _1, 2.0));
    std::cout << "d1 = 2 * d1:" << std::endl;
    boost::fusion::for_each(eig_d1, demo::print());
    std::cout << std::endl;
    boost::fusion::for_each(boost::fusion::zip(eig_d1, eig_d2, eig_d3),
                            demo::add<REALTYPE>());
    std::cout << "d3 = d1 + d2:" << std::endl;
    boost::fusion::for_each(eig_d3, demo::print());
    std::cout << std::endl;
    return EXIT_SUCCESS;
}