BOOST_FUSION_ADAPT_TPL_STRUCT 和模板数组大小

Question

感谢BOOST_FUSION_ADAPT_TPL_STRUCT，我正在尝试迭代 C++ 模板结构。我的结构包含固定大小的多维数组，其大小是模板参数。如果我们考虑修改 Boost 的示例以适应我的问题：

#include <iostream>
#include <string>
#include <boost/fusion/adapted/struct/adapt_struct.hpp>
#include <boost/fusion/include/adapt_struct.hpp>

// Example:
// http://www.boost.org/doc/libs/1_53_0/libs/fusion/doc/html/fusion/adapted/adapt_tpl_struct.html

namespace demo
{
    template<typename T, unsigned int SIZE1, unsigned int SIZE2, typename Name, typename Age>
    struct employee
    {
        Name name;
        Age age;
        T ar[SIZE1][SIZE2];
    };
}

// Any instantiated demo::employee is now a Fusion sequence
BOOST_FUSION_ADAPT_TPL_STRUCT(
    (T)(SIZE1)(SIZE2)(Name)(Age),
    (demo::employee) (T)(SIZE1)(SIZE2)(Name)(Age),
    (Name, name)
    (Age, age)
    (T, ar[SIZE1][SIZE2]))

int main()
{
    demo::employee<float, 2, 2, std::string, int> e;
    e.name = "Bob";
    e.age = 25;
    e.ar[0][0] = e.ar[1][0] = 0.1;
    e.ar[0][1] = e.ar[1][1] = 0.2;
}

编译失败。此外，如果我们只添加一个整数模板参数而不使用它作为数组大小，它也会失败。

BOOST_FUSION_ADAPT_TPL_STRUCT 也可以吗？如果没有，我该怎么办？

Answer 1

在与 Evgeny Panasyuk 进行了长时间的交谈后，我最终做了一些不同的事情。由于我希望能够对数据结构进行一些简单的算术运算，因此我决定使用Eigen::Map 而不是Boost::multi_array，因为它提供了广泛的运算符以及清晰的文档。

因此，较高级别的循环由boost::fusion::for_each 处理，较低级别的循环由 Eigen 处理。数组线性映射到特征向量。大小在data_eigen 的构造函数中传递。

#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/bind.hpp>

#include <Eigen/Core>

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;
    }
};

}

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> d;
    for (unsigned int i = 0; i < SIZE1; ++i)
        for (unsigned int j = 0; j < SIZE2; ++j)
        {
            d.ar1[i][j] = (i+1)*(j+1);
            d.ar2[i][j] = i + j;
        }

    // Eigen::Map + BOOST_FUSION_ADAPT_TPL_STRUCT
    demo::data_eigen<REALTYPE> eig_d(d);

    std::cout << "d:" << std::endl;
    boost::fusion::for_each(eig_d, demo::print());
    std::cout << std::endl;

    boost::fusion::for_each(eig_d, boost::bind<void>(demo::scalarMult(), _1, 2.0));
    std::cout << "2 * d:" << std::endl;
    boost::fusion::for_each(eig_d, demo::print());
    std::cout << std::endl;
}

Answer 2

来自doc：

序列 (template_param0)(template_param1)... 声明了使用的模板类型参数的名称。

虽然你有非类型模板参数 SIZE:

template<typename T, unsigned int SIZE, typename Name, typename Age>
struct employee

您可以将其转换为类型模板参数并使用boost::mpl::int_ 作为进位大小的包装器。

现在，您的代码是 compiled。

template<int Size>
struct Array
{
    template<typename T>
    struct Of
    {
        typedef T type[Size];
    };
};

namespace demo
{
    template<typename T, typename SIZE, typename Name, typename Age>
    struct employee
    {
        Name name;
        Age age;
        T ar[SIZE::value];
    };
}

// Any instantiated demo::employee is now a Fusion sequence
BOOST_FUSION_ADAPT_TPL_STRUCT(
    (T)(SIZE)(Name)(Age),
    (demo::employee) (T)(SIZE)(Name)(Age),
    (Name, name)
    (Age, age)
    (typename Array<SIZE::value>::template Of<T>::type, ar))

 //...
 demo::employee<float, int_<2>, std::string, int> e;