c++ - 如何在运行时构建 multi_array::index_gen

Question

在下面的代码中，ExtractSubArray 函数是完全通用的，而 ExtractSubArrayCornerAndExtent 需要在编写代码时了解维度(以构造RangeType 参数)。有什么方法可以编写通用的 ExtractSubArrayCornerAndExtent(没有 SFINAE 为每个维度使用不同的函数(这会很烦人，并且需要一组固定的可能维度)。

#include <boost/multi_array.hpp>

template <unsigned int Dimension>
boost::multi_array<double, Dimension> ExtractSubArray(const boost::multi_array<double, Dimension>& array, const typename boost::detail::multi_array::index_gen<Dimension, Dimension>& indices)
{
    using ArrayType = boost::multi_array<double, Dimension>;
    using IndexType = boost::array<double, Dimension>;

    typename ArrayType::template const_array_view<Dimension>::type view = array[indices];

    IndexType subArraySize;
    for(size_t dim = 0 ; dim < Dimension; ++dim) {
        subArraySize[dim] = indices.ranges_[dim].finish_ - indices.ranges_[dim].start_;
    }

    ArrayType subArray = view;

    return subArray;
}

template <unsigned int Dimension>
boost::multi_array<double, Dimension> ExtractSubArrayCornerAndExtent(const boost::multi_array<double, Dimension>& array, const boost::array<double, Dimension>& corner,
                              const boost::array<double, Dimension>& subarraySize)
{
    using ArrayType = boost::multi_array<double, Dimension>;
    using RangeType = typename ArrayType::index_range;

    // Here I have assumed Dimension=3 to produce the second argument. How do you construct this second argument when you don't know Dimension ahead of time.
    return ExtractSubArray<Dimension>(array, boost::indices[RangeType(corner[0],subarraySize[0])][RangeType(corner[1],subarraySize[1])][RangeType(corner[2],subarraySize[2])]);
}

int main()
{
    using ArrayType = boost::multi_array<double, 3>;
    using IndexType = boost::array<double, 3>;

    ArrayType myArray(IndexType({{3,3,3}}));

    std::vector<double> data(9);
    for(size_t i = 0; i < data.size(); ++i) {
        data[i] = i;
    }

    boost::detail::multi_array::index_gen<3,3> indices = boost::indices[ArrayType::index_range(0,1)][ArrayType::index_range(0,1)][ArrayType::index_range(0,1)];

    ArrayType subArray = ExtractSubArray<3>(myArray, indices);

    IndexType corner = {0,0,0};
    IndexType subarraySize = {1,1,1};
    ArrayType subArray2 = ExtractSubArrayCornerAndExtent<3>(myArray, corner, subarraySize);

    return 0;
}

Answer 1

编译时迭代通常必须用递归来实现。您可以使用递归模板构建索引对象。它看起来像这样:

#include <boost/multi_array.hpp>

template <typename T, size_t Dimension>
boost::multi_array<T, Dimension> ExtractSubArray(
   const boost::multi_array<T, Dimension>& array,
   const typename boost::detail::multi_array::index_gen<Dimension, Dimension>& indices)
{
   using ArrayType = boost::multi_array<T, Dimension>;
   using IndexType = boost::array<size_t, Dimension>;

   typename ArrayType::template const_array_view<Dimension>::type view = array[indices];

   IndexType subArraySize;
   for(size_t dim = 0 ; dim < Dimension; ++dim) {
      subArraySize[dim] = indices.ranges_[dim].finish_ - indices.ranges_[dim].start_;
   }

   ArrayType subArray = view;

   return subArray;
}

// Helper functor to build indices.
template<typename RangeArrayType, size_t Dimension>
struct IndicesBuilder {
   // Recursively invoke the functor for the next lowest dimension and
   // add the next range.
   static auto build(const RangeArrayType& range)
      -> decltype(IndicesBuilder<RangeArrayType, Dimension - 1>::build(range)[range[Dimension - 1]]) {
      return IndicesBuilder<RangeArrayType, Dimension - 1>::build(range)[range[Dimension - 1]];
   }
};

// Helper functor specialization to terminate recursion.
template<typename RangeArrayType>
struct IndicesBuilder<RangeArrayType, 1> {
   static auto build(const RangeArrayType& range)
      -> decltype(boost::indices[range[0]]) {
      return boost::indices[range[0]];
   }
};

template <typename T, size_t Dimension>
boost::multi_array<T, Dimension> ExtractSubArrayCornerAndExtent(
   const boost::multi_array<T, Dimension>& array,
   const boost::array<size_t, Dimension>& corner,
   const boost::array<size_t, Dimension>& subarraySize)
{
    using ArrayType = boost::multi_array<T, Dimension>;
    using RangeType = typename ArrayType::index_range;

    // Build a random-access container with the ranges.
    std::vector<RangeType> range;
    for (size_t i = 0; i < Dimension; ++i)
       range.push_back(RangeType(corner[i], corner[i] + subarraySize[i]));

    // Use the helper functor to build the index object.
    const auto index = IndicesBuilder<decltype(range), Dimension>::build(range);

    return ExtractSubArray<T, Dimension>(array, index);
}

int main() {
   using ArrayType = boost::multi_array<double, 3>;
   using IndexType = boost::array<size_t, 3>;

   ArrayType myArray(IndexType({{3,3,3}}));

   IndexType corner = {0,0,0};
   IndexType subarraySize = {1,1,1};
   ArrayType subArray2 = ExtractSubArrayCornerAndExtent(myArray, corner, subarraySize);

   return 0;
};

添加到您的代码中的是 IndicesBuilder 模板结构，它具有用于构建索引对象的一维的静态函数。该静态函数调用自身(使用不同的模板参数)来构建所有维度。递归由返回第一个维度的 IndicesBuilder 的特化终止。尾随返回类型万岁!

我编译并运行了这段代码，没有出现 fatal error ，但我没有做任何进一步的测试，因此可能存在一些小错误。不过，一般方法应该可行。