c++ - 自定义迭代器适用于 std::sort 但不适用于 tbb::parallel_sort？

Question

我正在尝试使用 tbb::parallel_sort 同时对 2 个数组进行排序。英特尔的文档在这里说 https://software.intel.com/en-us/node/506167 对迭代器和序列的要求与 std::sort 相同。。似乎并非如此。我的自定义迭代器与 std::sort 一起工作得很好，但与 tbb::parallel_sort 一起产生编译错误。请看下面的代码:

int main()//needs boost and tbb to compile
{
    int values_size = 6;
    int nums1[] = {5, 8, 7, 89, 56, 4};
    int nums2[] = {2, 1, 1, 4, 9, 2};

    //WORKS!
    std::sort(do_dual_sort.make_iter(nums1, nums2), 
    do_dual_sort.make_iter(nums1+values_size, nums2+values_size),
    do_dual_sort.make_comp_desc(nums1, nums2));

    //DOESN'T COMPILE
    tbb::parallel_sort(do_dual_sort.make_iter(nums1, nums2), 
    do_dual_sort.make_iter(nums1+values_size, nums2+values_size),
    do_dual_sort.make_comp_desc(nums1, nums2));

    for(unsigned int i = 0; i < values_size; i++) cout << "nums1[" << i << "] " << nums1[i] << " | nums2[" << i << "] "  << nums2[i] << "\n";
    return 0;
}

class dual_sort
{
public:
    template <class T, class T2>
    struct helper_type {
        public:
            typedef boost::tuple<typename iterator_traits<T>::value_type, typename iterator_traits<T2>::value_type> value_type;
            typedef boost::tuple<typename iterator_traits<T>::value_type&, typename iterator_traits<T2>::value_type&> ref_type;
    };

    template <typename T1, typename T2>
    class dual_iterator : public boost::iterator_facade<dual_iterator<T1, T2>,
                                                        typename helper_type<T1, T2>::value_type,
                                                        boost::random_access_traversal_tag,
                                                        typename helper_type<T1, T2>::ref_type> {
    public:
        explicit dual_iterator(T1 iter1, T2 iter2) : mIter1(iter1), mIter2(iter2) {}
        typedef typename iterator_traits<T1>::difference_type difference_type;
    private:
        void increment() { ++mIter1; ++mIter2; }
        void decrement() { --mIter1; --mIter2; }
        bool equal(dual_iterator const& other) const { return mIter1 == other.mIter1; }
        typename helper_type<T1, T2>::ref_type dereference() const { return (typename helper_type<T1, T2>::ref_type(*mIter1, *mIter2)); }
        difference_type distance_to(dual_iterator const& other) const { return other.mIter1 - mIter1; }
        void advance(difference_type n) { mIter1 += n; mIter2 += n; }

        T1 mIter1;
        T2 mIter2;
        friend class boost::iterator_core_access;
    };

    template <typename T1, typename T2>
    dual_iterator<T1, T2> make_iter(T1 t1, T2 t2) { return dual_iterator<T1, T2>(t1, t2); }

    template <class T1, class T2> struct iter_comp_desc {
        typedef typename helper_type<T1, T2>::value_type T;
        bool operator()(const T& t1, const T& t2) const { return get<0>(t1) > get<0>(t2); }
        bool operator()(const char*& t1, const char*& t2) const { return strcmp(get<0>(t1), get<0>(t2)) == 1; }
    };

    template <class T1, class T2> iter_comp_desc<T1, T2> make_comp_desc(T1 t1, T2 t2) { return iter_comp_desc<T1, T2>(); }

} do_dual_sort;

我得到的编译错误是:

error C2512: 'dual_sort::dual_iterator<T1,T2>' : no appropriate default constructor available
with
[
    T1=int *,
    T2=int *
]
tbb44_20150728oss\include\tbb/parallel_sort.h(201) : see reference to function template instantiation 'void tbb::internal::parallel_quick_sort<RandomAccessIterator,Compare>(RandomAccessIterator,RandomAccessIterator,const Compare &)' being compiled
with
[
    RandomAccessIterator=dual_sort::dual_iterator<int *,int *>,
    Compare=dual_sort::iter_comp_desc<int *,int *>
]
main.cpp(1125) : see reference to function template instantiation 'void tbb::parallel_sort<dual_sort::dual_iterator<T1,T2>,dual_sort::iter_comp_desc<T1,T2>>(RandomAccessIterator,RandomAccessIterator,const Compare &)' being compiled
with
[
    T1=int *,
    T2=int *,
    RandomAccessIterator=dual_sort::dual_iterator<int *,int *>,
    Compare=dual_sort::iter_comp_desc<int *,int *>
]

编辑:我使用的编译器是Visual Studio 2012。您可以尝试用 std 替换一些提升函数，使其在 g++ 上运行。

Answer 1

对于 RandomAccessIterator，reference必须是对 value_type 的引用.它不能是引用元组。

因此，您的双迭代器不是有效的 RandomAccessIterator。

许多算法仍然有效，但这并不能使您的代码有效。

要求相同并不意味着任何东西都适用于给定的 std::sort 实现也将与 tbb::parallel_sort 一起工作: std::sort 的给定实现不必强制执行标准中的所有要求。

无论文档如何，如果实现不适用于您的代码，它就不适用于您的代码。

最简单的方法可能是在原始数组中创建一个伪索引对(或迭代器)数组，然后对其进行排序。你只需要覆盖 <