Question

使用range-v3 library（由@EricNiebler），使编写算法代码更紧凑，例如这是如何生成一堆随机数：

#include <range/v3/all.hpp>
#include <iostream>
#include <vector>

int main() 
{
    using namespace ranges;

    auto const N = 10;
    std::vector<int> v; 
    v.reserve(N);

    v |= action::push_back(view::iota(0, N)); 
    random_shuffle(v);
    copy(v, ostream_iterator<>(std::cout, ","));
}

Live Example.

但是，我更倾向于使用假设action::random_shuffle()来扩展管道

v |= action::push_back(view::iota(0, N)) | action::random_shuffle();

这是我尝试编写这样的动作（不幸的是，编写新的range-v3代码比使用库更加冗长）

#include <functional> // bind, placeholders::_1

namespace ranges
{
    inline namespace v3
    {
        /// \addtogroup group-actions
        /// @{
        namespace action
        {
            struct random_shuffle_fn
            {
            private:
                friend action_access;

                static auto bind(random_shuffle_fn random_shuffle)
                RANGES_DECLTYPE_AUTO_RETURN
                (
                    std::bind(random_shuffle, std::placeholders::_1)
                )

                template<typename Gen>
                static auto bind(random_shuffle_fn random_shuffle, Gen && rand)
                RANGES_DECLTYPE_AUTO_RETURN
                (
                    std::bind(random_shuffle, std::placeholders::_1, bind_forward<Gen>(rand))
                )
            public:
                struct ConceptImpl
                {
                    template<typename Rng,
                        typename I = range_iterator_t<Rng>>
                    auto requires_(Rng&&) -> decltype(
                        concepts::valid_expr(
                            concepts::model_of<concepts::RandomAccessRange, Rng>(),
                            concepts::is_true(Permutable<I>())
                        ));
                };

                template<typename Rng>
                using Concept = concepts::models<ConceptImpl, Rng>;

                template<typename Rng,
                    CONCEPT_REQUIRES_(Concept<Rng>())>
                Rng operator()(Rng && rng) const
                {
                    ranges::random_shuffle(rng);
                    return std::forward<Rng>(rng);
                }

                template<typename Rng, typename Gen,
                    CONCEPT_REQUIRES_(Concept<Rng>())>
                Rng operator()(Rng && rng, Gen && rand) const
                {
                    ranges::random_shuffle(rng, std::forward<Gen>(rand));
                    return std::forward<Rng>(rng);
                }

                #ifndef RANGES_DOXYGEN_INVOKED
                template<typename Rng>
                void operator()(Rng &&) const
                {
                    CONCEPT_ASSERT_MSG(RandomAccessRange<Rng>(),
                        "The object on which action::random_shuffle operates must be a model of the "
                        "RandomAccessRange concept.");
                    using I = range_iterator_t<Rng>;
                    CONCEPT_ASSERT_MSG(Permutable<I>(),
                        "The iterator type of the range passed to action::random_shuffle must allow its "
                        "elements to be permuted; that is, the values must be movable and the "
                        "iterator must be mutable.");
                }
            #endif
            };

            /// \ingroup group-actions
            /// \relates sort_fn
            /// \sa `action`
            namespace
            {
                constexpr auto&& random_shuffle = static_const<action<random_shuffle_fn>>::value;
            }
        }
        /// @}
    }
}

Live Example无法编译，因为找不到某些隐藏在某处的operator()。

据我所见，我忠实地从类似的代码中转换了上述代码，例如action::sort()。唯一的区别是random_shuffle()有两个重载（一个采用随机生成器），而所有其他动作（包括sort）都有一个重载，其额外参数的默认值（比较器，谓词，投影仪等）。这转换为上面bind()的两个random_shuffle_fn静态成员函数，而所有其他操作只有一个bind()重载。

问题：如何为random_shuffle写一个范围-v3动作？

Answer 1

你有random_shuffle_function::operator()(Rng&&)的两个模糊重载，你的＆＃34;错误捕获＆＃34;需要限制过载只接受那些正确的重载拒绝的参数（我们真的需要C ++概念所以我再也不用SFINAE约束重载了）：

#ifndef RANGES_DOXYGEN_INVOKED
template<typename Rng,
    CONCEPT_REQUIRES_(!Concept<Rng>())>
void operator()(Rng &&) const
{
    CONCEPT_ASSERT_MSG(RandomAccessRange<Rng>(),
        "The object on which action::random_shuffle operates must be a model of the "
        "RandomAccessRange concept.");
    using I = range_iterator_t<Rng>;
    CONCEPT_ASSERT_MSG(Permutable<I>(),
        "The iterator type of the range passed to action::random_shuffle must allow its "
        "elements to be permuted; that is, the values must be movable and the "
        "iterator must be mutable.");
}
#endif

此外，您需要管道action::random_shuffle：

v |= action::push_back(view::iota(0, N)) | action::random_shuffle;

DEMO

Answer 2

git的最新版本已包含action::shuffle。它可以使用如下：

#include <random>
std::mt19937 gen;
...
v |= action::push_back(view::iota(0, N)) | action::shuffle(gen);

如何为random_shuffle编写range-v3动作？

2 个答案: