使用
range-v3 library(由@EricNiebler),使得编写算法代码更加紧凑,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,","));
}
但是,我更愿意使用假设的动作:: 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,因为某些operator()深藏在某处未找到.
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) .这转换为上面的random_shuffle_fn的两个bind()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编写range-v3动作？
解决方法
你有两个不明确的random_shuffle_function :: operator()(Rng&&)重载,你的“错误捕获”重载需要被限制为只接受那些正确的重载拒绝的参数(我们真的需要C Concepts所以我再也没有了到SFINAE约束重载)：
#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,您需要管道操作:: random_shuffle：
v |= action::push_back(view::iota(0,N)) | action::random_shuffle;
