转换的std ::元组到std ::阵列C ++ 11 [英] Convert std::tuple to std::array C++11

问题描述

如果我有的std ::元组LT;双，双，双＆GT; （其中类型是均一的），有一个股票的功能或构造转换为的std ::阵列＆LT;双＆GT; ？

If I have std::tuple<double, double, double> (where the type is homogeneous), is there a stock function or constructor to convert to std::array<double>?

编辑：：我能得到它递归模板code（我的回答下面草案公布）工作。这是处理最好的方法？看起来会有一个股票功能，为这个...或者，如果你有改进，我的回答，我会AP preciate它。我会留下未回答此问题（毕竟，我要的好的方式，不只是一个可行的方法），并且将preFER选择别人的[希望更好]的答案。

: I was able to get it working with recursive template code (my draft answer posted below). Is this the best way to handle this? It seems like there would be a stock function for this... Or if you have improvements to my answer, I'd appreciate it. I'll leave the question unanswered (after all, I want a good way, not just a workable way), and would prefer to select someone else's [hopefully better] answer.

谢谢你的建议。

推荐答案

转换一个元组数组未做使用递归，包括使用完美的转发（仅适用于移动类型有用）：

Converting a tuple to an array without making use of recursion, including use of perfect-forwarding (useful for move-only types):

#include <iostream>
#include <tuple>
#include <array>

template<int... Indices>
struct indices {
    using next = indices<Indices..., sizeof...(Indices)>;
};

template<int Size>
struct build_indices {
    using type = typename build_indices<Size - 1>::type::next;
};

template<>
struct build_indices<0> {
    using type = indices<>;
};

template<typename T>
using Bare = typename std::remove_cv<typename std::remove_reference<T>::type>::type;

template<typename Tuple>
constexpr
typename build_indices<std::tuple_size<Bare<Tuple>>::value>::type
make_indices()
{ return {}; }

template<typename Tuple, int... Indices>
std::array<
  typename std::tuple_element<0, Bare<Tuple>>::type,
    std::tuple_size<Bare<Tuple>>::value
>
to_array(Tuple&& tuple, indices<Indices...>)
{
    using std::get;
    return {{ get<Indices>(std::forward<Tuple>(tuple))... }};
}

template<typename Tuple>
auto to_array(Tuple&& tuple)
-> decltype( to_array(std::declval<Tuple>(), make_indices<Tuple>()) )
{
    return to_array(std::forward<Tuple>(tuple), make_indices<Tuple>());
}

int main() {
  std::tuple<double, double, double> tup(1.5, 2.5, 4.5);
  auto arr = to_array(tup);
  for (double x : arr)
    std::cout << x << " ";
  std::cout << std::endl;
  return 0;
}

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