在 C++17 中定义可变坐标（元组）类型？

Question

我想定义一个可变元组类型来表示坐标。例如，对于某些魔法类型：

template <unsigned int N>
struct CoordT {
  typedef std::tuple<_some_magic_> coord_type;
};

我希望 CoordT<3>::coord_type 成为 3 维坐标类型：

std::tuple<double, double, double>

.

但是我不知道如何使用模板编程来生成N 重复doubles.

谁能帮忙解释一下怎么写？

Answer 1

使用std::make_integer_sequence生成一个适当长度的包，然后将元素映射到双打：

template <size_t n>
struct TupleOfDoubles {
    template <size_t... i>
    static auto foo(std::index_sequence<i...>) {
        return std::make_tuple(double(i)...);
    }
    using type = decltype(foo(std::make_index_sequence<n>{}));
};

http://coliru.stacked-crooked.com/a/7950876813128c55

Answer 2

来不及玩了？

如果您可以接受如下声明（不需要定义）可变参数模板函数

template <std::size_t ... Is>
constexpr auto toIndexSeq (std::index_sequence<Is...> a)
   -> decltype(a);

并且coord_type是在CoordT特化中定义的，你可以这样写

template <std::size_t N,
          typename = decltype(toIndexSeq(std::make_index_sequence<N>{}))>
struct CoordT;

template <std::size_t N, std::size_t ... Is>
struct CoordT<N, std::index_sequence<Is...>>
 { using coord_type = std::tuple<decltype((void)Is, 0.0)...>; };

以下是完整的C++14编译示例

#include <tuple> 
#include <type_traits> 

template <std::size_t ... Is>
constexpr auto toIndexSeq (std::index_sequence<Is...> a)
   -> decltype(a);

template <std::size_t N,
          typename = decltype(toIndexSeq(std::make_index_sequence<N>{}))>
struct CoordT;

template <std::size_t N, std::size_t ... Is>
struct CoordT<N, std::index_sequence<Is...>>
 { using coord_type = std::tuple<decltype((void)Is, 0.0)...>; };


int main()
 {
   using t0 = std::tuple<double, double, double, double>;
   using t1 = typename CoordT<4u>::coord_type;

   static_assert( std::is_same<t0, t1>::value, "!" );
 }

Answer 3

如果您实际上不需要 std::tuple，而只需要像元组一样的东西，请使用 std::array:

template <unsigned int N>
struct CoordT {
  typedef std::array<double, N> coord_type;
};

std::array 有 std::get<I>, std::tuple_size, and std::tuple_element. Most library and language facilities which accept a tuple-like element will support std::array, such as std::apply and structured bindings.

的重载

Answer 4

一种非常简洁的方法是使用 std::tuple_cat and std::array:

template <unsigned int N>
struct CoordT {
  using coord_type = decltype(std::tuple_cat(std::array<double, N>{}));
};

std::tuple_cat 是 allowed to support tuple-like types such as std::array, but not guaranteed. However, every implementation I checked supports this.

在 C++17 中定义可变坐标（元组）类型？

defining a variadic coordinate (tuple) type in C++17?

c++

tuples

template-meta-programming

variadic-templates

c++17