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

Question

I wanted to define a variadic tuple type to represent coordinates. For example, for some magic type:

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

I'd like to have CoordT<3>::coord_type to be the 3-dimensional coordinate type:

std::tuple<double, double, double>

.

But I don't know how to use template programming to generate N repeated doubles.

Can anyone please help explain how to write it?

Answer 1

Use std::make_integer_sequence to generate a pack of the appropriate length, then map the elements to doubles:

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

If you don't literally need a std::tuple, but just need something which acts like a tuple, use std::array:

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

std::array has overloads for 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.