Associating an array with a variadic template - c++

Question

I'm now learning a little about templates and templates in C++11, C++14 and C++1z. I'm trying to write a variadic class template with an inside class that will associate an int to every template argument - and have a constexpr method that returns its array representation.
Let's say that I have ensured that the template cannot receive two of the same type as an argument. I was thinking about doing it somewhat like this:
template <typename... Types>
struct MyVariadicTemplate {
//we know that all types in Types... are different
template <int... Values>
struct MyInnerTemplate {
//I need to make sure that sizeof...(Values) == sizeof...(Types)
constexpr std::array<int, sizeof...(Values)> to_array() {
std::array<int, sizeof...(Values)> result = {Values...};
return result;
// this is only valid since C++14, as far as I know
}
};
};
this code should be valid (if it's not, I'd love to know why). Now, I'd like to add another inner template:
template <typedef Type>
struct AnotherInnerTemplate {};
that has a public typedef, which represents MyInnerTemplate with one on the position of Type in Types... and zeros elsewhere - and here I'm lost. I don't know how to proceed
I would appreciate any hint on how that can be done - and if I'm heading towards the wrong direction, I hope somebody can give me a hint on how to do that.

Answer 1

I think what you're looking for is something like this.
#include <array>
#include <cstddef>
#include <iostream>
#include <type_traits>
template <typename NeedleT, typename... HaystackTs>
constexpr auto get_type_index_mask() noexcept
{
constexpr auto N = sizeof...(HaystackTs);
return std::array<bool, N> {
(std::is_same<NeedleT, HaystackTs>::value)...
};
}
template <typename T, std::size_t N>
constexpr std::size_t ffs(const std::array<T, N>& array) noexcept
{
for (auto i = std::size_t {}; i < N; ++i)
{
if (array[i])
return i;
}
return N;
}
int
main()
{
const auto mask = get_type_index_mask<float, bool, int, float, double, char>();
for (const auto& bit : mask)
std::cout << bit;
std::cout << "\n";
std::cout << "float has index " << ffs(mask) << "\n";
}
Output:
00100
float has index 2
The magic happens in the parameter pack expansion
(std::is_same<NeedleT, HaystackTs>::value)...
where you test each type in HaystackTs against NeedleT. You might want to apply std::decay to either type if you want to consider, say, const int and int the same type.

Answer 2

template <int size, int... Values> struct AnotherImpl {
using Type = typename AnotherImpl<size - 1, Values..., 0>::Type;
};
template <int... Values> struct AnotherImpl<0, Values...> {
using Type = Inner<Values...>;
};
template <class T> struct Another {
using Type = typename AnotherImpl<sizeof...(Types) - 1, 1>::Type;
};
Full:
template <class... Types> struct My {
template <int... Values> struct Inner {
constexpr std::array<int, sizeof...(Values)> to_array() {
return std::array<int, sizeof...(Values)>{Values...};
}
};
template <int size, int... Values> struct AnotherImpl {
using Type = typename AnotherImpl<size - 1, Values..., 0>::Type;
};
template <int... Values> struct AnotherImpl<0, Values...> {
using Type = Inner<Values...>;
};
template <class T> struct Another {
using Type = typename AnotherImpl<sizeof...(Types) - 1, 1>::Type;
};
};
auto main() -> int {
My<int, float, char>::Another<int>::Type s;
auto a = s.to_array();
for (auto e : a) {
cout << e << " ";
}
cout << endl;
return 0;
}
prints:
1 0 0
Is this what you want?