C++ Insights

Source:

#include <tuple>
#include <type_traits>

template <class Haystack, class Needle>
struct contains;

template <class Car, class... Cdr, class Needle>
struct contains<std::tuple<Car, Cdr...>, Needle> : contains<std::tuple<Cdr...>, Needle>
{};

template <class... Cdr, class Needle>
struct contains<std::tuple<Needle, Cdr...>, Needle> : std::true_type
{};

template <class Needle>
struct contains<std::tuple<>, Needle> : std::false_type
{};

template <class Out, class In>
struct filter;

template <class... Out, class InCar, class... InCdr>
struct filter<std::tuple<Out...>, std::tuple<InCar, InCdr...>>
{
  using type = typename std::conditional<
    contains<std::tuple<Out...>, InCar>::value
    , filter<std::tuple<Out...>, std::tuple<InCdr...>>
    , filter<std::tuple<Out..., InCar>, std::tuple<InCdr...>>
  >::type::type;
};

template <class Out>
struct filter<Out, std::tuple<>>
{
  using type = Out;
};

template <class T>
using without_duplicates = typename filter<std::tuple<>, T>::type;

int main()
{
    static_assert(std::is_same<without_duplicates<std::tuple<int, char>>, std::tuple<int, char>>::value);
    static_assert(std::is_same<without_duplicates<std::tuple<int, char, int>>, std::tuple<int, char>>::value);
    static_assert(std::is_same<without_duplicates<std::tuple<int, int, char>>, std::tuple<int, char>>::value);
    static_assert(std::is_same<without_duplicates<std::tuple<int, char, char, int, char>>, std::tuple<int, char>>::value);
}

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