How to filter duplicate types from tuple C++

Question

How does one filter duplicate types from a tuple?

For example:

using Tuple = std::tuple<int, double, int, double, std::string, std::string>
using FilteredTuple = without_duplicates<Tuple>;

In which without_duplicates is implemented in such a way that it generates the following FilteredTuple type:

std::tuple<int, double, std::string>

Answer 1

#include <type_traits>
#include <tuple>

template <typename T, typename... Ts>
struct unique : std::type_identity<T> {};

template <typename... Ts, typename U, typename... Us>
struct unique<std::tuple<Ts...>, U, Us...>
    : std::conditional_t<(std::is_same_v<U, Ts> || ...)
                       , unique<std::tuple<Ts...>, Us...>
                       , unique<std::tuple<Ts..., U>, Us...>> {};

template <typename... Ts>
using unique_tuple = typename unique<std::tuple<>, Ts...>::type;

演示

Answer 2

这项应有的工作：

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
    , typename filter<std::tuple<Out...>, std::tuple<InCdr...>>::type
    , typename 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;

[现场示例]

它通过迭代构造输出元组来工作。在添加每种类型之前，请检查（使用谓词contains）它是否已经在输出元组中。如果没有，则添加（的“ else”分支std::conditional），否则不添加（的“ then”分支std::conditional）。