Filter types in a parameter pack

Question

Im trying create a filtered type of a variadic template/parameter pack and also preserve ordering.

// example what im trying to accomplish
template<typename... Args>
struct query
{
    using filtered = typename filtered<std::is_integral_v, Args...>
}

In this example filtered would filter all integral types eg.

query<int, A, B>::filtered == query<A, B>::filtered

How can I create a util like this or something that would achieve the same results

Answer 1

Comment by @Jarod42 has demo with working code

#include <tuple>
#include <type_traits>

template<template<class> class, template<class...> class, class...>
struct filter;
template<template<class> class Pred, template<class...> class Variadic>
struct filter<Pred, Variadic>
{
    using type = Variadic<>;
};

template<template<class> class Pred,
         template<class...> class Variadic,
         class T,
         class... Ts>
struct filter<Pred, Variadic, T, Ts...>
{
    template<class, class>
    struct Cons;
    template<class Head, class... Tail>
    struct Cons<Head, Variadic<Tail...>>
    {
        using type = Variadic<Head, Tail...>;
    };

    using type = typename std::conditional<
      Pred<T>::value,
      typename Cons<T, typename filter<Pred, Variadic, Ts...>::type>::type,
      typename filter<Pred, Variadic, Ts...>::type>::type;
};

And integration with my example

template<typename... Args>
struct query
{
    using filtered = filter<std::is_integral, std::tuple, Args...>::type
}