使用 C++ 中的模板过滤元组类型

Question

I need to check the types of the tuple with type traits. And if the type is appropriate it should stay, if it is not its continues.

For example:

using TUPLE = tuple<int, float,char, short, string, double, float>;
using TUPLE_INTEGRAL = filter_types_t<is_integral<void>, TUPLE>;
TUPLE_INTEGRAL --> tuple<int, char, short>

Up to now, ı did something like this,

template <typename Pred, typename Tuple> struct filter_types;

template <typename t_Predicate, typename ...Ts>
struct filter_types<t_Predicate, std::tuple<Ts...>>
{
    
    template<class E>
    using t_filter_impl = std::conditional_t<
        t_Predicate<E>::value,
        std::tuple<E>, std::tuple<>>;

    
    using filter_types_t = decltype(std::tuple_cat(std::declval<t_filter_impl<Ts>>()...));
};

But it's not compiling and gives an error.

Answer 1

Your code was almost correct. With these 2 things changed it works:

• t_Predicate needs to be a template template type since it is used as a template in the implementation.
• The using declaration needs to be split into 2 parts. Or at least that's how it is usually done and how you use it in the example.

Here is my working version:

#include <tuple>
#include <string>


template <template<typename>  typename t_Predicate, typename ...Ts> struct filter_types;

template <template<typename>  typename t_Predicate, typename ...Ts>
struct filter_types<t_Predicate, std::tuple<Ts...>>
{
    
    template<class E>
    using t_filter_impl = std::conditional_t<
        t_Predicate<E>::value,
        std::tuple<E>, std::tuple<>>;

    
    using type = decltype(std::tuple_cat(std::declval<t_filter_impl<Ts>>()...));
};

template <template<typename>  typename t_Predicate, typename ...Ts>
using filter_types_t = typename filter_types<t_Predicate, Ts...>::type;


using TUPLE = std::tuple<int, float,char, short, std::string, double, float>;
using TUPLE_INTEGRAL = filter_types_t<std::is_integral, TUPLE>;
static_assert(std::is_same_v<TUPLE_INTEGRAL, std::tuple<int, char, short>>);
static_assert(! std::is_same_v<TUPLE_INTEGRAL, std::tuple<int, float, char, short>>);