How do I find out if a tuple contains a type?

Question

Suppose I want to create a compile-time heterogenous container of unique types from some sequence of non-unique types. In order to do this I need to iterate over th

Answer 1

Here is a version that does not recursively instantiate the template to check for a matching type. Instead it uses SFINAE with indices-based meta-programming:

#include 
#include 

template 
struct index_sequence {
    typedef index_sequence next;
};

template 
struct make_index_sequence {
    typedef typename make_index_sequence::type::next type;
};

template <>
struct make_index_sequence<0> {
    typedef index_sequence<> type;
};

template 
using make_index_sequence_t = typename make_index_sequence::type;

template 
struct lookup;

template 
struct lookup>
{
private:
    struct null;

    template 
    static std::false_type
    apply(std::conditional_t::value, null, Args>...);

    template 
    static std::true_type apply(...);

    template 
    static auto apply_helper(Args&&...) ->
    decltype(apply...>(std::declval()...));
public:
    template 
    using value = decltype(
        apply_helper(
            std::declval<
                typename std::tuple_element::type
            >()...
        )
    );
};

template 
using has_type = decltype(
    typename lookup::value>
    >::template value{}
);

Live Demo