c++ - 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 the source type (some kind of tuple) and check whether each type already exists in my "unique" tuple.

My question is: How can I check whether a tuple (or a boost::fusion container) contains a type?

I'm open to using either the STL or boost.

Answer 1

#include <tuple>
#include <type_traits>

template <typename T, typename Tuple>
struct has_type;

template <typename T>
struct has_type<T, std::tuple<>> : std::false_type {};

template <typename T, typename U, typename... Ts>
struct has_type<T, std::tuple<U, Ts...>> : has_type<T, std::tuple<Ts...>> {};

template <typename T, typename... Ts>
struct has_type<T, std::tuple<T, Ts...>> : std::true_type {};

DEMO

And an additional alias, if the trait itself should be std::true_type or std::false_type :

template <typename T, typename Tuple>
using tuple_contains_type = typename has_type<T, Tuple>::type;