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; 

Answer 2

In C++17 you can do it like this:

template <typename T, typename Tuple> struct has_type;  template <typename T, typename... Us> struct has_type<T, std::tuple<Us...>> : std::disjunction<std::is_same<T, Us>...> {}; 

In C++11 you have to roll your own or / disjunction. Here's a full C++11 version, with tests:

#include <tuple> #include <type_traits>  template<typename... Conds> struct or_ : std::false_type {};  template<typename Cond, typename... Conds> struct or_<Cond, Conds...> : std::conditional<Cond::value, std::true_type, or_<Conds...>>::type {};  /* // C++17 version: template<class... B> using or_ = std::disjunction<B...>; */    template <typename T, typename Tuple> struct has_type;  template <typename T, typename... Us> struct has_type<T, std::tuple<Us...>> : or_<std::is_same<T, Us>...> {};  // Tests static_assert(has_type<int, std::tuple<>>::value == false, "test"); static_assert(has_type<int, std::tuple<int>>::value == true, "test"); static_assert(has_type<int, std::tuple<float>>::value == false, "test"); static_assert(has_type<int, std::tuple<float, int>>::value == true, "test"); static_assert(has_type<int, std::tuple<int, float>>::value == true, "test"); static_assert(has_type<int, std::tuple<char, float, int>>::value == true, "test"); static_assert(has_type<int, std::tuple<char, float, bool>>::value == false, "test"); static_assert(has_type<const int, std::tuple<int>>::value == false, "test"); // we're using is_same so cv matters static_assert(has_type<int, std::tuple<const int>>::value == false, "test"); // we're using is_same so cv matters