How can I make this variadic template code shorter

问题:

This is a code snippet that I am going to use in order to check whether the variadic template types are unique:

template <typename...>
struct is_one_of;

template <typename F>
struct is_one_of<F> {
    static constexpr bool value = false;
};

template <typename F, typename S, typename... T>
struct is_one_of<F, S, T...> {
    static constexpr bool value =
        std::is_same<F, S>::value || is_one_of<F, T...>::value;
};

template <typename...>
struct is_unique;

template <>
struct is_unique<> {
    static constexpr bool value = true;
};

template <typename F, typename... T>
struct is_unique<F, T...> {
    static constexpr bool value =
        is_unique<T...>::value && !is_one_of<F, T...>::value;
};

int main() {
    constexpr bool b = is_unique<bool, int, double>::value;
    constexpr bool c = is_unique<int, char, int>::value;
    static_assert(b == true && c == false, "!");
}

Is there any way to make this code shorter and/or more concise using features introduced in C++14 and C++1z? Or is there a better way to achieve the same effect using the new features?

In the case of C++1z I mean: features that are already available in the newest versions of Clang and GCC.

回答1:

#include <type_traits>

template <typename F, typename... Ts>
constexpr bool is_one_of = (std::is_same<F, Ts>{} || ...);

template <typename...>
constexpr bool is_unique = true;

template <typename F, typename... Ts>
constexpr bool is_unique<F, Ts...> = is_unique<Ts...> && !is_one_of<F, Ts...>;

DEMO

回答2:

We recently added std::disjunction to the C++1z draft, which can be used for is_one_of (and it stops instantiating as soon as it finds a match, see the link for more details):

template <typename F, typename... T>
  using is_one_of = std::disjunction<is_same<F, T>...>;

This is already implemented in GCC trunk. For older versions of GCC you can use the implementation detail __or_ instead:

template <typename F, typename... T>
  using is_one_of = std::__or_<is_same<F, T>...>;

Or implement disjunction by hand using C++11 facilities, as shown at the end of the proposal linked to above.

回答3:

I'd (now) suggest using the std::conj/disj/nega family of STL functions:

#include <type_traits>

template <typename H, typename... T>
struct is_one_of : std::disjunction<std::is_same<H, T>...> {};

template <typename H, typename... T>
struct is_unique : std::conjunction<std::negation<std::is_same<H, T>>..., is_unique<T...>> {};

template <typename H>
struct is_unique<H> : std::true_type {};

int main()
{
    static_assert(is_one_of<int, char, double, int, bool>::value);
    static_assert(is_unique<int, char, double, bool>::value);
    static_assert(!is_unique<int, int, char, double, bool>::value);
}

---

When fold-expressions, which were designed for these cases, are released into the language this will become trivial:

namespace stx = std::experimental;

template <typename H, typename... T>
struct is_one_of {
    static constexpr bool value = (stx::is_same_v<H, T> || ...);
};

template <typename H, typename... T>
struct is_unique {
    static constexpr bool value = (!stx::is_same_v<H, T> && ... && is_unique<T...>::value);
};

template <typename H>
struct is_unique<H> : std::true_type {};

回答4:

I'm in line with Brian Rodriguez's and Piotr Scontnincki's answers, as far as it concerns the fold expressions part. Until folding expressions are in, you could shrink the existing code a little bit by getting rid of the incomplete primary templates as follows:

template <typename...>
struct is_one_of {
    static constexpr bool value = false;
};

template <typename F, typename S, typename... T>
struct is_one_of<F, S, T...> {
    static constexpr bool value =
        std::is_same<F, S>::value || is_one_of<F, T...>::value;
};

template <typename...>
struct is_unique {
    static constexpr bool value = true;
};

template <typename F, typename... T>
struct is_unique<F, T...> {
    static constexpr bool value = is_unique<T...>::value && !is_one_of<F, T...>::value;
};