check variadic templates parameters for uniqueness

I want variadic template parameters must unique. I know when multi inheritance, identical classes inheritance is not allowed.

struct A{};
struct B: A, A{}; // error

Using this rule, I made a little code.

#include <type_traits>

template< class T> struct id{};
template< class ...T> struct base_all : id<T> ... {};

template< class ... T>
struct is_unique
{
     template< class ... U>
 static constexpr bool test( base_all<U...> * ) noexcept { return true; }

template< class ... U>
static constexpr bool test( ... ) noexcept { return false;}


static constexpr bool value = test<T...>(0);
};

int main()
{
    constexpr bool b = is_unique<int, float, double>::value; // false -- Why?
    constexpr bool c = is_unique< int, char, int>::value; // false

   static_assert( b == true && c == false , "!");// failed.
}

But my program is not worked as I expected. What's wrong?

//UPDATE: //Thanks, I fix my error: //

//     #include <type_traits>
//     #include <cstddef>
//    
//     template< class ... U> struct pack{};
//    
//     template< class T> struct id{};
//     template< class T> struct base_all;
//     template< class ... T> struct base_all< pack<T...> > : id<T>  ... {};
//        
//     
//    
//     template< class ... T>
//     struct is_unique
//     {
//           template< class P,  std::size_t  =  sizeof(base_all<P>) >
//          struct check;
//     
//       template< class ...U>
//      static constexpr bool test(check< pack<U...> > * ) noexcept { return true;}
//        
//        template< class ... U>
//        static constexpr bool test(...)noexcept { return false;}
//        
//        static constexpr bool value =  test<T...>(0);
//        };
//        
//        int main()
//        {
//            constexpr bool b = is_unique<int, float, double>::value; // true
//            constexpr bool c = is_unique< int, char, int>::value; // false
//             
//          static_assert( b == true && c == false , "!");// success.
//        }
//

Q: somebody can explain, why it's failed?

UPDATE2: My previous update was illegal :)). Legal form, but it compiled O(N) time.

#include <cstddef>
#include <iostream>
#include <type_traits>

namespace mpl
{

template< class T > using invoke = typename T :: type ;

template< class C, class I, class E > using if_t     = invoke< std::conditional< C{}, I, E> >;

template< class T > struct id{};
struct empty{};

template< class A, class B > struct base : A, B {};

template< class B , class ... > struct is_unique_impl;

template< class B > struct is_unique_impl<B>: std::true_type{};

template< class B, class T, class ... U>
struct is_unique_impl<B, T, U...> : if_t< std::is_base_of< id<T>, B>, std::false_type, is_unique_impl< base<B,id<T>>, U...> >{};


template< class ...T >struct is_unique : is_unique_impl< empty, T ... > {};



} // mpl    

int main()
{
    constexpr bool b = mpl::is_unique<int, float, double>::value;

    constexpr bool c = mpl::is_unique< int, char, int > :: value;

    static_assert( b == true   , "!");
    static_assert( c == false, "!");

    return 0;

}

Solution 1:

Passing a pointer to base_all<U...> merely requires the existence of a declaration of base_all<U...>. Without attempting the to access the definition, the compiler won't detect that the type is actually ill-defined. One approach to mitigate that problem would be to use an argument which requires a definition of base_all<U...>, e.g.:

template< class ...T> struct base_all
   : id<T> ...
{
    typedef int type;
};
// ...
template< class ... U>
static constexpr bool test(typename base_all<U...>::type) noexcept
{
    return true;
}

Although the above answers the question, it fail to compile: the multiple inheritance created isn't in a suitable context to be considered for SFINAE. I don't think you can leverage the rule on not allowing the same base inherited from twice. The relevant test can be implemented differently, though:

#include <type_traits>

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

Solution 2:

Another O(logN) instantiation depth solution. It still needs a major cleanup, comments, namespaces, renaming, and reduced code duplication.

Kudos again to Xeo, whose O(logN) instantiation depth version of gen_seq this (again) relies upon.

#include <cstddef>

    // using aliases for cleaner syntax
    template<class T> using Invoke = typename T::type;

    template<std::size_t...> struct seq{ using type = seq; };

    template<class S1, class S2> struct concat;

    template<std::size_t... I1, std::size_t... I2>
    struct concat<seq<I1...>, seq<I2...>>
      : seq<I1..., (sizeof...(I1)+I2)...>{};

    template<class S1, class S2>
    using Concat = Invoke<concat<S1, S2>>;

    template<std::size_t N> struct gen_seq;
    template<std::size_t N> using GenSeq = Invoke<gen_seq<N>>;

    template<std::size_t N>
    struct gen_seq : Concat<GenSeq<N/2>, GenSeq<N - N/2>>{};

    template<> struct gen_seq<0> : seq<>{};
    template<> struct gen_seq<1> : seq<0>{};

Aside from the indices sequence generation, this solution should even have O(1) instantiation depth. Instead of multiple inheritance, it uses a std::array<std::false_type, size> to do a O(1)-instantiation-depth OR via SFINAE.

Implementation of is_one_of. Note that "is one of" is the opposite concept of "are unique".

#include <array>

// check if `T` is in `Us...`
template<class T, class... Us>
struct is_one_of
{
    template<class T1>
    static constexpr auto SFINAE(int)
    -> decltype( std::array<std::false_type, sizeof...(Us)>
                 {{std::is_same<T1, Us>{}...}} )
    {  return {}; /* only to suppress warning */  }

    template<class...>
    static constexpr int SFINAE(...) { return 42; }

    template<class T1>
    static constexpr bool test()
    {
        return std::is_same<decltype(SFINAE<T1>(0)), int>{};
    }

    static constexpr bool value = test<T>();
    constexpr operator bool() const { return value; }
};

Implementation of are_unique:

namespace detail
{
    // `Any` type with a generic no-constraint ctor
    // to discard a number of arguments for a function template
    template<std::size_t>
    struct Any
    {
        template<class T>
        constexpr Any(T&&) {}
    };

    // `wrapper` is used as a substitute for `declval`,
    // and can keep track if `T` is a reference
    template<class T>
    struct wrapper { using type = T; };

    template<std::size_t I, class T, class... Us>
    struct is_one_of_pack
    {
        template<std::size_t... I1s>
        struct helper
        {
            template<class... Remaining>
            static constexpr bool deduce_remaining(Any<I1s>..., Remaining...)
            {
                // unique <-> is one of
                return not is_one_of<T, typename Remaining::type...>{};
            }
        };

        template<std::size_t... I1s>
        static constexpr bool deduce_seq(seq<I1s...>)
        {
            return helper<I1s...>::template deduce_remaining(wrapper<Us>()...);
        }

        static constexpr bool create_seq()
        {
            return deduce_seq(gen_seq<I+1>{});
        }

        using type = std::integral_constant<bool, create_seq()>;
    };

    template<class... Packs>
    constexpr auto SFINAE(int)
    -> decltype( std::array<std::true_type, sizeof...(Packs)>
                 {{typename Packs::type{}...}} )
    {  return {}; /* only to suppress warning */  }

    template<class...>
    static constexpr int SFINAE(...) { return 42; }

    template<class... Packs>
    constexpr bool test()
    {
        return std::is_same<decltype(SFINAE<Packs...>(0)), int>{};
    }

    template<class... Ts, std::size_t... Is>
    constexpr bool deduce_seq(seq<Is...>)
    {
        return test< is_one_of_pack<Is, Ts, Ts...>... >();
    }
}

template<class... Ts>
struct are_unique
: std::integral_constant<bool,
                         detail::deduce_seq<Ts...>(gen_seq<sizeof...(Ts)>{})>
{};

Usage example:

#include <iostream>
#include <iomanip>
int main()
{
    bool a = are_unique<bool, char, int>();
    bool b = are_unique<bool, char, int, bool>();
    bool c = are_unique<bool, char, bool, int>();
    std::cout << std::boolalpha;
    std::cout << a << std::endl;
    std::cout << b << std::endl;
    std::cout << c << std::endl;
}