How can I check if all members of two tuples are different?

Question

std::tuple<...>::operator!= returns true if at least one member of the two compared tuples is different.

I would need a function that would return true if all members of two compared tuples are different:

template <class... Args>
bool areAllMembersDifferent( const std::tuple<Args...>& left, const std::tuple<Args...>& right )
{
    bool allDiff = true;

    // iterate through the tuples are set allDiff to false if one member's is different than other's

    return allDiff;
}

Inspired from what I found on the web, I wrote this (adapted a function that was printing the tuple content):

template <std::size_t N, std::size_t, class = make_index_sequence<N>>
struct CheckTupleLoop;

template <std::size_t N, std::size_t J, std::size_t... Is>
struct CheckTupleLoop<N, J, index_sequence<Is...>> {
    template <class Tup>
    int operator()(bool& allDiff, const Tup &left,const Tup &right) {
        if ( std::get<J>(left) == std::get<J>(right) )
            allDiff = false;
        return 0;
    }
};

template <class... Args>
bool areAllMembersDifferent( const std::tuple<Args...>& left, const std::tuple<Args...>& right )
{
    bool allDiff = true;
    CheckTupleLoop<sizeof...(Args)>{}(allDiff,left,right);
    return allDiff;
}

But this is obviously not correct as the compiler reports me Error C2955 'CheckTupleLoop': use of class template requires template argument list

Any kind of implementation of bool areAllMembersDifferent in C++11 would be acceptable (using or not my first attempt approach).

Do you mean to check the content of two tuples? I got confused by *would return true if all members of **a** tuple are different* — Jonas, Nov 23 '16 at 10:06
@Jonas: Check if all members of the two tuples are different. Edited the post. — jpo38, Nov 23 '16 at 10:10
Boost.Hana has a `disjoint` function that checks whether tuples have elements in common. — TemplateRex, Nov 23 '16 at 22:51

Jarod42 · Accepted Answer · 2017-08-22T09:49:04.220

You may use the following:

namespace detail
{

template <std::size_t ... Is, typename Tuple>
bool areAllMembersDifferent(std::index_sequence<Is...>,
                            const Tuple& left,
                            const Tuple& right)
{
    bool res = true;

    const int dummy[] = {0, (res &= std::get<Is>(left) != std::get<Is>(right), 0)...};
    static_cast<void>(dummy); // Avoid warning for unused variable
    return res;
}

}

template <typename Tuple>
bool areAllMembersDifferent(const Tuple&left, const Tuple& right)
{
    return detail::areAllMembersDifferent(
        std::make_index_sequence<std::tuple_size<Tuple>::value>(), left, right);
}

Demo

Implementation for c++11 of std::make_index_sequence (which is C++14) can be found easily

In C++17, you may even simplify the helper function to:

namespace detail
{

template <std::size_t ... Is, typename Tuple>
bool areAllMembersDifferent(std::index_sequence<Is...>,
                            const Tuple& left,
                            const Tuple& right)
{
    return (std::get<Is>(left) != std::get<Is>(right) && ...);
}

}

Works great! Thanks. Hopefully one day I'll be able to write this kind of code without asking SO....;-) — jpo38, Nov 23 '16 at 10:18

score 2 · Answer 2 · answered Nov 23 '16 at 10:36

You may use the following C++11 compliant solution to achieve what you need:

template <size_t N>
struct CompareTuples
{
    template<class... Args>
    static bool areAllMembersDifferent(const std::tuple<Args...>& left, const std::tuple<Args...>& right)
    {
        return (std::get<N>(left) != std::get<N>(right)) && CompareTuples<N-1>::areAllMembersDifferent(left, right);
    }
};

template<>
struct CompareTuples<0>
{
    template<class... Args>
    static bool areAllMembersDifferent(const std::tuple<Args...>& left, const std::tuple<Args...>& right)
    {
        return (std::get<0>(left) != std::get<0>(right));
    }
};

template<class... Args>
bool areAllMembersDifferent(const std::tuple<Args...>& left, const std::tuple<Args...>& right)
{
    return CompareTuples<std::tuple_size<std::tuple<Args...>>::value-1>::areAllMembersDifferent(left, right);
}

score 0 · Answer 3 · answered Nov 23 '16 at 10:54

Jarod42's answer is quite reasonable, but here's my 2 cents:

#include <iostream>
#include <tuple>
#include <limits>

template <size_t index>
struct next_index
{
    static const size_t value = index - 1;
};

template <>
struct next_index<0>
{
    static const size_t value = 0;
};

template <class Tuple, size_t index>
bool is_same(const Tuple& left, const Tuple& right)
{
    if (index != 0)
        return is_same<Tuple, next_index<index>::value>(left, right) and std::get<index>(left) != std::get<index>(right);
    return std::get<index>(left) != std::get<index>(right);
}

template <typename Tuple>
bool areAllMembersDifferent(const Tuple& left, const Tuple& right)
{
    return is_same<Tuple, std::tuple_size<Tuple>::value - 1>(left, right);
}

int main() {
    std::cout << areAllMembersDifferent(std::make_tuple(12, '*', 4.2f), std::make_tuple(11, '#', 4.25f)) << std::endl;
    std::cout << areAllMembersDifferent(std::make_tuple(12, '*', 4.2f), std::make_tuple(11, '#', 4.2f)) << std::endl;
}

How can I check if all members of two tuples are different?

3 Answers3