Use the following general algorithms for set operations:

• std::set_union() to compute the union of two ranges into a third range:

        std::set_union(v1.cbegin(), v1.cend(),
                       v2.cbegin(), v2.cend(),
                       std::back_inserter(v3));
        // v3 = {1, 2, 3, 3, 4, 4, 5, 6, 8}

• std::merge() to merge the content of two ranges into a third one; this is similar to std::set_union() except that it copies the entire content of the input ranges into the output one, not just their union:

        std::merge(v1.cbegin(), v1.cend(),
                   v2.cbegin(), v2.cend(),
                   std::back_inserter(v3));
        // v3 = {1, 2, 2, 3, 3, 3, 4, 4, 4, 5, 6, 8}

• std::set_intersection() to compute the intersection of the two ranges into a third range:

        std::set_intersection(v1.cbegin(), v1.cend(),
                              v2.cbegin(), v2.cend(),
                              std::back_inserter(v3));
        // v3 = {2, 3, 4}

• std::set_difference() to compute the difference of two ranges into a third range; the output range will contain elements from the first range, which are not present in the second range:

        std::set_difference(v1.cbegin(), v1.cend(),
                            v2.cbegin(), v2.cend(),
                            std::back_inserter(v3));
        // v3 = {1, 4, 5}

• std::set_symmetric_difference() to compute a dual difference of the two ranges into a third range; the output range will contain elements that are present in any of the input ranges, but only in one:

        std::set_symmetric_difference(v1.cbegin(), v1.cend(),
                                      v2.cbegin(), v2.cend(),
                                      std::back_inserter(v3));
        // v3 = {1, 3, 4, 5, 6, 8}

• std::includes() to check if one range is a subset of another range (that is, all its elements are also present in the other range):

        std::vector<int> v1{ 1, 2, 3, 4, 4, 5 };
        std::vector<int> v2{ 2, 3, 3, 4, 6, 8 };
        std::vector<int> v3{ 1, 2, 4 };
        std::vector<int> v4{ };

        auto i1 = std::includes(v1.cbegin(), v1.cend(), 
                                v2.cbegin(), v2.cend()); // i1 = false
        auto i2 = std::includes(v1.cbegin(), v1.cend(), 
                                v3.cbegin(), v3.cend()); // i2 = true
        auto i3 = std::includes(v1.cbegin(), v1.cend(), 
                                v4.cbegin(), v4.cend()); // i3 = true