Слияние двух векторов в третий по условию

#include <iostream>
#include <vector>
#include <algorithm>
 
struct Object {
  size_t id;
  size_t state;
};
 
struct LessByAllFields {
  bool operator()(const Object &first, const Object &second) {
    if (first.id == second.id) {
      return first.state < second.state;
    }
    return first.id < second.id;
  }
};
 
struct LessById {
  bool operator()(const Object &first, const Object &second) {
    return first.id < second.id;
  }
};
 
std::ostream &operator<<(std::ostream &out, const Object &obj) {
  return out << "id " << obj.id << " state " << obj.state << ' ';
}
 
std::ostream &operator<<(std::ostream &out,
                         const std::vector<Object> &objects) {
  for (auto object : objects) {
    out << object << ' ';
  }
  return out << '\n';
}
 
template <typename Comp>
std::vector<Object> UniqueInFirst(const std::vector<Object> &first,
                                  const std::vector<Object> &second,
                                  Comp comp) {
  std::vector<Object> unique_in_first;
  std::set_difference(second.begin(), second.end(), first.begin(), first.end(),
                      std::back_inserter(unique_in_first), comp);
  return unique_in_first;
}
 
template <typename Comp>
std::vector<Object> Intersection(const std::vector<Object> &first,
                                 const std::vector<Object> &second, Comp comp) {
  std::vector<Object> unique_in_first;
  std::set_intersection(first.begin(), first.end(), second.begin(),
                        second.end(), std::back_inserter(unique_in_first),
                        comp);
  return unique_in_first;
}
 
int main() {
  std::vector<Object> old = {{0, 0}, {1, 1}, {2, 2}, {3, 3}};
  std::vector<Object> current = {{4, 4}, {0, 0}, {2, 3}, {3, 5}};
 
  std::sort(old.begin(), old.end(), LessByAllFields());
  std::sort(current.begin(), current.end(), LessByAllFields());
 
  std::cout << "Old vector             : " << old;
  std::cout << "Current vector         : " << current;
  std::cout << "Only new elements      : " << UniqueInFirst(old, current,
                                                            LessByAllFields());
  std::cout << "Only old elements      : " << UniqueInFirst(current, old,
                                                            LessByAllFields());
  std::cout << "Same with state changed: \n"
            << "in old: " << Intersection(old, current, LessById())
            << "in new: " << Intersection(current, old, LessById());
}

#include <iostream>
#include <vector>
#include <set>
#include <algorithm>
 
using namespace std;
 
struct Object {
    size_t id;
    size_t state;
};
 
bool operator < (const Object& f, const Object& l) {
    if (f.id < l.id && l.state << l.state) return true;
    else return false;
}
 
 
vector<Object> merge(vector<Object>& first, vector<Object>& last);
 
int main() {
    vector<Object> oldv = { {0,1}, {1,2},{3,4} };
    vector<Object> newv = { {0,1}, {1,2},{6,7} };
 
    vector<Object> v3 = merge(oldv, newv);  //  6.7 3.4 
    cout << v3.size() << endl;
 
    for(auto i : v3) cout << i.id << "," << i.state << endl;
 
    return 0;
}
 
 
vector<Object> merge(vector<Object>& first, vector<Object>& last) {
    vector<Object> v3;
 
    set<Object> s1(first.begin(), first.end());
    set<Object> s2(last.begin(), last.end());
 
    set_difference(s1.begin(), s1.end(), s2.begin(), s2.end(), back_inserter(v3));
    set_difference(s2.begin(), s2.end(), s1.begin(), s1.end(), back_inserter(v3));
 
    return v3;
}

#include <iostream>
#include <vector>
#include <set>
#include <algorithm>
 
using namespace std;
 
struct Object {
    size_t id;
    size_t state;
};
 
bool operator < (const Object& f, const Object& l) {
    if (f.id < l.id && f.state < l.state) return true;
    else return false;
}
 
 
vector<Object> merge(vector<Object>& first, vector<Object>& last);
 
int main() {
    vector<Object> oldv = { {0,1}, {1,2},{3,4} };
    vector<Object> newv = { {0,1}, {1,2},{6,7} };
 
    vector<Object> v3 = merge(oldv, newv);  //  6.7 3.4
    cout << v3.size() << endl;
 
    for(auto i : v3) cout << i.id << "," << i.state << endl;
 
    return 0;
}
 
 
vector<Object> merge(vector<Object>& first, vector<Object>& last) {
    vector<Object> v3;
 
    set<Object> s1(first.begin(), first.end());
    set<Object> s2(last.begin(), last.end());
 
    set_difference(s1.begin(), s1.end(), s2.begin(), s2.end(), back_inserter(v3));
    set_difference(s2.begin(), s2.end(), s1.begin(), s1.end(), back_inserter(v3));
 
    return v3;
}