Description : Constructs a sorted range beginning in the location passed by result with the set union of the two sorted ranges [first1, last1) and [first2, last2).
Example:
std::vector<int> v1 {3, 1, 5};
std::vector<int> v2 {1, 0};
// the destination needs to fit all merged values, we'll prune the extra elements later
std::vector<int> destination(v1.size() + v2.size());
// set_union requires sorted ranges
std::sort(v1.begin(), v1.end());
std::sort(v2.begin(), v2.end());
// returns iterator to the end of the constructed range
auto it = std::set_union(v1.begin(), v1.end(), // first range
v2.begin(), v2.end(), // second range
destination.begin());
// prune extra values
destination.resize(it - destination.begin());
// destination is now {0, 1, 3, 5}
for (auto value : destination) {
std::cout << value << " ";
}