c++ STL set_difference set_intersection set_union 操作

// set_symmetric_difference example
 #include <iostream>   // std::cout
 #include <algorithm>  // std::set_symmetric_difference, std::sort
 #include <vector>    // std::vector
  
 int main () {
  int first[] = {5,10,15,20,25};
  int second[] = {50,40,30,20,10};
  std::vector<int> v(10);           // 0 0 0 0 0 0 0 0 0 0
  std::vector<int>::iterator it;
  
  std::sort (first,first+5);   // 5 10 15 20 25
  std::sort (second,second+5);  // 10 20 30 40 50
  
  it=std::set_symmetric_difference (first, first+5, second, second+5, v.begin());
                         // 5 15 25 30 40 50 0 0 0 0
  v.resize(it-v.begin());           // 5 15 25 30 40 50
  
  std::cout << "The symmetric difference has " << (v.size()) << " elements:\n";
  for (it=v.begin(); it!=v.end(); ++it)
   std::cout << ' ' << *it;
  std::cout << '\n';
  
  return 0;
 }