Union Operation For std::set [duplicate]

Question

Is there no function in the standard library like this?

set<T> set::union(set<T> other)

Or even this?

set<T> getUnion(set<T> a, set<T> b)

set_union is the right function in name only. It can operate on vector also, which means it may not be as efficient as a set-only function.

I am not appending. Appending destroys the original set. I want a new set representing the union.

Answer 1

You can use the two-iterator std::set::insert template for this:

template <typename T>
std::set<T> getUnion(const std::set<T>& a, const std::set<T>& b)
{
  std::set<T> result = a;
  result.insert(b.begin(), b.end());
  return result;
}

---

Note: Following some of the comments suggesting I take one of the parameters by value because I need a copy anyway, I chose this implementation to avoid disallowing RVO, which is not allowed when returning parameter taken by value. To better deal with rvalue arguments, overloads of this function taking rvalue reverences and leveraging move semantics could be provided.

Answer 2

There's std::set_union.

The example from that page uses vectors and arrays, so it's pretty versatile:

// set_union example
#include <iostream>     // std::cout
#include <algorithm>    // std::set_union, 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_union (first, first+5, second, second+5, v.begin());
                                               // 5 10 15 20 25 30 40 50  0  0
  v.resize(it-v.begin());                      // 5 10 15 20 25 30 40 50

  std::cout << "The union has " << (v.size()) << " elements:\n";
  for (it=v.begin(); it!=v.end(); ++it)
    std::cout << ' ' << *it;
  std::cout << '\n';

  return 0;
}

Output:

The union has 8 elements:
 5 10 15 20 25 30 40 50