#include <iostream>
#include <vector>
vector<int> quickSort(vector<int> arr) {
    if (arr.size() <= 1) {
      return arr;
    } else {
      int pivot = arr[0];
      vector<int> less1;
      vector<int> more;
      for (int i = 1; i < arr.size(); i++) {
        if (arr[i] <= pivot) {
          less1.push_back(arr[i]);
        } else {
          more.push_back(arr[i]);
        }
      }
  return quickSort(less1) + vector<int>{pivot} + quickSort(more);
}


int main() {
  vector<int> arr = {12, 4, 5, 6, 7, 3, 1, 15};
  arr = quickSort(arr);
  for (int i = 0; i < arr.size(); i++) {
    cout << arr[i] << " ";
  }
  cout << endl;
  return 0;
}

How to overload + operator in the line return quickSort(less1) + vector<int>{pivot} + quickSort(more);

I have gone through Concatenating two std::vectors and https://www.geeksforgeeks.org/concatenate-two-vectors-in-cpp/

still did not get the idea.