An example of handling some simple edge cases

handle_errors.cpp

vector<pair<int, int>> findCells(const vector<pair<int, int>>& gameBoard, const int FLAG_CODE) {
  if (gameBoard.empty()) {
    return vector<pair<int, int>> {};
  }
  
  vector<pair<int, int>> cells;
  
  for (pair<int, int> cell : gameBoard) {
    if (cell.second == FLAG_CODE) {
      cells.push_back(cell);
    }
  }

  return cells;
}