Passing objects as parameters

Objects can be passed as parameters much like how we pass primitive data types. The difference is that because an object is passed, the function also has access to its public data members and member functions.

class Point {
private:
  int x_, y_;
public:
  int x() const { return x_; }
  void set_x(int x) { x_ = x; }
  int y() const { return y_; }
  void set_y(int y) { y_ = y; }
};

void display_horizontal_distance(const Point &p1, const Point &p2) {
  int distance = p1.x() - p2.x();
  std::cout << "The horizontal distance between the two points is: "
            << distance << "\n";
}

int main() {
  Point start, end;
  start.set_x(3);
  start.set_y(4);

  end.set_x(6);
  end.set_y(4);

  display_horizontal_distance(end, start);
  return 0;
}

In the example above, we see that two Point objects are passed to the display_horizontal_distance function. We are able to access all public data members and methods of each Point parameter from inside the function. Specifically, we use the x() methods of p1 and p2 then use them to compute the distance.

Returning objects

Objects can also be used as return types. You can imagine that this allows us to create objects from within the function and return them.

Point get_middle(const Point &p1, const Point &p2) {
  // Find distance between p1 and p2's x coordinates to find the middle and add
  // to the starting point to find the midpoint
  int mid_x = (p2.x() - p1.x()) / 2 + p1.x();

  // Use the same formula as above by use y coordinates
  int mid_y = (p2.y() - p1.y()) /2 + p1.y();

  Point mid_point;
  mid_point.set_x(mid_x);
  mid_point.set_y(mid_y);
  return mid_point;
}

int main() {
  Point start, end;
  start.set_x(3);
  start.set_y(4);

  end.set_x(6);
  end.set_y(4);

  Point mid = get_middle(start, end);
  cout << "Mid point is at " << mid.x() << ", " << mid.y() << "\n";
  return 0;
}

In the example above, we use the two Point parameters to find the x and y location between them. We use these values to create another Point object whose location is the mid point.