# Example: Polymorphism

## Description

Create a class POINT and its heir CIRCLE to demonstrate polymorphic attachment and dynamic binding.

## Source

Problem description from Rosetta Code: Polymorphism

Solution varies from Rosetta Code description (e. g., feature `out` is redefined in this solution, versus feature `print`.)

## Solution

```class POINT inherit ANY redefine out end create make, make_origin feature -- Initialization make (a_x, a_y: INTEGER) -- Create with values `a_x' and `a_y' do set_x (a_x) set_y (a_y) ensure x_set: x = a_x y_set: y = a_y end make_origin -- Create at origin do ensure x_set: x = 0 y_set: y = 0 end feature -- Access x: INTEGER assign set_x -- Horizontal axis coordinate y: INTEGER assign set_y -- Vertical axis coordinate feature -- Element change set_x (a_x: INTEGER) -- Set `x' coordinate to `a_x' do x := a_x ensure x_set: x = a_x end set_y (a_y: INTEGER) -- Set `y' coordinate to `a_y' do y := a_y ensure y_set: y = a_y end feature -- Output out: STRING -- Display as string do Result := "Point: x = " + x.out + " y = " + y.out end end```

```class CIRCLE inherit POINT rename make as point_make redefine make_origin, out end create make, make_origin, make_from_point feature -- Initialization make (a_x, a_y, a_r: INTEGER) -- Create with values `a_x' and `a_y' and `a_r' require non_negative_radius_argument: a_r >= 0 do point_make (a_x, a_y) set_r (a_r) ensure x_set: x = a_x y_set: y = a_y r_set: r = a_r end make_origin -- Create at origin with zero radius do Precursor ensure then r_set: r = 0 end make_from_point (a_p: POINT; a_r: INTEGER) -- Initialize from `a_r' with radius `a_r'. require non_negative_radius_argument: a_r >= 0 do set_x (a_p.x) set_y (a_p.y) set_r (a_r) ensure x_set: x = a_p.x y_set: y = a_p.y r_set: r = a_r end feature -- Access r: INTEGER assign set_r -- Radius feature -- Element change set_r (a_r: INTEGER) -- Set radius (`r') to `a_r' require non_negative_radius_argument: a_r >= 0 do r := a_r ensure r_set: r = a_r end feature -- Output out: STRING -- Display as string do Result := "Circle: x = " + x.out + " y = " + y.out + " r = " + r.out end invariant non_negative_radius: r >= 0 end```

```class APPLICATION create make feature {NONE} -- Initialization make -- Run application. local my_point: POINT my_circle: CIRCLE do create my_point.make_origin print (my_point.out + "%N") create {CIRCLE} my_point.make_origin print (my_point.out + "%N") create my_point.make (10, 15) print (my_point.out + "%N") create {CIRCLE} my_point.make (20, 25, 5) print (my_point.out + "%N") create my_circle.make (30, 35, 10) print (my_circle.out + "%N") create my_circle.make_from_point (my_point, 35) print (my_circle.out + "%N") end end```

## Output

```Point: x = 0 y = 0 Circle: x = 0 y = 0 r = 0 Point: x = 10 y = 15 Circle: x = 20 y = 25 r = 5 Circle: x = 30 y = 35 r = 10 Circle: x = 20 y = 25 r = 35```