{$MODE DELPHI} // class support
{$M+} // constructor support

type
  // Define a base TPolygon class :
  // This class is a triangle if 3 sides, square if 4 sides ...
  TPolygon = class
  private
    sideCount  : Integer;  // How many sides?
    sideLength : Integer;  // How long each side?
    shapeArea  : Double;   // Area of the polygon
  protected
     procedure setArea; Virtual; Abstract;  // Cannot code until sides known
  published
    property count  : Integer read sideCount;
    property length : Integer read sideLength;
    property area   : Double  read shapeArea;
    constructor Create(sides, length : Integer);
  end;

  // Define triangle and square descendents
  TTriangle = class(TPolygon)
  protected
    procedure setArea; override;   // Override the abstract method
  end;

  TSquare = class(TPolygon)
  protected
    procedure setArea; override;   // Override the abstract method
  end;

// Create the TPolygon object
constructor TPolygon.Create(sides, length : Integer);
begin
  // Save the number and length of the sides
  sideCount := sides;
  sideLength := length;

  // Set the area using the abstract setArea method :
  // This call will be satisfied only by a subclass
  setArea;
end;

// Implement the abstract setArea parent method for the triangle
procedure TTriangle.setArea;
begin
  // Calculate and save the area of the triangle
  shapeArea := (sideLength * sideLength*0.866) / 2;
end;

// Implement the abstract setArea parent method for the square
procedure TSquare.setArea;
begin
  // Calculate and save the area of the square
  shapeArea := sideLength * sideLength;
end;

var
  triangle : TTriangle;
  square   : TSquare;

begin
  // Create a triangle and a square
  triangle := TTriangle.Create(3, 10);
  square   := TSquare.Create(4, 10);
  // Show the areas of our polygons:
  Writeln('Triangle area = ',triangle.area);
  Writeln('Square   area = ',square.area);
end.