A comprehensive introduction to class-based design and polymorphism. This example explores defining class hierarchies, using 'virtual' and 'abstract' methods to define extensible interfaces, overriding constructors, and managing heterogeneous object collections using dynamic arrays of base class references.
<?pas
type
TShape = class
function Area : Float; virtual; abstract;
function Name : String; virtual; abstract;
end;
type
TCircle = class(TShape)
Radius : Float;
constructor Create(r : Float); begin Self.Radius := r; end;
function Area : Float; override; begin Result := PI * Radius * Radius; end;
function Name : String; override; begin Result := 'Circle'; end;
end;
type
TSquare = class(TShape)
Side : Float;
constructor Create(s : Float); begin Self.Side := s; end;
function Area : Float; override; begin Result := Side * Side; end;
function Name : String; override; begin Result := 'Square'; end;
end;
// Polymorphism in action
var shapes : array of TShape;
shapes.Add(new TCircle(5));
shapes.Add(new TSquare(4));
shapes.Add(new TCircle(2.5));
for var s in shapes do
PrintLn(Format(
'Shape: %s, Area: %.2f',
[ s.Name, s.Area ]
));
?>Shape: Circle, Area: 78.54 Shape: Square, Area: 16.00 Shape: Circle, Area: 19.63