type
  TCircle = class;
  TSquare = class;

  //visitor adds operations without
  //changing the element classes
  IVisitor = interface
    procedure VisitCircle(C: TCircle);
    procedure VisitSquare(S: TSquare);
  end;

  TShape = class
  public
    procedure Accept(
      const V: IVisitor); virtual; abstract;
  end;

  TCircle = class(TShape)
  public
    procedure Accept(
      const V: IVisitor); override;
  end;

  TSquare = class(TShape)
  public
    procedure Accept(
      const V: IVisitor); override;
  end;

  TAreaVisitor = class(TInterfacedObject,
    IVisitor)
    procedure VisitCircle(C: TCircle);
    procedure VisitSquare(S: TSquare);
  end;

procedure TCircle.Accept(const V: IVisitor);
begin
  V.VisitCircle(Self);
end;

procedure TSquare.Accept(const V: IVisitor);
begin
  V.VisitSquare(Self);
end;

procedure TAreaVisitor.VisitCircle(
  C: TCircle);
begin
  WriteLn('circle area');
end;

procedure TAreaVisitor.VisitSquare(
  S: TSquare);
begin
  WriteLn('square area');
end;

var
  Shapes: TArray<TShape>;
  Shape: TShape;
  Visitor: IVisitor;
begin
  Shapes := [TCircle.Create,
    TSquare.Create];
  Visitor := TAreaVisitor.Create;

  for Shape in Shapes do
    Shape.Accept(Visitor);
  //circle area
  //square area
end.