Is this valid Delphi code?

I played around how I can best wrap a Class (TBase) inside a Container that I have limited access to change.

Is this valid Delphi Code? It seems to work.

type
  TForm1 = class(TForm)
    procedure FormShow(Sender: TObject);
  private
    { Private-Deklarationen }
  public
    { Public-Deklarationen }
  end;

  TBase = class
    public
      bar: Integer;
  end;

  TSpecial = class(TBase)
    public
      foo: Integer;
  end;

  TBaseContainer = class
    public
      base: TBase;
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.FormShow(Sender: TObject);
var
  baseContainer: TBaseContainer;
begin
  baseContainer := TBaseContainer.Create;
  baseContainer.base := TBase.Create;
  baseContainer.base.bar := 5;

  TSpecial(baseContainer.base).foo := 3;

  ShowMessage(baseContainer.base.bar.ToString);
  ShowMessage(TSpecial(baseContainer.base).foo.ToString);
end;

What does the Compiler do in line "TSpecial(baseContainer.base).foo := 3;"? What memory is used?