How to override an inherited property?

Question

I have a class (TMyClass) which have a property (Items: TItems)

TItems = class;    

TMyClass = class(TComponent)
private
   FItems: TItems;
   procedure SetItems(const Value: TItems);
protected

public

protected
  property Items: TItems read FItems write SetItems;
end.

TExItems = class(TItems)
private
  FNewProb: Integer;
protected

public

published
  property NewProp: Integer read FNewProb write FNewProb;
end.

TExMyClass = class(TMyClass)
private
   FItems: TExItems;
   procedure SetItems(const Value: TItems);
protected

public

published
  property Items: TExItems read FItems write SetItems;
end.

The new "Items" property is inherited from TItems but when I installed the component the new property of TExItems which is "NewProb" did not appear and it looks like the "Items" property is still TItems not TExItems...how to override it?

Thanks

Modification : Here is the Real code

type TKHAdvSmoothDock = class;

TKHAdvSmoothDockItem = class(TAdvSmoothDockItem)
private
  FImageIndex: TImageIndex;
  procedure SetImageIndex(const Value: TImageIndex);
protected

public

published
  property ImageIndex: TImageIndex read FImageIndex write SetImageIndex default -1;
end;

TKHAdvSmoothDockItems = class(TAdvSmoothDockItems)
private
  FOwner: TKHAdvSmoothDock;
  FOnChange: TNotifyEvent;
  function GetItem(Index: Integer): TKHAdvSmoothDockItem;
  procedure SetItem(Index: Integer; const Value: TKHAdvSmoothDockItem);
protected
  function GetOwner: TPersistent; override;
public
  constructor Create(AOwner: TKHAdvSmoothDock);
  function Add: TKHAdvSmoothDockItem;
  function Insert(Index: Integer): TKHAdvSmoothDockItem;
  property Items[Index: Integer]: TKHAdvSmoothDockItem read GetItem write SetItem; default;
  procedure Delete(Index: Integer);
published
  property OnChange: TNotifyEvent read FOnChange write FOnChange;
end;

TKHAdvSmoothDock = class(TAdvSmoothDock)
private
  FImageChangeLink: TChangeLink;
  FImages: TCustomImageList;
  FItems: TKHAdvSmoothDockItems;
  procedure ImageListChange(Sender: TObject);
  procedure SetImages(const Value: TCustomImageList);
  procedure SetItems(const Value: TKHAdvSmoothDockItems);
  function GetItems: TKHAdvSmoothDockItems;
  { Private declarations }
protected
  procedure UpdateImagesFromImageList;
public
  constructor Create(AOwner: TComponent); override;
  destructor Destroy; override;
published
  property Images: TCustomImageList read FImages write SetImages;
  property Items: TKHAdvSmoothDockItems read GetItems write SetItems;
end;

Regards.

Solution 1

Property getters and setters can be virtual, and then overridden by inheriting classes, see below for your example updated. There's one caveat with you're example code and that's you're trying to change the type of the property, which is not allowed. I would advise you check for Value is TExItems in TExMyClass.SetItems but use the inherited Items property and cast to TExItems in all methods of TExMyClass and further inheritants.

TItems = class;

TMyClass = class(TComponent)
private
   FItems: TItems;
   procedure SetItems(const Value: TItems); virtual;
protected
  property Items: TItems read FItems write SetItems;
end;

TExItems = class(TItems)
private
  FNewProb: Integer;
protected

public

published
  property NewProp: Integer read FNewProb write FNewProb;
end;

TExMyClass = class(TMyClass)
private
   procedure SetItems(const Value: TItems); override;
end;

Solution 2

Properties cannot be virtual, so they cannot be overridden. They can be hidden, in that within the context of TExMyClass, references to Items will resolve to the property declared in that class, not the one declared in the ancestor.

If you have something whose static (declared, compile-time) type is TMyClass, Items will always refer to the one from that class, even if its run-time type is TExMyClass.

You could declare SetItems as protected and virtual in the base class, and then override it in the descendant instead of declaring a whole new property that happens to have the same name.

Solution 3

You can implement and override methods getItem and setItem;

Implement property Item only for TMyClass

property Items: TItems read getItems write setItemps;

For TMyClass:

public:  
function getItems : TItems; virtual;
procedure setItems(items: TItems); virtual;

For TExMyClass:

public:
function getItems : TItems; override;
procedure setItems(items: TItems); override;

function TExMyClass.getItems : TItems;
begin
  result := fItems;
end;

procedure TExMyClass.setItems(items : TItems);
begin
  self.itmes := items;
end;

so, TExMyClass.items.className = TExItems !

Solution 4

Technically, you can't override a property, but you can mimic override in several ways. See for example this answer for the most basic manners.

Now I do not have the code for TAdvSmoothDock so the rest is just guessing. When the property getter and setter of TAdvSmoothDock.Items are virtual, you could override them. But in more advanced components, and I suppose the ones from TMS are, then there is a good chance of TAdvSmoothDock having a protected CreateItem method which is called whenever there is need of a new item which could be overriden. If that is the case, then you should implement it like:

function TKHAdvSmoothDock.CreateItem: TAdvSmoothDockItem;
begin
  Result := TKHAdvSmoothDockItem.Create;
end;

And use it like:

TKHAdvSmoothDockItem(AKHAdvSmoothDock.Items[I]).ImageIndex := ...

View more solutions

Comments

[user1512094]

Thanks for this great reply but in the TExMyClass Class the Setter procedure "SetItems" takes Value from TItems...I want it to take TExItems ? when I changed its (Value parameter) type from TItems to TExItems the compiler gave me the following error : [DCC Error] xxxxxx.pas(34): E2037 Declaration of 'SetItems' differs from previous declaration how to fix this error?

[Stijn Sanders]

Like I said, changing the property type when inheriting is not allowed. Checking the type when setting and using Items as TExItems should work though.

[user1512094]

Thank you very much for the example! it is very important to read such examples before working with properties.... I was searching and I found something maybe i would help me but i need more explanation here is my question : cannot I use the "reintroduce" instead of "Override"? for the setter and getter?

[NGLN]

You could, but it is a bad idea because it would break inheritance: i.e. when the ancestor calls the setter or getter, yours wouldn't be called.

[thewaywewere]

Welcome to SO!. Your reply does not look like a good answer to the question. Once you have sufficient reputation you will be able to comment on any post. Also check this what can I do instead.

[Franco Michel Almeida Caixeta]

A good solution, but if the parent class is in units other than the daughter, the virtual property and the override must be in protected to work.