Override property type in descendant class

I have created 2 top level classes that I plan to use for a list and a list
item.  

Simplified version:

TListItem = Class
End;

TList = Class
  Private
    FFirst : TListItem;
  Protected
    Function GetFirst : TListItem; Virtual;
    Procedure SetFirst (AValue : TListItem); Virtual;
  Public
    Property First : TListItem Read GetFirst Write SetFirst;
End;  


I would now like to sub-class from these classes to make specific list types. 
For example, a Integer list.

First, I subclass TListItem like this to add a Data property of type Integer to
my list item:

TIntegerListItem = Class(TListItem)
  Private
    FData : Integer;
  Protected
    Function GetData : Integer; Virtual;
    Procedure SetData (AValue : Integer); Virtual;
  Public
    Property Data : Integer Read GetData Write SetData;
End;


I can now use my TList and TIntegerListItem together, however, anytime, I want
to access the Data property of the First item, I have to type cast or do an AS
like this:

(MyList.First As TIntegerListItem).Data
-- OR --
TIntegerListItem(MyList.First).Data

I want to create a new list type called TIntegerList that will hold
TIntegerListItem objects.  That way, when the I access the list items through
the list, I don't have to constantly typecast.  It would be ideal if I could do
something like this:

TIntegerList = Class (TList)
  Public
    Property First : TStringListItem;
End;

That way, I'd only have to list the property with a new type rather than
re-implementing all the Read Write methods, etc.  

Any suggestions on the best way to accomplish my goal here?

Josh Breitbach

That's what I was afraid of.  So there's no way to do it without re-defining the
interface and implementation of all those methods?

That's unfortunate.

Thanks,
Josh Breitbach
http://www.breittechnologies.com

Josh Breitbach wrote:

> 
> I have created 2 top level classes that I plan to use for a list and
> a list item.
> 
> Simplified version:
> 
> TListItem = Class
> End;
> 
> TList = Class
>   Private
>     FFirst : TListItem;
>   Protected
>     Function GetFirst : TListItem; Virtual;
>     Procedure SetFirst (AValue : TListItem); Virtual;
>   Public
>     Property First : TListItem Read GetFirst Write SetFirst;
> End;  
> 
> 
> I would now like to sub-class from these classes to make specific
> list types.  For example, a Integer list.
> 
> First, I subclass TListItem like this to add a Data property of type
> Integer to my list item:
> 
> TIntegerListItem = Class(TListItem)
>   Private
>     FData : Integer;
>   Protected
>     Function GetData : Integer; Virtual;
>     Procedure SetData (AValue : Integer); Virtual;
>   Public
>     Property Data : Integer Read GetData Write SetData;
> End;
> 
> 
> I can now use my TList and TIntegerListItem together, however,
> anytime, I want to access the Data property of the First item, I have
> to type cast or do an AS like this:
> 
> (MyList.First As TIntegerListItem).Data
> -- OR --
> TIntegerListItem(MyList.First).Data
> 
> I want to create a new list type called TIntegerList that will hold
> TIntegerListItem objects.  That way, when the I access the list items
> through the list, I don't have to constantly typecast.  It would be
> ideal if I could do something like this:
> 
> TIntegerList = Class (TList)
>   Public
>     Property First : TStringListItem;
> End;
> 
> That way, I'd only have to list the property with a new type rather
> than re-implementing all the Read Write methods, etc.
> 
> Any suggestions on the best way to accomplish my goal here?
> 

As long as we don't have generics in Win32 you either have to use a
code generator like

Typed list generator for Delphi
  http://cc.codegear.com/item/24490

or a template system like http://dn.codegear.com/article/27603

The problem cannot be solved using inheritance. 

-- 
Peter Below (TeamB)  
Don't be a vampire (http://slash7.com/pages/vampires), 
use the newsgroup archives :
http://www.tamaracka.com/search.htm

You must inherit TList and override GetFirst method using something like 
this:

// in the interface section of your descendant TList
...
   property First: TIntegerListItem read GetFirst write SetFirst;

...
// implementation of inherited GetFirst

function TNewList.GetFirst: TIntegerListItem;
begin
   Result := TIntegerListItem(inherited GetFirst);
end;


Leonardo M. Ramé
http://leonardorame.blogspot.com


Josh Breitbach escribió:
> I have created 2 top level classes that I plan to use for a list and a list
item.  
> 
> Simplified version:
> 
> TListItem = Class
> End;
> 
> TList = Class
>   Private
>     FFirst : TListItem;
>   Protected
>     Function GetFirst : TListItem; Virtual;
>     Procedure SetFirst (AValue : TListItem); Virtual;
>   Public
>     Property First : TListItem Read GetFirst Write SetFirst;
> End;  
> 
> 
> I would now like to sub-class from these classes to make specific list
types.  For example, a Integer list.
> 
> First, I subclass TListItem like this to add a Data property of type
Integer to my list item:
> 
> TIntegerListItem = Class(TListItem)
>   Private
>     FData : Integer;
>   Protected
>     Function GetData : Integer; Virtual;
>     Procedure SetData (AValue : Integer); Virtual;
>   Public
>     Property Data : Integer Read GetData Write SetData;
> End;
> 
> 
> I can now use my TList and TIntegerListItem together, however, anytime, I
want to access the Data property of the First item, I have to type cast or do an
AS like this:
> 
> (MyList.First As TIntegerListItem).Data
> -- OR --
> TIntegerListItem(MyList.First).Data
> 
> I want to create a new list type called TIntegerList that will hold
TIntegerListItem objects.  That way, when the I access the list items through
the list, I don't have to constantly typecast.  It would be ideal if I could do
something like this:
> 
> TIntegerList = Class (TList)
>   Public
>     Property First : TStringListItem;
> End;
> 
> That way, I'd only have to list the property with a new type rather than
re-implementing all the Read Write methods, etc.  
> 
> Any suggestions on the best way to accomplish my goal here?
> 
> Josh Breitbach

"Josh Breitbach" <josh@breittechnologies.com> wrote in message 
news:47d0249d$1@newsgroups.borland.com...
>
> That's what I was afraid of.  So there's no way to do it without 
> re-defining the interface and implementation of all those methods?
>
> That's unfortunate.
>

I just recently worked through the same issue. Your problem may be an 
indication of a design flaw. The properties of an object make the object and 
if you're having problems with that you may need to rethink whether or not 
that property should be in the base class. Also, you may find (as I did) 
that when you go to use the objects you won't need to have a generic 
property in the base class.