[fpc-devel] Language extension: absolute for classes

[Michalis Kamburelis]

Micha Nelissen wrote:
> Michalis Kamburelis wrote:
>> All you want is just to cover in class B identifier "Field" of class A.
>> So you should make "Field" a dummy function in class A (that just
>> returns a field value), and then you can redefine function name in
>> descendant classes. See the example below. Within the scope of class B
>> the "Field" function will return class G.
> 
> This is a smart hack, at best. It's abuse of function syntax IMHO (as if
> they were fields). Second, you cannot assign to a function.
> 
> If the functions were inline, it could be as optimal, I agree; then you
> have essentially localized the typecast to one place.
> 
> I just want to tell the compiler what I can guarantee, but what it
> cannot automatically infer.
> 

It can be done for properties too. And I agree, this can be considered
an "abuse" but not of the function syntax. It's an abuse of the rule
that you can always hide identifiers of parent class in a subclass.
Virtual methods are a typical clean solution to similar problems, but in
this case you want to change the method/property interface so we're left
with this unclean solution.

BTW That's why I agree with the cleaner approach of "overriding
properties" later in this thread.

Example below shows the same trick working for properties. Here "Field"
is a read/write property.

-----------
{$mode objfpc}

type
  TF = class
  end;

  TG = class(TF)
  end;

  TA = class
  private
    FField: TF;
  protected
    property Field: TF read FField write FField;
  end;

  TB = class(TA)
  private
    function GetField: TG;
    procedure SetField(Value: TG);
  protected
    property Field: TG read GetField write SetField;
  end;

function TB.GetField: TG;
begin
  Result := (inherited Field) as TG;
end;

procedure TB.SetField(Value: TG);
begin
  (inherited Field) := Value;
end;

begin
end.
-------

Michalis