... Sorry, forgot subject first....

I try to learn to use generics properly.
First, this is a class definition without generics:

  TMyDerived = class(TObjectList)
  private
    MyInt: integer;
  public
    constructor Create(FreeObjects: Boolean=True);
  end;

It works. Now I try to derive a class with generics:

  TMyGen = specialize TFPGObjectList<TControl>;
  TMyDerived = class(TMyGen)
  private
    MyInt: integer;
  public
    constructor Create(FreeObjects: Boolean=True);
  end;
It says:
 unit1.pas(40,24) Error: Duplicate identifier "FreeObjects"

Why? If I change "FreeObjects" to "aFreeObjects" then it works. Again why?

Is it even ok to derive classes from specialized classes like that?

Juha Manninen wrote

... Sorry, forgot subject first....

See my other mail...

Regards,
Sven

Now, if I do:

  myV := TMyDerived.Create(False);
  if myV is TFPGObjectList then
    ...

it gives:
unit1.pas(67,10) Error: Class or Object types "TMyDerived" and "TFPGObjectList" are not related

In my opinion they are related.

Maybe generics are good only for situations where you don't need to derive classes or test their inheritance.

Juha

They are not related. You should not think about generics as full classes. They are merely "templates". If you do a "specialize TFPGObjectList<SomeType>" the compiler will use the recorded declaration and implementation of the "TFPGObjectList", replace all occurences of the type "T" by "SomeType" and will parse the specialization as if you had written it like that yourself.

You'd need to check against your specialized "TMyGen" instead of "TFPGObjectList" then it would work.

In mode Delphi and my generic branch (I hope ^^) you are able to do it like this as well:

myV := TMyDerived.Create(False);
if myV is TFPGObjectList<TMyGen> then
   ...

Under the condition that "TMyDerived" is declared like in your previous mail.

Regards,
Sven

2011/11/24 Sven Barth

Thanks for the explanations.

In mode Delphi and my generic branch (I hope ^^) you are able to do it like

I think the syntax you gave should be
 if myV is TFPGObjectList<TControl> then

I will continue learning where to use generics and where not to use them.

Regards,
Juha

Yes, of course. Thank you for the correction.

I will continue learning where to usegenerics and where not to use them.

Have fun and feel free to ask further questions :)

Regards,
Sven