Hello,

I was surfing the internet and came across this structure Delphi:

TJSONPair = class sealed(TJSONAncestor)

So, what is this?

Link:

http://docwiki.embarcadero.com/Libraries/XE3/en/API:Data.DBXJSON.TJSONPair

Simple: You are not allowed to create child classes of sealed classes (the compiler enforces this). FPC supports this since 2.4.2.

Regards,
Sven

2013/10/7 Sven Barth

Perfect!

It tested it:

  TBaseClass = class
  end;
  TOtherClass = class sealed(TBaseClass)
  end;
  TFooClass = class(TOtherClass)
  end;

and:

unit1.pas(24,32) Error: Cannot create a descendant of the sealed class
"TOtherClass"

It is wonderful, thank you very much Sven! :)

Well, the opinion whether this feature is wonderful or not differs among FPC/Delphi users :P

Regards,
Sven

2013/10/7 Sven Barth

:D

May I know the usual use of such feature? I can only think of library vendors want to seal a class to prevent users from modifying it without paying for an upgraded/better version of such class.

Dennis

A well written class doesn't need to be sealed. As well as doesn't require an inheritance.

IIRC, "sealed" was added to delphi language at the time of the delphi .nettism. Correct me, if I'm wrong but Delphi-net is dead (replaced by Oxygen, which is not delphi or pascal).
However, since the keyword was introduced in the some point of the language, the support was provided in the next versions of the Delphi language/compiler.
Since FPC announces Delphi-compatibility on syntax level, the keyword was introduced in FPC as well.

Usefulness of the "sealed" (as some other .nettish syntax, i.e. class helpers) is questionable and further discussion is probably better for fpc-other list. So feel free to use it for good or bad.

thanks,
Dmitry

Actually, it's useful when used a platform-specific keyword (i.e. for declaring an external Java-class, which can be finalized). It was introduced for platform-specific needs (delphi.net), but became a part of the language.

thanks,
Dmitry

On Tue, Oct 8, 2013 at 12:14 AM, Dmitry Boyarintsev <

FPC/Delphi users :P

vendors want to seal a class to prevent users from modifying it without paying for an upgraded/better version of such class.

That's why I implied that the usefulness is highly debated...

Regards,
Sven

Does it prevent the creation of class helpers?

Sven Barth wrote

Simple: You are not allowed to create child classes of sealed classes (the compiler enforces this). FPC supports this since 2.4.2.

All good and well, but I really don't see the point of a class that you can't extend. The original developer can never foresee all possible use cases in the future. We already have issues with some classes defining fields or methods and private, when they would have been better suited as protected. Sealed classes are just going to make matters worse. It defeats the whole purpose of OOP.

Regards,
- Graeme -

Am 08.10.2013 09:12 schrieb "Mark Morgan Lloyd" <

(the compiler enforces this). FPC supports this since 2.4.2.

Does it prevent the creation of class helpers?

At least not in FPC. AFAIK I didn't test whether Delphi forbids it...

Regards,
Sven

08.10.2013 17:35, Graeme Geldenhuys пишет:

Just open the old mail thread - the question was discussed there with your participation (as I remember). There is no need to repeat it again.

Best regards,
Paul Ishenin