[fpc-devel] class abstract, class sealed implementation. please review.

[Florian Klaempfl]

Graeme Geldenhuys schrieb:
> 2009/10/19 Paul Ishenin <ip at kmiac.ru>:
>> ...
>> TNokiaPhone = class(TCellularPhone)
>> TNokia37xxPhone = class(TNokiaPhone)
>> TNokia3720Phone = class sealed(TNokia37xxPhone)
>>
>> TBasicPhone is ofcource an abstract class. It can have some basic fields
>> like color, weight, dimensions, ...
>> TNokia3720Phone is a final product which can't have any further derivations.
>> It is logically to mark this class as sealed to prevent ocasional errors.
> 
> Bad example! :-)  I have the Nokia 5800XM. You get that "final model"
> in two flavours. Red and Blue (phone color and software theme color).
> Then you also get revisions of that phone. Revision 1 had problems
> with the speaker. Revision 1.x (or 2.x - I don't know how they
> numbered them) have those hardware problems fixed. So it's the same
> model phone N5800XM, but it has various differences.

Without discussing details, I think a singleton is a good example where
sealed is useful.

> 
> This is exactly my point about sealed classes. When you design the
> product or class, you have NO way of know what will come in the
> future. So you need to stay flexible to change! Yet another OOP
> principal that "sealed" tries to prevent!

This might be true, but what's the difference with private etc.?