[fpc-devel] RFC: Delphi style class helpers

Dimitri Smits smitco at telenet.be
Thu Dec 30 20:02:57 CET 2010

----- "Jonas Maebe" <jonas.maebe at elis.ugent.be> schreef: 

On 21 Dec 2010, at 00:16, Jonas Maebe wrote: 

On 20 Dec 2010, at 21:37, Sven Barth wrote: 

- should abstract methods be forbidden? 
> Yes, since 
> a) they are virtual 
> b) even if it were possible to have virtual methods, there would be no way to override them with implementations that do something (since you cannot inherit from a class helper) 
Actually, it seems that inheritance is in fact allowed for Delphi-style class helpers: http://qc.embarcadero.com/wc/qcmain.aspx?d=76577 (it's not for Objective-C categories). But I'm still quite sure you cannot do anything that requires changes to the VMT of the helped class. 

There are also at some extra checks that must be added for constructors/destructors introduced via class helpers (Objective-C/Pascal doesn't have constructors or destructors, so it doesn't matter there): 
* http://docwiki.embarcadero.com/RADStudio/en/E2295_A_class_helper_cannot_introduce_a_destructor_(Delphi) 
* http://docwiki.embarcadero.com/RADStudio/en/E2296_A_constructor_introduced_in_a_class_helper_must_call_the_parameterless_constructor_of_the_helped_class_as_the_first_statement_(Delphi) 
* http://docwiki.embarcadero.com/RADStudio/en/E2358_Class_constructors_not_allowed_in_class_helpers_(Delphi) 



it seems that the virtual methods can be overridden (AOP style) using the TVirtualMethodInterceptor that comes with Delphi XE. It can also be on a per instance base (as opposed to on a per type) if I've read up on it correctly. 

I'd say that would be a better candidate for that purpose/intent/use case. 

kind regards, 
Dimitri Smits 

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