Re: Subclassing Catch-22

FROM : Marco Scheurer
DATE : Tue Apr 26 22:05:11 2005

On Apr 26, 2005, at 20:28, Todd Ransom wrote:

> I have a controller class that has methods like this:
>
> - (BOOL)doSomething;
> - (NSString *)getInformationRequiredToDoSomething;
>
> - (BOOL)doSomething {
>
>    NSString *info = [self getInformationRequiredToDoSomething];
>    ...
> }
>
> Which works fine until I subclass. In my subclass I would like 
> doSomething to call super for a subset of possible actions. But 
> when I call super it calls [self 
> getInformationRequiredToDoSomething] and returns its own info, not 
> super's.


You are not calling procedures but sending messages.

And "sending a message to super" really means: "sending a message to 
self, starting the search for the selector in the superclass".

So [super doSomething] will correctly send 
getInformationRequiredToDoSomething to self and in all probability, 
unless you've overriden doSomething in your subclass, this is just 
the same as [self doSomething].

> I thought this was a fairly common pattern in Cocoa -- overriding 
> super's methods to do something specific and deferring to super for 
> its own functionality. I feel like I am missing some simple rule 
> for safely subclassing that would prevent this problem. Is there a 
> way I can design these methods so I don't run into this problem?


This is not a "problem", just how all this is supposed to work: you 
should read the section about self and super at http://
developer.apple.com/documentation/Cocoa/Conceptual/ObjectiveC/
LanguageOverview/chapter_3_section_6.html .

marco


Marco Scheurer
Sen:te, Lausanne, Switzerland  http://www.sente.ch