Re: Subclassing Catch-22

FROM : Todd Ransom
DATE : Tue Apr 26 22:20:04 2005

I never said it was a problem or a bug in Cocoa. I know this is how 
it's supposed to work. I am asking if anyone knows of a pattern or 
method that would help me avoid this type of situation.

Todd Ransom
Return Self Software
http://returnself.com

On Apr 26, 2005, at 2:05 PM, Marco Scheurer wrote:

>
> 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
>