Re: Help on subclassing
Re: Help on subclassing
- Subject: Re: Help on subclassing
- From: Will Mason <email@hidden>
- Date: Thu, 1 Dec 2005 11:16:58 -0800 (PST)
What one typically does in this situation, since Objective-C does not support "pure virtual" methods, is to declare someMethod in class A and define it like this:
- (void) someMethod
{
[self subclassResponsibility: _cmd];
}
Then you just override the method in classes B and C. This will ensure that if the method ever does get called on an object of type A that it won't be allowed at runtime.
Cheers,
Will
----- Original Message ----
From: Joachim <email@hidden>
To: email@hidden
Sent: Thu Dec 1 13:01:22 2005
Subject: Re: Help on subclassing
Hi Jon,
Thanks for the input. When reading "The Objective-C Programming
Language" a few months back, I didn't think I would need protocols...
But still, it doesn't seem to do the trick.
It still requires class A to implement the someMethod, and that's
exactly what I don't want. That method will only exist in A's
subclasses, B and C. So I might want to make a protocol that both B
and C conforms to, but it still doesn't solve my problem:
I want to call [self someMethod] in my A class code, knowing that all
A class code, including someMethod, will only be executed within a B
or C object, so that someMethod only needs to be present in the B and
C subclasses.
If I'm doing what you suggest below, I get a compile error, saying
that A doesn't fully implement the 'myProtocol' protocol, because
someMethod is not present in A.
Other ideas will be much appreciated.
Thank you,
Joachim
On 30-Nov-05, at 4:13 PM, Jon Adelman wrote:
>
Declare a protocol for the methods that you are going to be using
>
in both B and C
>
>
@protocol myProtocol
>
>
- (NSRect) someMethod;
>
>
@end
>
>
>
Make A Subscribe to that protocol
>
>
@interface A : NSObject <myProtocol>
>
>
@end
>
>
>
>
>
On 30-Nov-05, at 4:09 PM, Joachim wrote:
>
>
> Esteemed experts,
>
>
>
> I'm working with 2 classes, B and C, that are quite similar to
>
> each other. One of them is a preview of the other. They both
>
> inherit from their superclass, A, that will never be instantiated
>
> (like an abstract class). All functionality that is the same for
>
> both B and C is of course placed in A, and the methods that differ
>
> are put in the respective B and C subclasses.
>
>
>
> From within the generic code (in the A class), I'd like to call
>
> code that's declared and will be run in the respective B and C
>
> subclasses. That is, from within A I'd like to call [self
>
> someMethod], where someMethod only exists in B and C - not in A.
>
> How do I do this?
>
>
>
> In my attempts, the compiler warns that A may not respond to
>
> someMethod which is fine, because I know it will at runtime. And
>
> it works as expected at runtime. But if someMethod returns a
>
> value, the compiler throws an error:
>
>
>
> NSRect myRect = [self someMethod];
>
> -> error: invalid initializer
>
>
>
> (or
>
>
>
> NSRect myRect;
>
> myRect = [self someMethod];
>
> -> error: incompatible types in assignment)
>
>
>
> My workaround is to declare the method in A where I just return a
>
> "dummy" value, because it's never used anyway. But it's not very
>
> nice to look at and shouldn't be necessary. Any solution to this?
>
>
>
> Thanks you for your help,
>
> Joachim
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