Method return types are covariant though, yes? So you should always be able to return a more-derived type from a method that is declared to return a less-derived type. If I read your example right, ClassA is "Animal" and ClassB is "Cat" (in covariance-blog-post lingo). So that is legal, unless I'm missing something.
Perhaps the issue is that the @interface for your overriding class (the one that implements the override of "propA") does not redeclare that method as returning (ClassB *)? I am away from Xcode right now (and as such have immediately begun to doubt everything I know about programming and the universe in general), but I reckon if you don't declare that in the overridden @interface then other public callers are just going to dumbly see the superclass @interface. But that's not a variance problem (again, assuming I'm not messing something up here), that's just that you need to declare it. Peter On Mon, Jan 4, 2016 at 11:40 AM, Jeff Kelley <slauncha...@gmail.com> wrote: > Hi Dave, > > For myClassA, you will always have to cast the return value to > myClassB if you know that’s what will be returned. For a great blog post on > the subject, I give you Mike Ash: > > > https://mikeash.com/pyblog/friday-qa-2015-11-20-covariance-and-contravariance.html > > > Jeff Kelley > > slauncha...@gmail.com | @SlaunchaMan <https://twitter.com/SlaunchaMan> | > jeffkelley.org <http://jeffkelley.org/> > > On Jan 4, 2016, at 11:24 AM, Dave <d...@looktowindward.com> wrote: > > > > Hi, > > > > In Objective-C, is it possible to override a property and have it return > a different type to the base class? > > > > I have a base class with the following property defined: > > > > @property (nonatomic,retain) ClassA* > propA; > > > > and the following getter: > > > > > > -(ClassA*) propA > > { > > return someValueOfClassA; > > } > > > > > > I’d like to override this in my subclass, as so: > > > > -(ClassB*) propA > > { > > ClassA* myClassA; > > ClassB* myClassB; > > > > myClassA = super.propA; > > myClassB = [[ClassB alloc] initWithClassA:myClassA]; //ClassB is a > subclass of Class A > > > > return myClassB; > > } > > > > I can do this ok, but when I access the property: > > > > // myClassA is already setup OK > > ClassB* myClassB; > > > > myClassB = myClassA.propA; //Warning on this line > saying return type is not ClassB > > > > and I have to coerce the value to be ClassB to get rid of the warning. > Is there any way to avoid this? > > > > Thanks a lot > > Dave > _______________________________________________ > > Cocoa-dev mailing list (Cocoa-dev@lists.apple.com) > > Please do not post admin requests or moderator comments to the list. > Contact the moderators at cocoa-dev-admins(at)lists.apple.com > > Help/Unsubscribe/Update your Subscription: > https://lists.apple.com/mailman/options/cocoa-dev/vast.grapes%40gmail.com > > This email sent to vast.gra...@gmail.com > _______________________________________________ Cocoa-dev mailing list (Cocoa-dev@lists.apple.com) Please do not post admin requests or moderator comments to the list. Contact the moderators at cocoa-dev-admins(at)lists.apple.com Help/Unsubscribe/Update your Subscription: https://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com This email sent to arch...@mail-archive.com
