Objective-C, protocols and subclasses

Question

Let's say that I have the following protocols defined:

// basic protocol for an User Interface object:

@protocol UIObjectProtocol <NSObject>
@property (assign) BOOL touchable;
@end

// basic protocol for an User Interface object that is held by a holder object:

@protocol UIHeldObjectProtocol <UIObjectProtocol>
@property (readonly) id holder;
@end

And the following class hierarchy:

// base class for an User Interface object, which synthesizes the touchable property

@interface UIObject : NSObject <UIObjectProtocol> {
   BOOL _touchable;
}
@end

@implementation UIObject
@synthesize touchable=_touchable;
@end

At this point, everything is OK. Then I create a UIObject subclass named UIPlayingCard. Inherently, UIPlayingCard conforms to the UIObjectProtocol since it's superclass does it too.

Now suppose I want UIPlayingCard to conform to UIHeldObjectProtocol, so I do the following:

@interface UIPlayingCard : UIObject <UIHeldObjectProtocol> {
}
@end

@implementation UIPlayingCard
-(id)holder { return Nil; }
@end

Note that the UIPlayingCard conforms to UIHeldObjectProtocol, which transitively conforms to UIObjectProtocol. However I'm getting compiler warnings in the UIPlayingCard such that:

warning: property 'touchable' requires method '-touchable' to be defined - use @synthesize, @dynamic or provide a method implementation

which means that the UIPlayingCard superclass conformance to the UIObjectProtocol wasn't inherited (maybe because the @synthesize directive was declared in the UIObject implementation scope).

Am I obligated to re-declare the @synthesize directive in the UIPlayingCard implementation ?

@implementation UIPlayingCard
@synthesize touchable=_touchable; // _touchable now must be a protected attribute
-(id)holder { return Nil; }
@end

Or there's another way to get rid of the compiler warning? Would it be the result of bad design?

Thanks in advance,

This has nothing to do with your problem, but it's generally not the best idea to use "Apple's" prefixes for your own classes/protocols/constants/whatevers: You never know what big fruit will include in the next release... — danyowdee
I agree with you danyowdee, in fact I'm not using the 'UI' prefix in my application and I do follow all these good practices listed by you. I just suppressed the real prefix (the company prefix, for instance) here in the post since it has no relation with the problem. — Eduardo Coelho

Jeremy W. Sherman Jeremy W. Sherman · Accepted Answer · 2011-03-02T16:05:39

You comment that _touchable would then have to be protected, but you did not include an @private compiler directive, so _touchable is already protected.

Clang does not seem to give this warning in this case, so the most straightforward solution:

Change your compiler from GCC to Clang.

This will have plenty of side-benefits, as well, including much more intelligible error and warning messages.

It is a bug that GCC complains about this. The implementation is in fact inherited, as you can verify with a simple main function exercising UIPlayingCard's touchable property. So, your second solution:

Ignore the warning as bogus. If none of your tests are failing, what do you care, anyway?

Objective-C, protocols and subclasses

4 Answers