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,

Answer 1

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?