How to release attribute which is protocol?

Question

Hi, everyone.

I have working for iPhone development for a while. First time, I so surprised with memory-management in objective-c :). but now I got it a little bit.

The question is, sometime, I use protocol as an attribute of a class, because I think its definition very similar to 'interface' in C# or Java. like below.

@protocol Shield
   ...
@end

// Interface
@interface Dragon {
    id<Shield> shield
    NSString * name;
}

@property (nonatomic,retain) id<Shield> shield;
@property (nonatomic,retain) NSString * name;

@end

but I alway release any attribute object in dealloc() method. like below.

-(void)dealloc {
   [name release];
   [shield release];  // <--- Totally impossible. xcode said '-release not found in protocol'
   [super dealloc];
 }

As you see, I couldn't release the protocol. So would this cause me future memory issue? Do you have another way to handle this solution to advice me?

Answer 1

You need to define your protocol as adhering to the NSObject protocol, like this:

@protocol Shield <NSObject>
   ...
@end

It's simple when you know how! ;-)

Edit: Also, you're correct - protocols in Objective-C are equivalent to interfaces in Java and C#.

Another edit: It might strike you as odd having to do this, but Objective-C allows for multiple root objects, so you can't actually guarantee that every object will descend from NSObject. Since release is an NSObject method, you have to define your protocol as also adhering to the <NSObject> protocol before you can be sure it'll be able to respond to the release method.

Answer 2

1-the proper thing to do instead of [shield release] is setting

self.shield = nil;

2-Also change

@property (nonatomic,retain) id<Shield> shield;

to

@property (nonatomic,assign) id<Shield> shield;

Then you are fine.

edit:

The reason that you avoid retaining delegates is that you need to avoid a retain loop:

A creates B A sets itself as B's delegate … A is released by its owner

If B had retained A, A wouldn't be released, as B owns A, thus A's dealloc would never get called, causing both A and B to leak.

You shouldn't worry about A going away b/c it owns B and thus gets rid of it in dealloc.

http://stackoverflow.com/questions/918698/why-are-objective-c-delegates-usually-given-the-property-assign-instead-of-retain

please see uitableview class reference for an example protocol declaration:

@property(nonatomic, assign) id < UITableViewDelegate> delegate