Question

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.

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

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