Question

I thought I had a pretty good handle on memory management for objective-c, but I can’t figure out the following situation:

---

@protocol MyProtocol
@end

---

@interface MyObject : NSObject {
    id<MyProtocol> reference;
}
@property (nonatomic, retain) id<MyProtocol> reference;
@end

---

@implementation MyObject 
@synthesize reference;
-(void) dealloc {
    [reference release];
    [super dealloc];
}
...
@end

---

This gives me a “warning: ‘-release’ not found in protocol(s)“.
Can I safely ignore this error? Or am I doing something horribly wrong?

Answer 1

Yes you can safely ignore this error. An object declared as type id<MyProtocol> may not inherit from NSObject (you do not have to use the Cocoa libraries to program in Objective-C and there are other root classes even in Cocoa such as NSProxy). Since retain (and release, autorelease) are declared in NSObject, the compiler cannot know that the instance declared as type id<MyProtocol> responds to these messages. To get around this, Cocoa also defines the NSObject protocol which mirrors the NSObject API. If you declare your protocol as

@protocol MyProtocol <NSObject>
@end

indicating that MyProtocol extends the NSObject protocol, you’ll be set.